P0074
Ambient Air Temperature Sensor Circuit A Intermittent/Erratic.
- Generic
- Powertrain
- Generic code
- Low severity
How this code is built
- P
- SystemPowertrain
- 0
- Defined bySAE, the same on every car
- 0
- AreaFuel and air metering, auxiliary emission controls
- 74
- FaultThe specific fault in that area
What it means
P0074 means the control unit has seen the ambient air temperature sensor signal (circuit A) drop out, jump or behave erratically rather than staying steady. The signal may briefly go open or short, or change faster than outside temperature realistically could. The sensor is usually behind the front bumper or grille, and on some models its reading is passed over the vehicle network from another control unit.
Symptoms
- Engine warning light on, often with no change in how the car drives
- Outside temperature display on the dash jumps around, shows dashes or reads implausibly high or low
- Automatic climate control behaves oddly, such as unstable cabin temperature or fan speed
- Slightly rough or slow starting in cold weather on some models
- Fault appears and disappears, especially after rain, washing or driving over bumps
Possible causes
- Loose, corroded or contaminated connector at the ambient temperature sensorlikely
- Faulty ambient air temperature sensor with an internal intermittent faultlikely
- Damaged, chafed or water-affected wiring between the sensor and the control unit, often near the front bumperlikely
- Poor earth or reference voltage supply shared with other sensorspossible
- Sensor reading shared over the network from another module, with a communication or module-side fault (model dependent)possible
- Fault inside the engine or body control unit input circuitrare
How to diagnose
- Read all stored and pending codes and the freeze frame data, noting temperature, engine state and conditions when the fault set; look for other temperature-sensor or network codes
- Watch the live ambient temperature value and compare it with a known outside temperature; check it is plausible and does not jump or drop out
- Locate the sensor for this model and inspect it, its connector and the surrounding wiring for damage, corrosion, water ingress or bumper repair damage
- With the engine off and the connector unplugged, check terminals for tightness and corrosion, then wiggle-test the harness and connector while watching the live reading
- Measure the sensor resistance and compare it with the manufacturer's temperature table, gently warming or cooling it to see if the reading changes smoothly
- Check the wiring for continuity, short to earth, short to supply and resistance between the sensor and control unit, following the model's wiring diagram
- Check the sensor supply and earth at the connector, and check for network faults if the model gets the reading from another module
- Clear the code after repair and road test, rechecking the live value and confirming the code does not return
How to fix
- Clean and secure the connector, repairing or replacing any corroded or bent terminals
- Replace the ambient air temperature sensor if its resistance is out of range or erratic
- Repair or replace damaged wiring and protect it from chafing and water
- Restore a faulty earth or supply connection affecting the sensor circuit
- Repair the network or module fault if the reading is supplied by another control unit, as per the manufacturer's procedure
- Replace or repair the control unit only after the sensor, wiring and supplies have all been proved good
Can I keep driving?
The car can normally be driven as usual while you book it in, as this fault rarely affects safety. Get it checked reasonably soon, because the climate control and temperature display may be unreliable and other faults could be masked by the warning light being on.
Guide written by AI from the code's definition. Confirm with a diagnosis before replacing parts.