|Code||Fault Location||Probable Cause|
|P0074|| Outside air temperature sensor -circuit intermittent |
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|Wiring, poor connection, outside air temperature sensor, ECM|
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What Does Code P0074 Mean?
Aside from controlling the engine, the engine control module (ECM) is in constant communication with other modules, such as those can control the anti-lock braking system, automatic transmission, and climate control system. With regards to climate control, the ECM needs to know when to activate the air-conditioning compressor, which command is received from the climate control module (CCM). Similarly, the CCM needs to know coolant temperature, ambient air temperatures, and air-conditioning compressor function, which signals it gets from the ECM.
On vehicles equipped with automatic climate control, one simply selects the desired cabin temperature, or temperatures, depending on how many zones the system is equipped to handle. The CCM simply handles the rest, including application of heat or air conditioning, blend door positions, vent door positions, fan speed, and others.
To illustrate, if the driver selects 72 °F in automatic, the CCM takes into account outside and inside temperatures and coolant temperature when calculating how much heating or cooling to apply. If it’s 45 °F outside and 50 °F inside, the CCM will apply a heat from the heater core, until the inside temperature reaches the desired 72 °F. On the other hand, if it’s 85 °F outside, and 100 °F inside, because you just got in your car on a hot summer day, the CCM will command maximum air conditioning and fan speed to cool off the cabin.
To make these calculations, the CCM counts on a number of signals, such as outside ambient air temperature, one or more inside air temperatures sensors, air-conditioning clutch operation, engine coolant temperature, and others. If there is a problem with one of the signals, the CCM sets a diagnostic trouble code (DTC) in memory. Depending on system design, a CCM DTC may need to be read directly from the CCM or it may command a DTC to be stored in the ECM. In case the CCM or ECM detects a problem with the outside ambient air temperature sensor circuit, it will set DTC P0074 Ambient Air Temperature (AAT) Sensor Circuit. Additionally, the AAT signal may be used to display the exterior temperature, perhaps in the instrument cluster or in the climate control head unit itself.
Note: Regardless of the code and the system affected, the word “Intermittent” refers to the fact that there is a sporadic, unpredictable, or, well, intermittent fault in that system. The most likely causes of codes that relate to intermittent faults include poor connections in both live and ground circuits, as well defective switches, relays, and sensors.
Intermittent faults can be extremely difficult to trace and fix, since code readers generally do not specify in which part of the circuit the problem lays. For instance, the fault could be an intermittent interruption of the reference voltage (power supply), or, as often happens, the problem could be an intermittent break in the signal voltage to the PCM (Powertrain Control Module) due to a failing sensor, wiring issues, or defective electrical connector(s).
Note that the word “intermittent” often means different things to different applications; some systems will set a code the first time an intermittent fault occurs, while others will only do so after several failure cycles. In some cases, it may be necessary to allow the fault to worsen before an accurate and definitive repair can be made.
What are the common causes of code P0074 ?
Depending on year, make, and model, DTC P0074 may have number of causes. The most common is sensor or wiring damage. The AAT sensor is typically located in front of the radiator and air-conditioning condenser, inside a fender, or behind the bumper cover. Because of its exposed position, it is more susceptible to damage from collision, road debris, and corrosion. Even a minor bump from another vehicle could damage the sensor, connector, or the wiring.
What are the symptoms of code P0074 ?
The ECM may or may not illuminate the malfunction indicator lamp (MIL). It does one good to note that the AAT sensor has nothing to do with engine operation, that is, it is not the IAT (intake air temperature) sensor, which the ECM uses to fine-tune fuel trim.
Whether or not the MIL is illuminated, you will not notice any drivability issues, but you may notice malfunctions in the climate control system, such as failure of the “Auto” mode, air-conditioning compressor “failure,” or inconsistent heating or cooling. Also, exterior temperature displays, such as those in the instrument cluster, overhead console, or in the climate control head unit, may also malfunction.
How do you troubleshoot code P0074 ?
Because this DTC refers to a circuit problem, we will need to inspect the AAT sensor, connector, and wiring harness. Usually, a sluggish AAT will not set a DTC, and an open- or short-circuit may set “low-input” or “high-input” DTCs. You can check the AAT sensor with a DVOM (digital volt-ohm meter), thermometer, and a little patience.
- Sensor Check – If you note inconsistent resistance readings or corrosion in the connector, replace the sensor. If the sensor is fine, suspect a wiring problem.
- For two-wire sensors, use your DVOM to measure the resistance across the terminals of the AAT sensor. Check the resistance against the current temperature and the resistance chart in the repair manual. Use a hair dryer to increase the temperature of the AAT sensor, while you watch the resistance change. Make sure the resistance change is smooth, with no dropouts.
- For three-wire sensors, you need to check the sensor while connected and with the key in the “on” position. Check for 5 V reference and a good ground, and a variable voltage on the signal wire, which you can check against the temperature/voltage chart. Check your repair manual or electrical wiring diagram for pins and voltages.
- Wire Harness and Connector Check
- Check the wire harness for damage. Then, make sure the AAT connector is in good condition, no bent or backed-out pins, corrosion, or signs of water entry. Make sure the AAT connector is firmly seated. Repair as necessary.
- With the key on and AAT connector firmly seated, check for 5 V reference voltage and variable signal voltage. Three-wire AAT sensors also need a good ground, on the third wire. Check your repair manual or electrical wiring diagram for pinout, wire routing, and expected signal. Repair as necessary.
Codes Related to P0074
- P0070 Ambient Air Temperature Sensor Circuit Malfunction
- P0071 Ambient Air Temperature Sensor Range/Performance
- P0072 Ambient Air Temperature Sensor Circuit Low Input
- P0073 Ambient Air Temperature Sensor Circuit High Input
- P0074 Ambient Air Temperature Sensor Circuit Intermittent
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