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P codepowertrainMedium severity

P0426Catalyst Temperature Sensor Range/Performance (Bank 1)

The Bank 1 catalyst temperature sensor signal is implausible — stuck, sluggish, or inconsistent with the temperature the PCM expects.

Definition

The PCM continuously compares the Bank 1 catalyst temperature sensor against a modeled exhaust temperature and against other temperature sensors after a cold soak. P0426 sets when the signal is electrically valid but irrational — for example, it stays fixed while the engine warms, responds too slowly, or disagrees with ambient after the vehicle has sat overnight. The MIL illuminates and catalyst protection and monitoring strategies are degraded; on diesel applications an inaccurate reading can disturb regeneration timing.

Common causes

  • Drifted or lazy sensor element that responds slowly to temperature change
  • Soot or carbon buildup insulating the sensor tip
  • Intermittent high resistance in the connector or harness
  • Sensor loose or partially backed out of its exhaust boss
  • Exhaust leak near the sensor tip skewing the local temperature
  • Heat-fatigued wiring causing an erratic signal

Common misdiagnoses

  • Catalytic converter replaced for a sensor rationality fault
  • Sensor replaced when a corroded connector is the real problem
  • Oxygen sensors replaced based on the catalyst-range code number

Troubleshooting steps

  1. 1.Perform a cold-soak comparison

    After 6-8 hours parked, the catalyst temperature PID should read within roughly 10 F of ECT and IAT; a large offset points to a drifted sensor.

  2. 2.Watch response during warm-up

    Start the engine and confirm the reading climbs steadily; a value that lags far behind or never moves indicates a lazy or stuck element.

  3. 3.Wiggle-test the wiring

    Monitor the PID while flexing the harness and connector; any sudden jump reveals an intermittent connection.

  4. 4.Inspect the sensor tip and mounting

    Remove the sensor and check for heavy soot coating, physical damage, or evidence it was not fully seated in the exhaust.

  5. 5.Check for exhaust leaks near the sensor

    A leak at a nearby joint pulls in outside air and lowers the temperature the sensor sees relative to the PCM model.

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