Which C15 do you have? The photographs and every location below are a C15 ACERT – the emissions engine built from 2004. The earlier pre-emissions C15 (the 6NZ and its relatives) carries far fewer sensors and does not share this layout, so check your engine serial prefix before hunting for a part that may not be fitted to your engine.
- Fuel injectors (under valve cover center of cylinder head)
- Camshaft position sensor (CMP) (located on side at top of timing cover)
- Oil pressure sensor (driver’s side of engine block)
- Fuel Temp Sensor (on top of fuel filter near primer pump)
- Crankshaft position sensor (CKP) (located at bottom side of timing cover)
- Engine coolant temperature sensor (ECT) (located next to thermostat housing)
- Accelerator Pedal Position Sensor (APP) (attached to accelerator pedal)
- Boost Pressure Sensor (attached to intake manifold)
- Inlet Air Temp Sensor (attached near air inlet on intake manifold)
- Atmospheric Pressure Sensor (driver’s side of cylinder block)
The engine oil temperature is not in the list above because there is no oil temperature sensor on this engine to point at – the ECM works the figure out for itself. It is covered at the end, because plenty of people come looking for it.
3. Oil Pressure Sensor
Monitors the lubrication oil pressure immediately after the oil passes through the primary filtration system to ensure the proper lubrication of all sliding engine parts. The ADEM ECM continuously evaluates this voltage return against engine speed (RPM) to verify that engine lubrication parameters stay within safe operating ranges and to activate the low oil pressure warning or engine shutdown protection systems. Sensor faults or sudden signal losses typically cause immediate dashboard warning buzzers, a dropped oil gauge reading, severe engine power derating, or a sudden, uncommanded electronic engine shutdown to prevent catastrophic engine damage.
Where it is. The oil pressure sensor, number 3 in the list and the driver’s-side photograph above, is located on the lower driver’s side of the cylinder block. It is threaded horizontally directly into the primary main engine oil gallery channel, positioned horizontally behind the vertical oil filter base housing and sitting vertically just above the engine oil pan mounting flange. Look for a brass-bodied sensor with a hexagonal base and an integrated 3-pin triangular electrical connector socket pointing slightly outward toward the vehicle frame rail.
Getting at it. Replacement is a simple 20-minute driveway job that is best performed when the engine is completely cold to avoid burns from hot engine components. No structural engine brackets, rigid fuel lines, or airlines need to be unbolted or removed, although sliding a drain pan under the area is recommended to catch the small amount of oil that will seep out of the block gallery when the sensor is unscrewed. The job requires a 1-1/16 inch deep socket or standard open-ended combination wrench, a drive ratchet, and a lint-free shop rag to wipe down the surrounding block surface before removing the old sensor.
Codes: SPN 100 with FMI 3 or FMI 4, indicates an engine oil pressure sensor signal voltage return that is either open or short-circuited to the engine ground.
Fitting it. This sensor relies on straight machine threads and a dedicated high-temperature Viton rubber O-ring nested around the base of the hex head to seal pressurized engine oil within the main oil gallery. Never install the sensor completely dry or wrap the brass machine threads in standard white PTFE Teflon tape or apply liquid RTV silicone, as stray fragments can tear off, enter the pressurized oil stream, where they can clog critical oil galleries. Instead, apply a clean smear of fresh engine oil to the rubber O-ring to prevent it from pinching or tearing when threaded into the cast iron block. Thread the sensor into the block by hand to ensure smooth engagement, and then torque the hex base to 10 Nm (88 lb-in) to prevent damaging the sensor or the soft threads in the engine casting.
How it typically fails. Being mounted directly to the engine block’s primary oil gallery subjects this sensor to near constant, low-frequency engine vibrations and over extended periods, vibration and thermal stress can rupture the sensor’s internal sealing diaphragm, which causes pressurized engine oil to leak past the terminal pins into the external wire harness plug to create a short circuit.
5. Crankshaft Position Sensor (CKP)
Monitors the precise rotational speed and fractional position of the crankshaft to establish the foundational timing baseline for the ADEM ECM. During startup and normal engine operation, the ECM continuously tracks the wave pulses from this sensor and correlates them directly against the input from the Camshaft Position Sensor (CMP). This mechanical-to-electronic synchronization allows the computer to verify exact camshaft/crankshaft correlation, map the specific top dead center (TDC) compression stroke of cylinder number one to control the high-pressure fuel injectors. If the correlation signal drops out, slips out of sync due to excessive gear lash, or fails completely, the engine will develop an immediate crank-no-start condition, severe mid-range misfiring, rough idling, erratic dashboard tachometer readings, or an abrupt engine shutdown as the computer cuts all fueling to prevent mechanical engine damage.
Where it is. The crankshaft position sensor, number 5 in the list and the driver’s-side photograph above, is located at the lower front driver’s side of the engine block. It is threaded horizontally directly into the lower outer quadrant of the cast aluminum front timing gear cover housing, positioned horizontally forward of the gear-driven air compressor and sitting vertically just above the main engine oil pan rail flange. Look for a cylindrical plastic and steel magnetic probe body secured by a single hold-down bolt flange, fitted with a 2-pin rectangular connector plug that routes straight up into the primary timing harness trunk line.
Getting at it. Replacement is a moderately simple 30-minute driveway job, though clearances can be tight depending on the vehicle’s front axle and steering shaft configurations. No structural engine brackets, coolant hoses, or fuel lines need to be unbolted or swung out of the way, but working through the driver’s side wheel well offers the cleanest line of sight. The job requires a 10mm socket or a 1/2-inch wrench for the single bracket fastener, a 1/4-inch drive ratchet handle with a 6-inch extension bar, a small pick tool to release the harness safety clip, and a lint-free shop rag to wipe down the timing cover around the sensor.
Codes: SPN 190 with FMI 2, 3, or 8, or primary correlation codes SPN 651 through 656, indicates a crankshaft speed signal that is erratic, missing, un-synchronized, or suffering from a serious circuit fault.
Fitting it. This sensor relies on a high-temperature rubber barrel O-ring around its cylindrical barrel to seal engine oil inside the front timing gear train. Never install the sensor completely dry or use RTV silicone adhesive or white PTFE thread tape on the smooth plastic probe barrel, as these compounds could alter the sensor’s critical depth spacing relative to the timing gear teeth and exceed the magnetic gap tolerance. Instead, apply a clean smear of fresh engine oil to the rubber O-ring to ensure it slides smoothly into the aluminum timing cover bore without binding or tearing. When installing the hold-down bolt, tighten it to 12 Nm (9 lb-ft) using hand tools, as over-tightening can easily fracture the composite plastic mounting ear or strip out the soft aluminum threads inside the front cover casting.
After you fit it. After replacing this component or the timing train gears, a mandatory Timing Calibration procedure must be performed via Caterpillar Electronic Technician (Cat ET) software to recalibrate the electronic offset between the crank and cam signals.
How it typically fails. Being positioned low on the front timing cover subjects this sensor to intense, continuous mechanical vibration from the crankshaft damper pulley alongside harsh road debris and moisture splashing up from the front tires. Over extended periods, continuous thermal cycles from the engine block bake the plastic sensor housing brittle, leading to hairline cracks that allow fine metallic engine debris and engine oil to enter the internal magnetic pickup coil and short out the circuit. Furthermore, if the front gear train develops excessive backlash or if a gear is misaligned by even half a tooth during a repair, the resulting cam/crank sensor signal mismatch will cause the ECM to flag a correlation fault code and initiate a low-power limp mode due to the compromised injection timing accuracy.
6. Engine Coolant Temperature Sensor (ECT)
Monitors the temperature of the engine coolant to manage essential thermal maps across all engine speed and load variations. The ADEM ECM uses this information to control cold-mode startup fueling maps, govern cooling fan clutch engagement thresholds, and activate safety overrides to save the engine block if coolant/engine temperatures spike beyond acceptable maximum thresholds. Sensor faults or out-of-range sensor signals typically cause severe white or grey smoke on cold startups, extended engine cranking times, the engine cooling fan locking up at 100% duty cycle continuously, an inoperative dashboard temperature gauge, or or in some cases an uncommanded engine shutdown.
Where it is. The engine coolant temperature sensor, number 6 in the list and the close-up photograph above, is located at the top front driver’s side of the engine block. It is threaded horizontally directly into the cast iron water temperature housing assembly, positioned immediately adjacent to the primary dual-thermostat housing regulators and sitting vertically directly above the heavy gear-driven air compressor. Look for a brass-bodied probe with a hexagonal base and a black, two-pin rectangular Deutsch DT connector plug integrated into the front leg of the primary engine sensor wiring harness loom.
Getting at it. Replacement is a 30 to 45-minute shop floor job that requires dropping the coolant level below the thermostat housing line before unthreading the component to prevent a massive coolant dump over the alternator and air compressor. No major structural engine brackets, fuel lines, or airlines need to be unbolted or removed to access the sensor. The job requires a 1-inch deep socket or open-ended combination line wrench, a drive ratchet, and a clean drain bucket to catch spilling coolant.
Codes: SPN 110 with FMI 3 or FMI 4, indicates an engine coolant temperature sensor signal that is either open or short-circuited to ground.
Fitting it. This sensor features straight male machine threads and seals against the pressurized cooling jacket using a high-temperature Viton rubber O-ring or an embedded copper crush gasket. Never wrap the sensor threads in standard white PTFE Teflon tape or apply household RTV silicone adhesive, as loose fragments can break loose into the cooling stream, travel directly to the narrow passages inside the oil cooler or heater core, preventing effective coolant circulation. Instead, install the sensor with a thin, uniform coating of specialized high-temperature liquid thread sealant or anti-seize applied strictly to the male threads. Thread the sensor into the casting by hand first to avoid cross threading, then torque the brass hex to 20 Nm (15 lb-ft) to prevent damaging the new sensor.
How it typically fails. Being positioned directly alongside the thermostat housing subjects this sensor to severe, continuous thermal cycling. Over extended periods, repeated expansion and contraction causes the internal silicone potting compound to degrade and shrink, allowing pressurized engine coolant to seep past the pins into the external harness plug, which causes a short circuit between connector pins.
8. Boost Pressure Sensor
Monitors the absolute pressure of the compressed intake air to optimize combustion efficiency. The ECM reads this voltage feedback to continuously balance the air-to-fuel ratio, regulate high-pressure fuel injection timing, and coordinate exhaust emission controls. Sensor faults or abnormal pressure signals typically cause a severe drop in engine power, black exhaust smoke under acceleration, sluggish throttle response, and a hard-starting condition.
Where it is. The boost pressure sensor, number 8 in the list and the driver’s-side photograph above, is located on the upper driver’s side of the engine. It is threaded horizontally directly into the cast aluminum intake manifold plenum runner, positioned immediately behind the air compressor bracket assembly, and sitting vertically directly above the fuel primer pump structure. Look for a small plastic-bodied sensor with a brass hexagonal base, and a 3-pin triangular electrical connector harness plug pointing directly toward the rear flywheel housing channel.
Getting at it. Replacement is an easy, 15-minute driveway job that does not require the removal of engine components, structural brackets, coolant plumbing lines, or rigid steel fuel lines. The job requires a 1-1/16 inch deep socket and ratchet, or a standard open-ended combination wrench, a small pocket screwdriver to depress the harness connector locking tab, and a clean shop rag to wipe down the area around the sensor.
Codes: SPN 102 with FMI 3 or FMI 4, indicates an absolute manifold boost pressure signal voltage return that is either completely open or shorted directly to the engine ground.
Fitting it. This sensor relies on a heavy-duty, high-temperature Viton rubber O-ring around the base of its straight male threads to seal pressurized boost air within the intake manifold plenum chamber. Never install the sensor completely dry or wrap the threads in standard white PTFE Teflon tape or apply liquid RTV silicone, as frayed tape or dried silicone beads can break off and slide into the internal pressure orifice, preventing the sensor from operating correctly. Instead, apply a clean, light smear of fresh engine oil to the rubber O-ring to prevent it from pinching or tearing during installation. Torque the sensor to 10 Nm (88 lb-in) to prevent stripping out the aluminum threads in the intake manifold.
How it typically fails. Being mounted directly to the engine’s primary fresh-air intake plenum subjects this sensor to severe, high-amplitude mechanical vibration and continuous oil mist traveling through the closed crankcase breather tube. Over extended periods this atomized engine oil mixes with microscopic carbon soot particles to form a thick, tar-like sludge coating that fills and plugs the tiny air sampling orifice on the tip of the sensor probe, insulating the internal crystal from actual manifold pressure fluctuations and causing the sensor to report frozen or out-of-range pressure values to the ECM.
9. Inlet Air Temperature Sensor (IAT)
Monitors the temperature of the compressed intake air after it has passed through the charge air cooler assembly. The ADEM ECM reads this variable resistance input to calculate changes in air density, allowing the computer to continuously adjust fuel injection timing and volumes, manage emissions compliance, and protect the cylinder head from excessive thermal stress under high load. Sensor faults or out-of-range signals typically cause sudden drops in engine horsepower, excessive black smoke from the exhaust caused by improper air-fuel matching, rough idling, poor cold-weather driveability, and an immediate check engine light illumination.
Where it is. The inlet air temperature sensor, number 9 in the list and the driver’s-side photograph above, is located on the upper driver’s side of the engine. It is threaded horizontally directly into the secondary elbow or runner wall of the cast aluminum intake manifold plenum, positioned immediately behind the main vertical charge air intake pipe and sitting horizontally forward of the rear valve cover section. Look for a small brass-bodied probe with a plastic hex head, featuring an integrated 2-pin rectangular connector plug that taps straight into the top deck of the engine sensor wiring harness loom.
Getting at it. Replacement is an easy, 15-minute driveway job and no major structural brackets, fuel lines, or cooling paths need to be unbolted or removed to gain easy access to the sensor. The job requires a 1-inch deep socket or a standard open-ended combination line wrench, a ratchet, a small pocket screwdriver to depress the harness plug security clip, and a clean shop rag to wipe away oil mist from the manifold bore.
Codes: SPN 105 with FMI 3 or FMI 4, indicates an inlet air manifold temperature signal voltage return that is either open or short-circuited to the engine ground.
Fitting it. This sensor relies on straight male machine threads and uses a high-temperature Viton rubber O-ring or a thin copper crush washer seated at the base of the hex head to isolate the pressurized intake air. Never wrap the sensor threads in standard white PTFE Teflon tape or apply household RTV silicone adhesive, as loose fragments can break loose into the high-velocity intake stream and migrate straight into the engine cylinders. Instead, install the sensor with a thin, uniform coating of fresh engine oil or clean liquid thread sealant applied strictly to the male threads. Thread the sensor into the soft aluminum manifold casting by hand first to prevent cross-threading, then torque the hex base to 20 Nm (15 lb-ft) to prevent damaging the new sensor or the soft aluminum threads in the intake manifold.
How it typically fails. Being positioned directly inside the main fresh-air intake tract exposes this sensor to continuous thermal soaking and an ongoing mist of atomized engine oil from the crankcase breather tube. Over extended periods, this oil film combines with fine soot particulates blown back from the valves to form a thick, crusty sludge layer that completely cakes the exposed thermistor bulb on the tip of the sensor probe. This sludge coating acts as an insulator, slowing the sensor’s response time to air temperature shifts and forcing the ECM to utilize incorrect density values that hurt engine efficiency and cause poor fuel economy.
10. Atmospheric / Barometric Pressure Sensor
Monitors the ambient barometric (aka atmospheric) pressure to establish a constant atmospheric baseline calculation for the ADEM ECM. The ECM continuously reads this voltage return as a reference value to accurately calculate fuelling strategies as the altitude changes to manage emissions and engine power output. Sensor faults or out-of-range signals typically cause sluggish engine performance at higher altitudes, poor fuel economy, an unexpected reduction in turbocharger boost response, or a check engine light accompanied by false codes for other sensors due to a lost reference baseline.
Where it is. The atmospheric / barometric pressure sensor, number 10 in the list and the driver’s-side photograph above, is located on the lower driver’s side of the cylinder block. It is threaded horizontally directly into the cast iron block casting, positioned horizontally directly behind the primary vertical engine fuel filter housing base and sitting vertically above the main engine oil pan rail. Look for a brass-bodied sensor with a hexagonal base and an integrated 3-pin triangular electrical connector plug that taps straight into the primary engine harness loom just behind the fuel line loops.
Telling it apart from the oil pressure sensor. The sensor immediately to the right of the fuel filter canister is the barometric / atmospheric pressure sensor. The sensor further to the right of the fuel filter is the oil pressure sensor, number 3 in the list and the driver’s-side photograph above. Note that the barometric / atmospheric pressure sensor screws into a “blind” hole in the engine casting. In this position, the sensor is not influenced by the compressed intake air as it would have been had it been mounted in the inlet air tract.
Getting at it. Replacement is a simple 15-minute driveway job that can be completed with minimal mechanical experience. No major structural brackets, fuel lines, or cooling lines need to be unbolted or removed to access the sensor bore, although clearing the wiring loom from its adjacent retention clips makes tool access much easier. The job requires a 1-1/16 inch deep socket or a standard open-ended combination wrench, a ratchet, a small flathead pocket screwdriver to release the connector harness safety clip, and a clean shop rag to wipe away road dust from the block surface.
Codes: SPN 108 with FMI 3 or FMI 4, indicates an atmospheric pressure signal voltage return that is either open or short-circuited to the engine ground circuit.
Fitting it. This sensor relies on straight male machine threads and a dedicated high-temperature Viton rubber O-ring seated around the base of the hex head to isolate the internal electronics from the engine block environment. Never install the sensor completely dry or wrap the threads in standard white PTFE Teflon tape or apply liquid RTV silicone compounds, as stray fragments or dried adhesive can block the extremely small, and critical reference port on the sensor body and ruin its calibration. Instead, apply a clean smear of fresh engine oil to the rubber O-ring to prevent it from pinching or tearing during installation. Thread the sensor into the engine block by hand to ensure a clean thread fit, then torque the hex base to 10 Nm (88 lb-in) to prevent damaging the new sensor.
How it typically fails. Being mounted on the lower quadrant of the engine block behind the fuel filter base exposes this sensor to continuous, low-frequency engine vibrations and harsh environmental elements splashing up from the front wheel wells. Over extended periods, repeated thermal cycling and exposure to road spray make the plastic connector housing brittle, leading to hairline cracks that allow moisture intrusion to short out the internal electrical circuit board. Additionally, because the sensor relies on an open ambient port to measure atmospheric air pressure, fine road dust, grease, or oil mist can slowly accumulate and plug the tiny breathing orifice, thus causing incorrect, implausible, or static readings.
Engine Oil Temperature (no separate sensor)
This engine does not use a discrete sensor to actively monitor the engine oil temperature; instead, the ECM derives and calculates the oil temperature value within its internal processing maps. The ECM continuously generates this thermal profile by cross-referencing signals from the Engine Coolant Temperature (ECT) sensor, number 6 in the list and the close-up photograph above, active engine load variables, engine speed (RPM), and fueling duration calculations. If the ECM detects or infers abnormal engine oil degradation or temperature spikes, the ECM adjusts variable engine oil pump parameters and activates power derate protocols to prevent engine damage. Because there is no physical sensor, real-world failures display as erratic gauge readings, constant false high-temperature alerts on the dashboard, or active engine protection power derates triggered by corrupted calculations rather than a physical part failure.
Why there is nothing to unbolt. In practical terms, the entire oil temperature data structure exists purely as an algorithm written into the electronic software architecture inside the main engine Electronic Control Module (ECM) housing assembly. Therefore, diagnosing and resolving an oil temperature fault is a specialized software and electrical diagnostic shop job that requires approximately 15 to 30 minutes of electronic troubleshooting rather than mechanical wrenching.
Codes: SPN 175 with FMI 3 or FMI 4, indicates an engine oil temperature calculation fault where underlying sensor circuits (such as the ECT sensor) are sending out-of-range voltage signals.
What an SPN 175 actually means. Because the oil temperature value is derived mathematically by the ECM, a physical open or short circuit code cannot occur on a non-existent oil sensor wire. Instead, an active SPN 175 fault code indicates that the ECM has detected corrupted internal data tables, or that a severe electrical open/short circuit has occurred on the Engine Coolant Temperature sensor circuit (SPN 110), forcing the oil temperature algorithm to default to a high-voltage or low-voltage safety value.
If you have an aftermarket gauge. If the vehicle features an aftermarket dashboard oil temperature gauge that is reading erratically or pinned at zero, inspect the bottom of the oil pan sump for a secondary, OEM chassis-manufacturer sensor (installed by Kenworth or Peterbilt), which must be sealed strictly with a fresh rubber O-ring and hand-tightened into the pan port to avoid stripping the aluminum or steel pan threads.
How it typically fails. In heavy-duty field operations, this virtual data channel typically breaks down due to component degradation in one or more primary measuring circuits. Over extended periods, the most common root cause of an oil temperature code is a failed or faulty Engine Coolant Temperature sensor (#6) that is reporting a static or incorrect thermal profile, which corrupts the ECM’s internal mathematical lookup tables. Additionally, if the vehicle’s wiring harness chafes against the cylinder head or air compressor brackets, it can induce a high resistance into the coolant sensor circuit, causing the calculated oil temperature reading to instantly skyrocket and trick the ECM into running the engine cooling fan continuously or initiating a preventative power-derate mode.
Models Found on Caterpillar Agricultural/Industrial applications as well as HD: (1999 and later)
- Kenworth
- Peterbilt