P0072: Ambient Air Temperature Sensor Circuit Low Voltage

What this code means

P0072 means the ECM may be seeing the ambient air temperature sensor signal as lower than expected, which can make the temperature calculation look hotter than actual.

What the vehicle may do

The driver may see a service A/C message.
The outside temperature display may be inaccurate or slow to change.
The system can use a default value when the fault is present.

Possible fault areas

Possible ambient air temperature sensor issue.
Possible signal circuit issue.
Possible low reference circuit issue.
Possible connector or terminal condition.
Possible ECM-side circuit interpretation issue.

Diagnostic path

Open on P0072

On this 2014 to 2018 GMC Sierra 1500 with the 5.3 gas engine, P0072 means the ambient air temperature sensor circuit is being seen low by the ECM. In plain language, the module may be interpreting the outside air temperature signal as hotter than it should be. The driver may see a service A/C message, the system can use a default value, and the displayed outside temperature may not update the way you expect because that display is filtered and usually changes while driving, not while parked. For this diagnosis, think in broad fault areas: the sensor behind the grille, the signal circuit, the low reference circuit, connector condition, and possible module-side issues.

Understand the circuit and code logic

The ambient air temperature sensor is monitored by the engine control module using a signal circuit and a low reference circuit. The module watches the voltage drop across the sensor. Cold air gives the sensor higher resistance, and warm air gives it lower resistance. The ECM converts that voltage into a temperature value and sends it to the display module. For P0072, the code sets when the ECM detects the sensor signal is more than 88°C (190°F). The monitor is gated by normal operating basics: ignition ON, and the 12 V battery voltage greater than or equal to 10.2 V.

Start with the basic checks

Before going into pinpoint testing, start with the basic system checks and follow a structured diagnostic approach. With ignition ON or the vehicle in Service Mode, first look at whether P0072, or an ambient temperature companion code, is currently set. If it is set, go into circuit and system testing. If it is not set, check for any other ECM or temperature-related codes. If other codes are present, check what they mean first. If there are no other codes, let the vehicle cool completely to ambient temperature before comparing scan data.

Compare temperature inputs before circuit testing

Once the vehicle is fully cooled down, use actual ambient temperature as your reference. Verify the scan tool Intake Air Temperature parameter is within 15°C (27°F) of actual ambient temperature. If it is not, follow the intake air temperature sensor diagnostic path instead. If it is within 15°C (27°F), then check the ECM Ambient Air Temperature parameter. That value also needs to be within 15°C (27°F) of actual ambient temperature. If it is not, continue into the ambient sensor circuit testing. If it is within range, the check is OK at that point.

Check the low reference circuit first

For circuit testing, turn ignition OFF, let all vehicle systems power down, and disconnect the harness connector at the B9 Ambient Air Temperature Sensor. Be patient here; it may take up to 2 min for every module to shut down. Test for less than 10 Ω between the low reference circuit terminal B, or terminal 2, and ground. If that reading is 10 Ω or greater, keep the ignition OFF and disconnect the appropriate harness connector at the K20 Engine Control Module. Then test the low reference circuit end to end for less than 2 Ω. If it is 2 Ω or greater, repair the open or high resistance in that circuit. If it is less than 2 Ω, the diagnostic path calls for replacing the K20 Engine Control Module.

Use scan data to prove the signal side

If the low reference to ground test was less than 10 Ω, turn ignition ON and look at the ECM Ambient Air Temperature parameter. With the sensor disconnected, it should read colder than -30°F (-34°C). If it reads -30°F (-34°C) or warmer, turn ignition OFF, disconnect the X1 harness connector at the K20 Engine Control Module, and test for infinite resistance between the signal circuit terminal A, or terminal 1, and ground. If you have less than infinite resistance, repair the short to ground on the signal circuit. If the circuit tests at infinite resistance, the diagnostic path calls for replacing the K20 Engine Control Module.

Simulate the sensor response

If the scan tool reads colder than -30°F (-34°C), install a 3 A fused jumper wire between the signal circuit terminal A, or terminal 1, and the low reference circuit terminal B, or terminal 2. Now verify the scan tool Ambient Air Temperature parameter reads warmer than 180°F (82°C). If it reads 180°F (82°C) or colder, turn ignition OFF, disconnect the X1 harness connector at the K20 Engine Control Module, then turn ignition ON. Test for less than 1 V between the signal circuit and ground. If the test shows 1 V or greater, repair the short to voltage on the signal circuit. If it is less than 1 V, turn ignition OFF and test the signal circuit end to end for less than 2 Ω. If that circuit is 2 Ω or greater, repair the open or high resistance. If it is less than 2 Ω, the diagnostic path calls for replacing the K20 Engine Control Module.

Connector treatment and code check

If the jumper test makes the scan tool read warmer than 180°F (82°C), treat the connector terminals with dielectric grease, then reconnect the B9 Ambient Air Temperature Sensor. Reproduce the operating conditions that run the monitor, and verify P0072 does not reset. If the code or symptom is still not corrected, continue with testing of the B9 Ambient Air Temperature Sensor. If the code stays gone, that part of the path is OK. Also remember that when the ECM no longer detects the malfunction, the code can become history.

Static test the ambient temperature sensor

For the component static test, place the B9 Ambient Air Temperature Sensor in a stable, temperature-controlled area for at least 10 minutes. Then measure resistance between the sensor signal terminal A, or terminal 1, and the low reference terminal B, or terminal 2. Compare that reading to the specified Ambient Air Temperature Sensor Resistance information. That specification is required for the decision, so do not guess a resistance value. If the reading is not in the specified range, replace the B9 Ambient Air Temperature Sensor. If it is in the specified range, the component test is OK.

Verify the repair and close

After the repair is complete, verify the repair and confirm the code stays gone under the conditions that run the monitor. Keep verification separate from the circuit tests: first prove the low reference, then prove the signal circuit, then prove the sensor response. Bottom line: P0072 is handled by comparing believable temperature data, checking the two-wire sensor circuit in order, and only condemning a component or module after the circuit tests support it. For more diagnostic training, visit stepdiagnostics.com.