Damaged, corroded or loose sensor connector; Faulty ambient air temperature sensor B (internal open or short circuit); Chafed, broken or shorted wiring between sensor and control unit, often near the front bumper or grille; Water ingress or road debris damage to the sensor or its wiring at the front of the car; Poor reference voltage or ground supply to the sensor circuit; Fault in the control unit or in a module that shares the temperature signal over the network.

P00D9

Ambient Air Temperature Sensor Circuit B.

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
D9: FaultThe specific fault in that area

What it means

P00D9 means the control unit has detected a fault in the circuit of the second ambient air temperature sensor, referred to as sensor B. It sets when the signal voltage or resistance from that sensor is outside the expected range, or does not make sense compared with other temperature readings. Which sensor is 'B' and where it sits depends on the model, so check manufacturer data for the vehicle.

Symptoms

Warning light on the dashboard, possibly the engine management light
Outside temperature display missing, showing dashes or reading wildly wrong
Climate control or automatic air conditioning behaving oddly
Slightly rough or slow cold starting on some models
Cooling fan or intake air temperature related strategies running differently on some models

Possible causes

Damaged, corroded or loose sensor connectorlikely
Faulty ambient air temperature sensor B (internal open or short circuit)likely
Chafed, broken or shorted wiring between sensor and control unit, often near the front bumper or grillelikely
Water ingress or road debris damage to the sensor or its wiring at the front of the carpossible
Poor reference voltage or ground supply to the sensor circuitpossible
Fault in the control unit or in a module that shares the temperature signal over the networkrare

How to diagnose

Read all stored codes and freeze frame data, and note whether the fault is current or intermittent
Compare the ambient temperature readings in live data with the other temperature readings on a cold car that has been standing for several hours
Visually inspect the sensor, its mounting position and the wiring for damage, corrosion, water or crash repairs
Disconnect the connector and check the pins for corrosion, bent terminals and a good fit
Measure the sensor resistance and compare it with the manufacturer's value at the current temperature
With the sensor unplugged, check the reference voltage and ground at the connector against the wiring diagram
Wiggle-test the harness with live data running to find an intermittent break or short
Check the wiring continuity and insulation from sensor to control unit, and look for any manufacturer service information for the model

How to fix

Clean or replace the damaged connector or terminals and ensure a secure fit
Replace the ambient air temperature sensor if its resistance is out of specification
Repair or replace damaged wiring, using proper sealed repairs
Restore correct reference voltage or ground supply by repairing the affected circuit
Replace or repair the control unit or related module only after the wiring and sensor have been proved good and the manufacturer's procedure confirms it

Can I keep driving?

Low severity

The car can normally be driven as the fault mainly affects temperature display and comfort functions. Get it checked soon, as it may affect other systems on some models and the warning light could hide other faults.

Guide written by AI from the code's definition. Confirm with a diagnosis before replacing parts.