P0429

Code

Generic P — Powertrain

Catalyst Heater Control Circuit/Open Bank 1

Brand: Generic

Type: P — Powertrain

Views: UK: 31 EN: 44 RU: 26

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Page language: EN

Causes

Blown fuse or tripped circuit breaker for heater circuit
Open or damaged wiring/connector between PCM and catalyst heater (Bank 1)
Corroded or poorly connected connector at heater or ground
Failed heater element inside the catalytic converter (open circuit)
Faulty relay or external power supply to heater circuit
PCM/ECM output driver failure or internal short/open

Symptoms

Check Engine Light (MIL) on with code P0429 stored
Possible reduced warm-up performance of emissions system (longer time to reach normal catalyst temperature)
No noticeable change in driveability in many cases (often no obvious drivability symptom)
Possible difficulty passing cold-start emissions or longer tailpipe heating time

What to check

Read freeze frame and stored data with a scan tool; attempt an active test to command the heater ON/OFF if available
Visually inspect fuses, relays, and wiring for the catalyst heater circuit (Bank 1) — look for melted insulation, chafing, heat damage
Inspect connectors at the catalyst heater and PCM for corrosion, bent pins, or poor mating
Back-probe heater power and control pins with ignition ON and with PCM commanding heater ON; check for battery voltage and switching/grounding
Measure resistance of the heater element with harness disconnected (see signal_params for expected range)
Perform a wiggle test while monitoring circuit voltage/current to find intermittent opens

Signal parameters

Heater element resistance (with harness disconnected): low ohms — typically in the low single digits up to a few tens of ohms depending on design (consult vehicle spec)
Supply voltage to heater power pin (key ON, heater commanded ON): approximately battery voltage (~11–14V)
Control output from PCM: either a switched ground or switched 12V; may be steady or PWM when active — verify with oscilloscope/scan tool
Current draw when heater is energized: typically measurable with a clamp meter; expected to be a few amps for most designs (very large currents may indicate a short)
Open-circuit condition: infinite/very high resistance or no current when heater commanded ON

Diagnostic algorithm

Retrieve freeze-frame and complete DTC list with a scan tool. Note whether the code is stored as active/pending and record any related codes.
Attempt an active/command test (if supported) to force the catalyst heater ON while monitoring voltage/current at the heater connector. Observe if the PCM supplies the expected signal and whether current flows.
Visually inspect fuses and relays for the heater circuit; verify fuse continuity and inspect relay operation.
With ignition OFF, disconnect the catalyst heater connector at the converter and measure heater element resistance across the heater pins. Compare to manufacturer specification; an open (infinite) reading indicates a failed heater element.
With harness disconnected, check for short to battery or ground from the heater supply and control circuits using an ohmmeter.
Back-probe the harness connector with the heater connected. With ignition ON and heater commanded ON, check for battery voltage at the power feed and proper switching/ground at the control pin. If no command signal, inspect PCM grounds and connectors.
Inspect the entire wiring route for heat damage, chafing, rodent damage, or pushed-through insulation, particularly near the exhaust and underbody. Repair any damaged wiring using proper high-temperature wiring techniques.
Repair or replace corroded connectors and re-measure. If wiring and power/ground are good but heater element is open, replace the catalytic converter assembly (or heater component if replaceable separately).
If wiring and heater test good but no PCM output when commanded, check PCM power/ground and communications; consult wiring diagrams and consider PCM/ECM replacement only after ruling out harness and connector faults.
Clear codes and perform a road or drive-cycle verification to ensure the code does not return.

Likely causes

Open or damaged wiring/connector to catalyst heater (most common)
Blown fuse or faulty relay supplying heater power
Failed heater element (open circuit) in the catalytic converter
Corroded/poor ground or connector pin contamination
PCM/ECM driver fault (least common)

Fault status

⚠️ Status

PCM detected an open or fault in the catalyst heater control circuit for Bank 1 — heater not drawing expected current or no circuit continuity when commanded.

🟡 Repair difficulty: Medium

⏱️ Diagnostic time: 0.5-2.0 hours