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P0538A/C Evaporator Temperature Sensor Circuit High

The A/C evaporator temperature sensor circuit is reading a higher voltage than possible, typically from an open circuit or disconnected sensor.

Definition

P0538 sets when the evaporator temperature sensor signal voltage rises above the valid range, which on an NTC thermistor circuit reads as an impossibly cold evaporator — the same signature as an open circuit or unplugged sensor. Believing the core is already at or below freezing, most systems refuse to engage or quickly disengage the A/C compressor, so the typical complaint is no cold air or a compressor that will not stay on. The fault is purely a comfort/HVAC issue with no driveability effect.

Common causes

  • Open (failed) evaporator temperature sensor
  • Unplugged or partially seated sensor connector after HVAC service
  • Broken signal or ground wire in the under-dash harness
  • Corroded terminals creating an open connection
  • Signal wire shorted to voltage (less common)
  • Control module fault (rare)

Common misdiagnoses

  • A/C compressor or clutch replaced for a no-engagement complaint
  • Refrigerant recharge
  • A/C pressure switch
  • Climate control head unit

Troubleshooting steps

  1. 1.Confirm the pegged reading

    An evaporator temp PID stuck at its coldest value (often -40) or at reference voltage confirms an open/high circuit.

  2. 2.Check the connector

    Reach the sensor at the HVAC case and verify it is plugged in and latched — sensors are commonly left off after evaporator or dash repairs.

  3. 3.Jumper the sensor terminals

    With the harness connector jumped, the PID should snap to its hottest reading; if it does, the harness and module are good and the sensor is open.

  4. 4.Trace the open if jumpering changes nothing

    Ohm both wires end-to-end from the sensor connector to the module to find the broken or corroded segment.

  5. 5.Replace or repair and verify

    Install a new sensor or repair the harness, then confirm the compressor engages and the evaporator temp drops into the normal 1-4 C control range.

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