Comprehensive diagnostic guide for OBD-II code P0073

Title: P0073 - Ambient Air Temperature Sensor Circuit High Input (often described as high input on the ambient air temperature sensor circuit)

What This Code Means

• P0073 is a powertrain diagnostic trouble code
• OBD-II DTCs (including P0073) are generated when the on-board diagnostic system detects a fault in monitored parameters and stores a trouble code for diagnostic reference. This is part of how emissions-related and powertrain-related faults are tracked and reported.

Symptoms

• MIL (Check Engine Light) illuminated or flashing depending on vehicle state and drive cycle
• Inconsistent ambient air temperature readings on the vehicle's dash display or on a scan tool (when the vehicle shows ambient temp values that don't match actual conditions)
• Potential impact on fuel trim calculations, slight idle issues, or irregular throttle response if the PCM uses a faulty ambient temp input for fueling decisions
• In some cases, no obvious symptoms beyond the illuminated MIL; the vehicle may run normally once the fault is corrected but the code remains stored until cleared

Important Notes

• The ambient air temperature sensor input is used by the PCM/ECU to adjust fueling and other engine control strategies. A circuit high input can affect fueling calculations and may trigger related checks. However, the impact varies by vehicle and how the PCM uses the sensor data.
• Emissions-related monitoring and readiness can be affected by sensor faults; the code belongs to the powertrain/catalyst/efficiency monitoring family, per the OBD-II overview and powertrain code categorization. (Source notes: OBD-II sections on Diagnostic Trouble Codes, Powertrain Codes, Emissions Testing)

Probable Causes

Note: do not give NHTSA complaint frequencies for P0073. When such data is unavailable , probabilities are presented based on typical field experience and general code behavior.

• Faulty ambient air temperature sensor (AAT sensor) itself (high or stuck input)

  • Likelihood: high

• Damaged, corroded, or moisture-contaminated sensor harness/connector

  • Likelihood: high

• Short to 5V reference or improper ground on the sensor circuit

  • Likelihood: medium to high

• PCM/ECU sensor input circuit or software issue

  • Likelihood: low to medium

• Environmental or external factors (direct sun exposure, engine bay heat affecting a compromised sensor)

  • Likelihood: low to medium

Step-by-Step Diagnosis

1) Confirm and document

• Use a good OBD-II scan tool to confirm P0073 and pull freeze-frame data. Note the reported ambient temperature value in the freeze-frame data and compare it with actual ambient conditions. Also review engine coolant temperature, RPM, and load values in the frame, as these can help interpret whether the ambient reading is plausibly high or out of range for the circumstances.
• Confirm the exact definitions in your vehicle's service information because P0073 can be defined as ambient air temperature sensor circuit high input in many items, but the exact fault description can vary by manufacturer. The general concept is that the ambient sensor circuit is reading abnormally high. (Source notes: OBD-II DTC and Powertrain Code sections)

2) Inspect the ambient air temperature sensor and wiring

• Visually inspect the sensor for physical damage, oil/contaminant ingress, or heat soak issues.
• Inspect wiring harness and connectors for fraying, corrosion, moisture, bent pins, or loose connections. Pay particular attention to the sensor signal wire and the 5V reference wire and the ground path.
• Look for signs of heat or chafing where the harness runs near hot engine components.

3) Verify the 5V reference and ground integrity

• With the ignition on (engine off) and the sensor connector disconnected, back-probe the sensor circuit to verify that the PCM provides a 5V reference on the reference line when appropriate, and that a proper ground is present on the sensor ground circuit.
• With sensor connected, verify the signal circuit voltage behavior across a range of ambient temperatures if you can safely do so. A high input would typically show elevated readings; an actual ambient air temperature test can help determine if readings are plausible.

4) Sensor resistance/behavior test (NTC thermistor behavior)

• The ambient air temperature sensor is typically a thermistor that changes resistance with temperature. If you have the vehicle's service information, compare the sensor's observed resistance to the expected resistance versus temperature.
• If the sensor is reading high voltage (or high input) and the resistance vs. temperature appears out of spec, replace the sensor.

5) Check for short-to-Vref or short-to-ground on the signal path

• Disconnect the sensor and observe whether the fault code reappears or the signal line reverts to a known state. If the high-input condition disappears when the sensor is unplugged, the problem is likely in the sensor or wiring rather than the PCM.
• Inspect for any shorts to 5V or ground along the harness, including aftermarket wiring or ad-hoc repairs that might have compromised the circuit.

6) Rule out PCM/software issues (last resort)

• If sensor and wiring checks are clean and the fault persists, consider PCM software/ calibration issues or a defective PCM input stage. Some manufacturers require a software update or reflash if a fault cannot be resolved with sensor/harness repairs. This is usually pursued after all hardware-related causes are ruled out.

7) Repair, recheck, and verify

• Replace the ambient air temperature sensor if the sensor test indicates failure or if readings remain abnormally high after wiring repair.
• Repair or replace wiring/connector assemblies showing damage or corrosion. Ensure a secure, clean, and properly seated connector.
• After repair, clear the codes with the scan tool and perform a drive cycle to confirm the code does not return and that the ambient reading now aligns with actual ambient conditions.

Recommended testing resources and references

• OBD-II fundamentals and DTC mechanism:
  • OBD-II Diagnostic Trouble Codes and Powertrain Codes sections describe how DTCs are generated, stored, and used for fault diagnosis. This provides the context for how P0073 fits into the broader engine management and emissions monitoring framework.
• Specific code description in open-source listings:
  • An open-source listing describes P0073 as i.e., ambient air temperature sensor circuit high. This supports the interpretation that P0073 is related to a high input signal on the ambient temp sensor circuit. )
• Emissions and readiness context:
  • Emissions testing sections discuss how monitors are used and how failures may affect readiness status, which is relevant when diagnosing codes that influence fuel trim and sensor input like P0073.

Safety Considerations

• Work with the engine off and the ignition off whenever possible when inspecting wiring and connectors. If you must test live circuits, take appropriate safety precautions to avoid electric shock or short circuits.
• Be careful around hot engine components; the ambient sensor is in the engine bay area where hot components can cause burns.
• When handling electrical testing, disconnect battery ground when performing invasive checks to prevent short circuits, then reconnect battery after testing.

What to document in your repair report

• Vehicle year, make, model, engine, and PCM type
• Exact code(s) reported and freeze-frame values (ambient temp reading, engine temp, RPM)
• Sensor location and wiring harness condition findings
• Measured voltages and/or resistances (5V reference presence, ground continuity, signal wire behavior across temperature range)
• Parts replaced (sensor, connectors, wiring harness) and any software/ECU updates performed
• Vehicle test drive results and whether the code reappeared

Plain-language takeaway

• P0073 points to the ambient air temperature sensor circuit showing a higher-than-expected signal. Start with a thorough inspection of the sensor and its wiring, verify the 5V reference and ground, test the sensor's resistance with temperature, and replace the sensor or repair the wiring if faults are found. If hardware checks are clean, suspect ECU input issues or software that may require update. Always recheck after repair and clear codes to confirm stability.

• OBD-II overview and diagnostic trouble codes - general description of how DTCs work and that powertrain codes are used for engine-related faults.

• Emissions Testing context - context on how OBD-II monitoring relates to emissions and readiness. (OBD-II - Emissions Testing)

• Open Source OBD2 code listing - P0073 described as high input ambient air temperature sensor circuit (Circuito Alto - Sensor temp. ar ambiente).

This diagnostic guide was generated using verified reference data:

• Wikipedia Technical Articles: OBD-II
• Open-Source OBD2 Data: N/A (MIT)

Content synthesized from these sources to provide accurate, real-world diagnostic guidance.

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