P00D9 — Ambient Air Temperature Sensor Circuit B Malfunction

What P00D9 means

The engine control module watches the signal voltage from the ambient air temperature sensor. The sensor is a thermistor, so its resistance and signal voltage change with air temperature. P00D9 is set when the circuit B signal is out of range or does not make sense, for example from an open circuit or a short to ground or voltage.

Symptoms

• Check Engine light on
• Wrong or missing outside temperature on the dash display
• Slightly worse fuel economy
• Rough idle or hesitation (not always)
• Automatic climate control behaving oddly (on some vehicles)

Possible causes

1. Failed ambient air temperature sensor
2. Damaged, chafed or corroded wiring in the sensor circuit
3. Corroded or loose connector terminals at the sensor
4. Open or poor ground for the sensor
5. Missing or unstable reference voltage
6. Rarely, a faulty PCM or software issue

How to diagnose

1. Scan all codes and freeze frame data, clear the code and see if it returns
2. Compare the scan tool ambient temperature reading to actual temperature and to IAT and coolant temperature on a cold engine
3. Inspect the sensor, connector and wiring for damage, moisture and corrosion
4. Unplug the sensor and check for reference voltage (typically about 5 V) and a good ground
5. Measure sensor resistance with a multimeter and compare to the manufacturer spec at a known temperature
6. Test the wiring between the sensor and the PCM for opens and shorts
7. If wiring and sensor are good, check the PCM and look for software updates

Common fixes

• Replace the ambient air temperature sensor
• Repair or replace damaged wiring or the connector
• Restore the sensor's power supply or ground
• Update or reprogram the PCM if the manufacturer calls for it

Questions and answers

Can I drive with code P00D9?

Yes, usually. The fault is not a safety risk, but it is best not to put off diagnosis, since bad temperature data can affect engine operation and climate control.

What most often causes P00D9?

Most often it is a failed ambient air temperature sensor or damaged wiring and a corroded connector. Start by inspecting the sensor and its harness.

How do I know the sensor itself is bad?

Compare its scan tool reading to the real temperature, then measure its resistance. If the values are off while wiring and supply are fine, replace the sensor.

This is general information about the code. The meaning can differ between makes and models: a scan tool and the repair manual for your car will show the exact cause.