P0071

A trouble code records what a control module measured. It does not identify which part failed. Test before replacing anything.

Safe to drive

Safe to drive. Expect the outside temperature display and automatic climate control to behave incorrectly until it is repaired.

Urgency

Not urgent

Will it pass emissions

Not affected

If ignored

Automatic climate control keeps mistuning itself, and the A/C may refuse to engage if the module believes it is below freezing. On many vehicles this fault stores a code without lighting the check-engine lamp, since ambient temperature is not an emissions monitor — so scan for stored codes rather than waiting for a light. Where the lamp is lit, other faults still set and store normally; the lit lamp just makes a new one easy to overlook. On vehicles that use ambient temperature in transmission or cooling fan strategy, calibration drifts slightly off, though the effect is usually small.

Check engine light

Solid

These are the causes this code can have. For the order to work through them in, see what to check first.

Ordering reflects how often each cause is responsible in general, not a probability for your vehicle. Confirm by testing.

In order. Each of these is cheaper or faster than what follows it, and each one can make the rest unnecessary.

1. What the scan tool reports for ambient, intake air and coolant temperature after an overnight soak

   All three should sit within a few degrees of each other and of the actual outside temperature. This one comparison usually identifies which sensor is lying, and it costs nothing.

2. The sensor's physical location for debris, mud, road salt or a plastic bag blocking airflow

   These sensors typically mount low in the front airflow path. Anything trapping heat or blocking air over the element produces exactly this fault, and clearing it is free.

3. Whether the front bumper, grille or condenser area has been repaired or modified

   After collision repair or an aftermarket bumper install, the sensor is often remounted where it soaks heat from the radiator or condenser instead of sitting in clean airflow.

4. Whether the reported temperature drops after several minutes of highway driving

   A reading that is high when parked but falls once air moves over the sensor means the sensor works and the problem is its mounting or heat exposure, not the element.

5. The sensor connector for corrosion

   These connectors live in the spray zone behind the bumper. Corrosion adds resistance, and added resistance on a thermistor circuit reads directly as a temperature offset.

1. Compare all temperature inputs on a fully cold vehicle

   Tools Scan tool, accurate thermometer

   After the vehicle has sat overnight, read ambient, intake air and engine coolant temperature on the scan tool before starting the engine. Compare all three against an accurate thermometer left near the vehicle. The outlier is your suspect.

   Expected

   All three readings within a few degrees of each other and of actual ambient. Any that is well off is the fault candidate

2. Locate and visually inspect the sensor

   Tools Service information, flashlight

   Ambient sensors usually sit behind the grille, under the front bumper, or in a mirror housing. The exact location varies by make and model, so use service information rather than searching blind. Check for packed debris, corrosion, a bent bracket or a mounting position exposed to condenser heat.

   Expected

   A sensor in clean, unobstructed airflow ahead of the radiator and condenser, with no debris packed around it

3. Watch the reading change with vehicle speed

   Tools Scan tool with data logging

   Log the ambient reading from a cold start through several minutes of steady driving above roughly 30 mph. Note whether it settles toward true ambient or stays elevated.

   Expected

   A reading that converges on actual outside temperature once air is moving over the sensor

4. Verify the sensor's own resistance against temperature

   Tools Digital multimeter, thermometer

   Unplug the sensor and measure its resistance at a known temperature, then warm it in your hand or with an ice pack and confirm the resistance moves in the correct direction and by a reasonable amount. These are negative temperature coefficient thermistors, so resistance falls as temperature rises. The specific resistance values are vehicle-specific.

   Expected

   Resistance that changes smoothly and substantially with temperature, with no dead spots or open readings

5. Check the circuit for added resistance

   Tools Digital multimeter, wiring diagram

   With the sensor unplugged, check the signal and ground wires back to the module for corrosion, high resistance or a chafed section. A poor ground on a thermistor circuit shifts the reading in one direction consistently, which looks exactly like a drifted sensor.

   Expected

   Near-zero resistance on the ground path and clean continuity on the signal wire

6. Determine whether the value arrives over the network

   Tools Scan tool with multi-module data access

   On many vehicles the ambient reading is produced by one module and shared with others by network message. Compare the value shown by the instrument cluster, the climate control and the powertrain module. If they disagree, the fault is in the message path or the receiving module, not the sensor.

   Expected

   All modules reporting the same ambient value

7. Confirm the repair through a full temperature swing

   Tools Scan tool

   After the repair, clear the code and check the reading on a cold morning start and again after highway driving. This code often needs a cold soak to re-run its comparison, so a same-hour verification proves little.

   Expected

   Ambient tracking a thermometer through the whole cycle, and no return of the code

The sensor itself is one of the cheaper parts on the vehicle. The cost driver is access — a sensor clipped behind the grille is minutes of labor, while one integrated into a mirror assembly or buried behind a bumper cover is much more. Where the value is generated in another module, diagnostic time dominates the bill.

• Replacing the sensor when it was simply packed with road debris or mounted where it absorbs condenser heat. The part is cheap, but the code returns because the cause was never addressed.
• Assuming the reading is wrong when the intake air temperature sensor is the one that has drifted. The code is a comparison, so either side of the comparison can be at fault.
• Clearing the code and testing on the same warm afternoon. The rationality check usually needs a long cold soak to run again.
• Waiting for a check-engine light to confirm the fault. On many applications this code stores without illuminating the lamp, so it is found by scanning, not by watching the dash.
• Chasing a sensor on a vehicle where the value is broadcast over the network and the real fault is a communication problem or a failed receiving module.
• Overlooking a corroded connector. Added circuit resistance reads as a temperature offset that is indistinguishable from a drifted element until it is measured.

• Ambient Air Temp Sensor

• P00DAAmbient Air Temperature Sensor Circuit B Range/PerformanceSeverity level 3 of 5: Diagnose soon.moderate
• P0070Ambient Air Temperature Sensor Circuit ASeverity level 3 of 5: Diagnose soon.moderate
• P0072Ambient Air Temperature Sensor Circuit A LowSeverity level 3 of 5: Diagnose soon.moderate

Definition

Generic code set (SAE J2012 / ISO 15031-6) via the OBDexopen database, dedicated to the public domain under CC0-1.0.

Diagnostic guidance

Written and reviewed in-house. See our editorial policy.

Review status

Reviewed

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