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P codepowertrainMedium severity

P0425Catalyst Temperature Sensor (Bank 1)

A malfunction has been detected in the Bank 1 catalyst temperature sensor or its wiring circuit.

Definition

The PCM supplies a reference voltage to the Bank 1 catalyst temperature sensor and uses its signal to verify catalyst light-off and protect the converter from overheating; on diesels this sensor family also manages aftertreatment temperatures. P0425 is the general circuit-fault code, set when the signal opens, shorts, or falls outside expected electrical limits. Driveability is normally unaffected, but the MIL turns on, catalyst temperature monitoring is suspended, and the vehicle will fail an emissions test.

Common causes

  • Heat-damaged wiring or connector where the harness routes near the exhaust
  • Failed catalyst temperature sensor element (open or shorted internally)
  • Corroded or loose terminals in the sensor connector
  • Chafed harness shorting the signal wire to ground
  • ECM connector or input-circuit fault (rare)

Common misdiagnoses

  • Catalytic converter replaced for what is purely an electrical sensor fault
  • Oxygen sensors replaced because the code is in the catalyst range
  • Sensor replaced repeatedly when the real fault is a broken harness wire

Troubleshooting steps

  1. 1.Inspect the harness along the exhaust

    Look for melted insulation, chafing, and heat-hardened wiring where the sensor lead passes the exhaust; this is the most common failure point.

  2. 2.Check the sensor connector

    Unplug the sensor and look for green corrosion, backed-out terminals, or heat distortion of the connector body.

  3. 3.Measure sensor resistance

    Compare the element's resistance at a known temperature to the service-manual chart; an open or zero-ohm reading condemns the sensor.

  4. 4.Verify reference voltage and ground

    With the sensor unplugged and key on, confirm the reference (typically 5 volts) on one terminal and a solid ground path on the other.

  5. 5.Watch live data through a warm-up

    The catalyst temperature PID should start near ambient and rise smoothly into the several-hundred-degree range; a flatlined or pegged value confirms a circuit fault.

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