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P codepowertrainMedium severity

P0429Catalyst Heater Control Circuit (Bank 1)

An electrical fault has been detected in the control circuit for the Bank 1 electrically heated catalyst.

Definition

The PCM controls the Bank 1 catalyst heater through a relay or driver stage and monitors circuit feedback to verify the heater switches on and off as commanded. P0429 sets when the control circuit is open, shorted, or the feedback voltage does not match the commanded state. With the heater inoperative the catalyst is slow to light off, so cold-start emissions rise; the MIL comes on and the vehicle will fail an emissions inspection even though it drives normally.

Common causes

  • Failed catalyst heater relay
  • Blown fuse in the heater power feed
  • Open or shorted wiring in the control or feedback circuit
  • Open heater element pulling the feedback out of range
  • Failed PCM driver stage (rare)

Common misdiagnoses

  • Catalytic converter assembly replaced when only the relay or a fuse has failed
  • PCM replaced before the relay and wiring are tested
  • Oxygen sensors replaced based on the catalyst-range code alone

Troubleshooting steps

  1. 1.Check the fuse and relay first

    Verify the heater fuse is intact and swap or bench-test the relay; these are the cheapest and most common failures in this circuit.

  2. 2.Command the heater with a scan tool

    Use bidirectional controls (or a cold start) to activate the heater and listen or meter for the relay clicking and the circuit going live.

  3. 3.Test the control circuit wiring

    Confirm the PCM's control signal reaches the relay coil and that the relay output delivers battery voltage to the heater connector.

  4. 4.Measure the heater element

    Check resistance across the heater terminals; an open element can set this code by disturbing the feedback the PCM expects.

  5. 5.Inspect grounds and connectors

    A corroded ground for the high-current heater circuit causes voltage-drop faults; verify less than 0.2 V drop on the ground side under load.

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