|Trouble Code||Fault Location||Probable Cause|
|P0112||Intake air temperature (IAT) sensor -low input||Wiring short to earth, IAT sensor, ECM|
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What Does Code P0112 Mean?
We already know that an engine needs fuel, air, and spark to run, but to run efficiently and most effectively, the engine control unit powertrain control module (ECU or PCM) needs to control these elements precisely. Depending on engine speed, engine load, coolant temperature, and air temperature, the ECU fine tunes fuel volume, ignition timing, variable valve timing (if applicable), and some emissions controls.
Intake air temperature (IAT) is a measurement of the ambient air coming into the engine at any given moment. This is important, since warmer air is less dense and vaporizes fuel faster, so less fuel needs to be injected. Conversely, colder air is denser and vaporizes fuel slower, so more fuel needs to be injected to get the same amount of power. The IAT sensor is therefore located in the air stream, usually after the air filter. Sometimes, the IAT sensor is integrated in the MAF (mass air flow) meter, otherwise it will be a separate sensor.
The IAT is a thermistor, which works by modifying a 5 V reference signal from the ECU. As temperature goes up, the IAT sensor’s resistance goes down, and vice versa. The voltage that the ECU reads on the other side is calculated as the temperature of the intake air. Different vehicles will have different values, but one typical Ford IAT sensor reads as in the table above. Many manufacturers use a similar scheme, resistance dropping as temperature rises, while some use the opposite scheme, resistance increasing as temperature rises.
The ECU’s internal memory table will recognize the voltage, not only to measure intake air temperature, but also to identify any fault. If the IAT sensor sends back a voltage far outside the acceptable range, the ECU will set a diagnostic trouble code (DTC). In case the IAT sensor reading returns an unusually high temperature, typically over 300 °F (149 °C) or less than 0.1 V at the ECU, the ECU will ignore the reading, set DTC P0112 – Intake Air Temperature Low Input, and illuminate the MIL or CEL (malfunction indicator lamp or check engine light). DTC P0112 may be accompanied by other IAT- and MAF-related DTCs, such as P0110, P0101, or P0102.
Note: Circuit Low Input codes are often the result of low battery voltages (that can have many possible causes), bad connections across electrical connectors or previously repaired wiring, as well as corrosion in electrical connectors. Other possible causes of low input voltages include poor installation of aftermarket components, poor quality aftermarket components like fuses, relays, and switches, and modification of the electrical system that could include the use of conductors that are not rated for use in a particular application. However, poor connections often result in high resistances in some parts of the circuit, which is why it is important to perform resistance and continuity checks during the diagnostic procedure.
What are the common causes of code P0112?
Depending on year, make, and model, DTC P0112 may have number of causes. Here are some of the most common.
- Defective IAT Sensor – As with all solid-state components, the IAT sensor may cease to function properly, be it due to manufacturing processes or damage.
- Contaminated Sensor – If the IAT sensor is especially dirty, such as due to a poorly-installed air filter or excessive positive crankcase ventilation (PCV) blow-by, the sensor may deteriorate or won’t react as fast to temperature changes. This type of failure would likely be accompanied by DTC P0110 or P0111.
- Improper Installation – The IAT sensor needs to be in the full flow of air into the engine intake. If the sensor is touching a metal surface, such as IAT sensors installed in the intake manifold, it could ruin the sensor or throw off its reaction to temperature changes.
- Stored Vehicles – Stored vehicles are particularly susceptible to rodent damage. In building their nests and making room for runways, rodents may gnaw through wires, among other things, interrupting circuits critical to your engine’s operation.
Diagnosing DTC P0112 is not a particularly difficult task. The sensor itself is simple in operation, which makes its analysis easy. On the other hand, if you have to trace an open or short circuit in the wiring harness, this will take time and patience. Depending on year, make, and model, the IAT may be reasonably priced, less than $100 in many cases, but if it is part of the MAF sensor, this will increase the price significantly. Have a professional double check your work before you condemn the MAF/IAT. Similarly, if you have done all your checks and suspect the ECU may be at fault, have your work double checked by a professional, before you condemn an expensive ECU.
What are the symptoms of code P0112?
Other than the MIL with just DTC P0112 in memory, you may not notice any drivability issues, though some engines may hesitate when cold, or ping when hot. Of course, with the MIL on, you’ll notice a drop in fuel economy, as well.
How do you troubleshoot code P0112?
This is a simple circuit, just two wires running from the ECU to the IAT sensor. If you have access to a scan tool, you can use it to read temperatures directly, otherwise you can use a DVOM (digital volt-ohm meter), thermometer, and a short pin, such as a thin wire or paper clip. A resistance table for your specific year, make, and model, will be especially helpful. You can often find these online, or perhaps if you ask your local trusted automobile technician. Given that DTC P0112 refers to low voltage input to the ECU, we need to look at the voltage coming out of the IAT sensor and make sure that it’s getting to the ECU.
- Sensor Check – With the key in the off position, disconnect the IAT sensor. With your DVOM set to Ohms Ω, measure the resistance across the terminals of the sensor.
- If you read an open circuit or short circuit (∞ Ω or 0.0 Ω), the sensor is at fault. Replace the sensor and you should be good to go.
- If you read resistance, check it against the temperature / resistance chart. If the engine is cold, then the IAT temperature / resistance should be fairly close to ambient temperature, which you can measure with a thermometer. If the resistance is correct at ambient temperature, you may still have a sensor problem. A sensor that reads properly at ambient temperature may go out of range at other temperatures. You can check for sensor problems by checking the resistance after you change its temperature, such as by putting it in a pot of boiling water or in the freezer.
- Circuit Check – If the sensor itself appears to be properly responding to temperature changes, then you can proceed with circuit checks.
- With KOEO (key on, engine off), disconnect the IAT sensor, and check for voltage in the connector.
- One of the wires should read 5 V reference. If you have no 5 V reference, you may have an open circuit or an ECU fault.
- The other wire should read 0 V. If you can properly identify the signal wire, check for resistance to ground, which should read 0.0 Ω. An open circuit likely means you have a wiring problem. On the other hand, 0.0 Ω could still indicate a short to ground.
- If there is no 5 V reference voltage, there could be an open circuit between the ECU and the sensor, a short circuit to ground, or the ECU itself may be at fault. If there is no signal voltage on the other line, then you could have a defective sensor, open circuit, or short to ground.
- At the ECU, check for reference and signal voltage. If there is no 5 V reference, the ECU may be at fault. If there is low or no signal voltage, check for corrosion or an open circuit in the wires to the IAT sensor.
- Turn the key to the off position, disconnect the ECU connector, and check for resistance. If the resistance reading is the same as the sensor check, then the ECU may be at fault. If you measure 0.0 Ω or ∞ Ω, check for open or short circuit in the wiring to the IAT sensor.
- Disconnect the sensor and measure resistance again, it should now read as an open circuit, ∞ Ω. If you read any resistance at all, check for short circuits in the wiring to the IAT sensor.
- Install a short wire, in the IAT sensor connector, and check for resistance in the circuit.
- The proper reading should be 0.0 Ω. Any reading over 1 Ω could indicate corrosion or an open circuit.
- Check for resistance to ground. Any reading under 1,000 kΩ could indicate a short to ground in one of the wires to the IAT sensor.
- With KOEO (key on, engine off), disconnect the IAT sensor, and check for voltage in the connector.
- If you have access to a scan tool and live data, you can perform many of these checks simply by reading ECU IAT readings. With KOEO, read the temperature with the sensor connected, and then disconnected. If there is no change, you have a circuit problem. If there is a change, you have a sensor problem.
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