P0074 trouble code definition
P0074 is the code for the Ambient Air Temperature Sensor Circuit Intermittent
What the P0074 code mean
This code means the Ambient Air Temperature (AAT) sensor electrical circuit has a fault which is not constant. This results in the signal from the sensor to the Engine Control Unit (ECU) to go on and off.
What causes the P0074 code
This code has several causes:
The sensor itself is faulty and has an internal problem that causes the circuit to go on and off.
The electrical wiring can be damaged causing the signal from the sensor to fail when the circuit is broken.
The electrical connector of the sensor to the wiring harness is loose, damaged, or occasionally failing.
What are some of the symptoms of P0074 code?
The possible symptoms of the AAT sensor intermittent circuit can cause the ECU to miscalculate the ambient air temperature to control the fuel-to-air ratio of the engine and will cause the engine to run rough and erratic when the circuit is not working. The Check Engine Light would then come on once the ECM detects the intermittent condition.
How does a mechanic diagnose the P0074 code?
The Check Engine Light would normally come on the dashboard display of the vehicle first. The mechanic must use an OBD-II scanner to correctly diagnose the fault code.
With the ignition turned on, the mechanic must then check the AAT sensor to see if internal continuity is broken or intermittent. If the sensor appears to be okay, then the wiring and connector should be carefully checked for bare or broken wiring, faulty connection, or damaged components. They should also check that there is a signal of 0-5 volts output from the sensor.
Common mistakes when diagnosing the P0074 code
Do not assume the sensor is bad until the wiring and connector are checked to ensure there are no problems. The mechanic must check the wiring and sensor with the ignition turned on and use a multimeter to check voltages in and out of the sensor. There is a 0-5 volt signal that will indicate the ambient air temperature. If there is no voltage, then it has a circuit or sensor failure that must be determined first.
How serious is the P0074 code?
The vehicle will still drive, but the Check Engine Light will be on even if the AAT sensor circuit is operating correctly. The vehicle will run erratically with possible loss of power, high fuel consumption, and poor performance, especially under acceleration and at speed. It is important to have the problem diagnosed as soon as possible to prevent engine damage or other problems such as exhaust and catalytic converter systems.
Often times, if the Check Engine Light came on immediately at start up, the OBD-II system can be reset and the vehicle will operate normally.
What repairs can fix the P0074 code?
The most common repairs to address the P0074 code is as follows:
A certified technician will verify the code with a scanner. Reset the fault codes and perform a road test.
If the P0074 code returns, then follow the test procedure below.
First, inspect the wiring for damage, loose connection, and verify with a multimeter for continuity while physically moving the wiring around by hand to see if the circuit breaks.
Next, assuming the wiring and connections appear to be in good condition, the sensor itself should be checked for continuity and possible wiring issues by moving the wiring by hand. If continuity is consistent then go to the next step.
With the ignition on, check output voltage per the manufacturer's specifications which normally is .5 volts to 4.5 volts. If the output is not within this range, the AAT sensor is faulty.
If all components check out okay, per the procedure, start the engine and monitor output voltage with a multimeter. If the output voltage is erratic or the meter goes to zero or above 5 volts, replace the AAT sensor.
Additional comments for consideration regarding the P0074 code
Many vehicles with mileage over 100,000 have momentary sensor problems that usually occur during start up or prolonged stress situations on the drive train. If the Check Engine Light comes on and the vehicle seems to be operating normally, the OBD-II system can be reset using the scanner and the problem may not reoccur. This is why it is important to verify the fault and reset it before doing any repairs.
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