P0073

What Does Code P0073 Mean?

DTC P0073 signifies a detected malfunction within the Ambient Air Temperature (AAT) sensor circuit, specifically indicating a "circuit high" condition. The AAT sensor is a thermistor, a temperature-sensitive resistor, which provides the Engine Control Module (ECM) or Powertrain Control Module (PCM) with data regarding the external air temperature. The ECM/PCM supplies a reference voltage, typically 5 volts, to the sensor and monitors the voltage drop across it to calculate the ambient temperature. In a normal operating range, as the ambient air temperature increases, the resistance of the AAT sensor (typically a Negative Temperature Coefficient or NTC thermistor) decreases, causing the voltage signal returned to the ECM/PCM to increase. Conversely, as temperature decreases, resistance increases, and the signal voltage decreases.

A "circuit high" condition for P0073 means the ECM/PCM is receiving an excessively high voltage signal from the AAT sensor circuit, often at or near the 5-volt reference voltage. This high voltage typically corresponds to an implausibly low or "frozen" temperature reading, such as -40°F (-40°C), which the ECM/PCM identifies as an out-of-range value. This specific condition commonly indicates an open circuit within the AAT sensor itself, a break in the signal wire between the sensor and the ECM/PCM, or a short of the signal wire to a voltage source. The AAT sensor’s input is critical for various vehicle systems, including automatic climate control for interior temperature regulation, exterior temperature display, and in some applications, it may contribute to engine fan control or cold-start enrichment strategies, although its primary role is often related to convenience and HVAC.

Common Symptoms

• Check Engine Light (CEL) Illumination: The Malfunction Indicator Lamp (MIL) will be illuminated on the dashboard.
• Inaccurate Ambient Temperature Display: The external temperature display on the instrument cluster or infotainment system will typically show an extremely low temperature, often defaulting to -40°F or -40°C.
• Malfunctioning Automatic Climate Control: The automatic climate control system may operate erratically or not function correctly, as it relies on accurate AAT data for temperature blending, fan speed, and A/C compressor engagement logic.
• Cooling Fan Operation Anomaly: In some vehicles, if the AAT input is used for supplementary cooling fan control, the electric cooling fans might run continuously or not engage when expected.

What Causes the Code P0073?

• Faulty Ambient Air Temperature (AAT) Sensor: An internal open circuit or degraded thermistor element within the sensor itself, causing it to transmit an erroneously high voltage signal.
• Open Circuit in AAT Sensor Wiring: A physical break, corrosion, or severe chafing in the signal wire or the ground/return wire connecting the AAT sensor to the ECM/PCM.
• Damaged or Corroded Electrical Connector: Poor connection at the AAT sensor connector due to bent pins, corrosion, or a loose terminal, leading to an intermittent or complete open circuit.
• Short to Voltage: The AAT sensor signal wire is inadvertently shorted to a constant voltage source (e.g., the 5V reference line or another power wire) within the wiring harness, causing a consistently high voltage signal.
• Faulty Engine Control Module (ECM/PCM): Although rare, an internal fault within the ECM/PCM’s sensor input circuit could misinterpret the AAT signal or fail to provide the correct reference voltage, leading to a false "circuit high" detection.

How to Diagnose and Troubleshoot

Diagnosing P0073 requires a methodical approach, utilizing a digital multimeter (DMM) and an OBD-II scanner.

1. Verify the DTC and Freeze Frame Data: Connect an OBD-II scanner and confirm P0073 is present. Review freeze frame data to understand the conditions (engine speed, load, temperature) when the code was set, which can provide valuable context.
2. Monitor Live Data: Access the live data stream with the OBD-II scanner and observe the Ambient Air Temperature (AAT) sensor reading. If the reading is fixed at an extremely low value (e.g., -40°F/-40°C) and does not change regardless of actual ambient temperature, it strongly indicates a fault in the sensor or its circuit.
3. Visual Inspection:
   • Locate the AAT sensor. Its typical locations include behind the front grille, within the front bumper assembly, or integrated into one of the side mirrors.
   • Inspect the sensor and its electrical connector for any visible signs of physical damage, corrosion, bent pins, or rodent damage to the wiring harness.
   • Ensure the connector is securely seated.
4. AAT Sensor Circuit Voltage Check (Ignition ON, Engine OFF):
   • Disconnect the AAT sensor’s electrical connector.
   • Using a DMM, set to DC voltage, identify the reference voltage wire (usually 5V) and the ground wire according to the vehicle’s wiring diagram.
   • Measure the voltage between the 5V reference wire terminal in the harness connector and a known good chassis ground. It should read approximately 5 volts.
   • Measure the voltage between the sensor’s ground/return wire terminal in the harness connector and a known good chassis ground. It should read close to 0 volts.
   • If the 5V reference is absent or incorrect, inspect the wiring back to the PCM.
5. AAT Sensor Resistance Test (Sensor Disconnected):
   • Set the DMM to Ohms (Ω).
   • Measure the resistance directly across the two terminals of the AAT sensor itself.
   • Compare this reading to the specified resistance-to-temperature chart found in the vehicle’s factory service manual for the specific AAT sensor. At typical room temperature (e.g., 68°F / 20°C), the sensor should exhibit a specific resistance value (often several kilohms). An "OL" (open loop) or infinite resistance reading on the DMM indicates an internal open circuit within the sensor, confirming it is faulty.
6. Wiring Harness Continuity and Short Circuit Tests (Ignition OFF, PCM and Sensor Disconnected):
   • With both the AAT sensor and the PCM’s main connector disconnected (to isolate the wiring), use the DMM set to continuity or Ohms.
   • Test for continuity on both the signal wire and the ground/return wire from the AAT sensor connector to the corresponding pins at the PCM connector. There should be very low resistance (near 0Ω). An "OL" reading indicates an open circuit in the wire.
   • Test for a short to ground: Measure resistance between the signal wire terminal at the AAT connector and a known good chassis ground. Resistance should be infinite ("OL").
   • Test for a short to voltage: Measure resistance between the signal wire terminal at the AAT connector and any known power supply wire (e.g., 12V constant or ignition ON). Resistance should be infinite.
7. Wiggle Test: While monitoring live data or with the DMM connected for voltage/resistance checks, gently manipulate or "wiggle" the wiring harness near the AAT sensor and the PCM connector. Observe if the AAT reading changes or if any voltage fluctuations occur, which could indicate an intermittent connection or a short.

Recommended Repairs and Solutions

Once the diagnostic steps have pinpointed the root cause, the appropriate repair can be undertaken:

1. Replace the Ambient Air Temperature Sensor: If diagnostic testing confirms the AAT sensor itself has an internal open circuit (infinite resistance) or its resistance values are significantly outside of the manufacturer’s specifications at given temperatures, replacement is the most direct solution. Ensure the replacement part is an OEM equivalent or meets OEM specifications for accurate operation.
2. Repair or Replace Damaged Wiring: If an open circuit or short to voltage is identified within the wiring harness, the damaged section must be repaired.
   • For localized damage, use automotive-grade solder and heat shrink tubing for a durable, weather-resistant repair.
   • If the damage is extensive, consider replacing the entire segment of the wiring harness or the pigtail connector.
3. Replace Corroded or Damaged Connector: If the electrical connector at the AAT sensor is found to be corroded, has bent or pushed-out pins, or is physically broken, it should be replaced. Aftermarket connector pigtails are often available for this purpose. Ensure proper crimping and sealing for a reliable connection.
4. Clear DTCs and Verify Repair: After completing the repair, clear the P0073 code from the ECM/PCM using an OBD-II scanner. Perform a road test under varying conditions to ensure the code does not return and that the AAT reading in live data is accurate and responsive to changes in ambient temperature. Verify the functionality of affected systems like automatic climate control.
5. PCM Replacement (Last Resort): Only consider replacing the ECM/PCM after all other potential causes (sensor, wiring, connectors) have been exhaustively tested, ruled out, and confirmed to be in perfect working order. This is a very uncommon cause for a P0073 code and should only be pursued when a definitive diagnosis points to an internal PCM fault.