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P0537A/C Evaporator Temperature Sensor Circuit Low

The A/C evaporator temperature sensor circuit is reading a lower voltage than possible, usually from a short to ground.

Definition

The control module feeds the evaporator thermistor through a pull-up resistor and reads the divided voltage; P0537 sets when that voltage drops below the valid threshold, which for an NTC thermistor corresponds to an impossibly hot reading. Because the module believes the evaporator is warm, it may run the compressor continuously, allowing the core to ice over — vent airflow gradually weakens and cooling fades on longer drives, then recovers after the ice melts. Some systems instead disable the compressor entirely as a fail-safe. Engine operation is unaffected.

Common causes

  • Evaporator temperature sensor shorted internally
  • Signal wire chafed and shorted to ground under the dash
  • Moisture or condensation intrusion into the sensor connector
  • Harness pinched during dash, evaporator, or radio installation work
  • Control module fault (rare)

Common misdiagnoses

  • A/C compressor
  • Refrigerant overcharge or undercharge
  • Expansion valve replaced for freeze-up symptoms
  • Blower motor for the low-airflow complaint

Troubleshooting steps

  1. 1.Check the PID reading

    A sensor pegged at its maximum hot value (or minimum voltage) confirms a circuit-low condition rather than a real temperature.

  2. 2.Unplug the sensor

    With the connector open, the PID should swing to its coldest/open reading; if it does, the module and harness pull-up are good and the sensor is shorted.

  3. 3.Test for a harness short

    If the reading stays pegged with the sensor unplugged, disconnect the module and ohm the signal wire to ground; near 0 ohms confirms a chafed wire.

  4. 4.Inspect for moisture

    Look for condensation drips from the HVAC case wetting the connector; dry, clean, and dielectric-grease the terminals.

  5. 5.Replace the sensor or repair the wire

    Fit a new thermistor or repair the grounded section, then verify the evaporator temp PID tracks normally with the A/C running.

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