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P codepowertrainMedium severity

P0424Heated Catalyst Temperature Below Threshold (Bank 1)

The electrically heated catalytic converter on Bank 1 did not reach its expected operating temperature during the warm-up period after a cold start.

Definition

On vehicles equipped with an electrically heated catalyst, the PCM energizes the catalyst heater at cold start and watches catalyst temperature through a dedicated temperature sensor to confirm rapid light-off. P0424 sets when Bank 1 catalyst temperature stays below the calibrated threshold for the allowed warm-up window. There are usually no driveability symptoms, but the MIL illuminates, cold-start emissions rise sharply, and the vehicle will fail an emissions inspection.

Common causes

  • Failed catalyst heater element (open or high resistance)
  • Blown heater fuse or failed heater relay in the high-current feed
  • Damaged, chafed, or corroded wiring to the catalyst heater
  • Faulty catalyst temperature sensor under-reporting actual temperature
  • Exhaust leak upstream of the catalyst cooling the exhaust stream
  • Deteriorated catalyst substrate that is slow to light off

Common misdiagnoses

  • Catalytic converter replaced when the heater circuit or its fuse is the actual fault
  • Oxygen sensors replaced simply because the code lives in the catalyst range
  • Thermostat or coolant temperature sensor replaced for a perceived slow warm-up
  • PCM replaced before the heater relay and wiring are load-tested

Troubleshooting steps

  1. 1.Record freeze frame data

    Note coolant temperature, run time, and catalyst temperature at the moment the code set to confirm the failure occurred during the cold-start heating phase.

  2. 2.Check the heater fuse and relay

    Inspect the dedicated high-amperage fuse and bench-test the heater relay; catalyst heater circuits can carry well over 20 amps.

  3. 3.Measure heater element resistance

    Disconnect the heater and measure across the element; most read under a few ohms, and an open (OL) reading condemns the heater.

  4. 4.Verify power delivery during warm-up

    Start the engine cold and confirm battery voltage and current draw at the heater connector while the PCM commands heating.

  5. 5.Rationality-check the catalyst temperature sensor

    After an overnight soak the sensor should read within about 10 F of ambient and climb steadily with engine run time.

  6. 6.Inspect for upstream exhaust leaks

    Check the manifold, gaskets, and flex pipe; a leak ahead of the catalyst lowers exhaust temperature and delays light-off.

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