OBD-II trouble code
P0429: Catalyst Heater Control Circuit (Bank 1)
The circuit that drives the catalyst heater on bank 1 is not behaving as commanded. Every other code in this block watches something; this one drives something. That single difference changes the electrical environment, the test method and the eventual bill.
Quick facts
- System
- Powertrain
- Category
- Emissions / Catalyst
- Severity
- Medium severity
- Drivable
- Usually safe to drive short-term
- Repair cost range
- $30 – $3,000
- DIY difficulty
- Advanced DIY
Browse every code in P0400–P0499, or start from the full code library.
What does P0429 mean?
P0424 through P0428 are all measurement codes — a thermometer that has stopped reporting, or a temperature that arrived too late. P0429 sits at the other end of the system. It concerns an output: a control circuit through which the module energises a heating element built into the catalytic converter, and a feedback path by which the module confirms the command actually took effect. The code sets when those two disagree — the heater was commanded on and the circuit did not respond, or it was commanded off and current is still flowing.
The first thing to understand is that this is not a signal circuit. A catalyst heater is a load, not a signal. The diagram names its fuse, its relay or driver, and its supply. There is no universal current and no universal on-time on this page. The fuse and the relay are tested because they are in that circuit. This page does not rank what share of these codes they account for.
If the diagram shows a heater supply above the 12-volt system, follow that system's disconnect procedure before the connector is opened or probed. Do not assume a 48-volt rail, an orange sheath, or a safe touch voltage. The diagram is the architecture.
Where the parts listing shows the heating element as part of the converter assembly, an open element is that assembly. Check the listing before a converter is ordered. A fuse, a relay, a connector, or a ground that the diagram names can store this code with the element intact. Test those first because they are the circuit, not because of a price ranking.
The vehicle will drive normally. The consequence of leaving it is measured in cold-start emissions, which is the part of the journey where a converter that has not lit off is doing nothing at all.
Common causes
- Blown fuse in the heater supply the diagram names
- Failed heater relay, or a relay with contacts welded closed so current flows when it should not
- Catalyst heater element failed open, replaced with the converter only when the parts listing includes the element in that assembly
- Corroded or heat-damaged heater connector at the converter
- Poor ground for the heater circuit, raising resistance until the module sees the command fail
- Damaged supply cable where it passes close to the exhaust or a suspension component
- Module output driver failed, either open or shorted
- Charging system unable to support the load, so heater voltage sags below the threshold the module accepts
Symptoms
- Check engine light with P0429 stored
- Failed emissions test, particularly on cold-start measured cycles
- A cold-start difference only where the heater is what brings the catalyst up to its operating region
- Normal drivability once the engine is warm
- Sometimes stored alongside P0424, since a heater that is not working delays light-off
- A repeatedly blown fuse for the heater circuit, if the fault is a short in the supply
Diagnostic steps
- 1.Read the diagram before the connector is opened. If the heater supply is above the 12-volt system, follow that system's disconnect procedure and do not back-probe it live.
- 2.Check the heater fuse and the relay the diagram names. They are in the circuit. They are not a share of cases.
- 3.Swap in a known good relay of the same part number, or test the relay's coil and contact resistance directly, before going any further into the circuit.
- 4.Measure heater resistance against the specification for this element. An open reading, with the supply and ground already proven, condemns the element. There is no universal ohm value.
- 5.Check for voltage drop across the supply cable and the ground under load rather than statically. The heater circuit can pass a continuity test and still drop voltage when the heater is commanded on. The allowed drop is the specification for this vehicle.
- 6.Inspect the heater connector at the converter for heat discoloration, a melted housing, and spread terminals. A connector with heat discoloration, a melted housing, or spread terminals is tested for voltage drop with the heater commanded on.
- 7.Command the heater on with a bidirectional scan tool if the vehicle supports it and watch current draw. Commanded state against measured current is the fastest confirmation of where the break sits.
- 8.Replace the converter only when the parts listing shows the element is part of that assembly and the fuse, relay, wiring, connector, ground, and driver have passed.
Repair cost
$30 – $3,000
The low end of the printed bracket is the fuse or the relay the diagram names. The high end is the converter assembly when the parts listing includes the heater in that assembly. Read the bracket as a planning span, not a quoted price.
Estimate your repair
Run the numbers for your vehicle
Open the Repair Cost Estimator with catalytic converter replacement preselected. Adjust labor rate and vehicle category to fit your situation.
DIY vs shop
This is an advanced DIY job. It typically requires specialty tools, scan-tool access, lifting equipment, or careful sequencing to avoid causing new failures. Plan for extended downtime and have a backup vehicle. Most owners are better served by a shop that has done this repair before.