OBD-II trouble code
P0423: Heated Catalyst Efficiency Below Threshold (Bank 1)
The heated-catalyst efficiency monitor on bank 1 is below its threshold. The wiring diagram shows whether an electric heater is fitted. Where it is not, the diagnosis is the oxygen-storage monitor, the downstream sensor, and exhaust leaks. Where it is, the heater circuit is tested before the converter is condemned.
Quick facts
- System
- Powertrain
- Category
- Emissions / Catalyst
- Severity
- Medium severity
- Drivable
- Usually safe to drive short-term
- Repair cost range
- $200 – $2,500
- DIY difficulty
- Intermediate DIY
Browse every code in P0400–P0499, or start from the full code library.
What does P0423 mean?
P0423 lives in the same catalyst-efficiency family as P0420 and P0422, but its SAE definition calls out a 'heated catalyst.' Some vehicles use an electrically heated catalytic converter, a heating element that brings the substrate up to its operating region after a cold start. Bosch patent US6550236 describes three-way catalyst operation above approximately 250°C. Whether this vehicle has that heater is the wiring diagram, not the word heated in the code title. On those vehicles, the catalyst reaching efficient operation depends partly on that heater working. Where the upstream sensor is a switching zirconia sensor, Bosch describes about 0.1 volts lean to about 0.9 volts rich, and a healthy catalyst keeps the downstream trace from copying that swing. A wideband upstream sensor does not switch. Bosch's ME7 monitor instead measures oxygen storage against a borderline catalyst. The threshold is the calibration. P0423 is that efficiency result on the heated-catalyst monitor for bank 1.
The downstream sensor, an exhaust leak ahead of it, and a misfire or rich condition stored with the code are checked before the converter. If the diagram shows an electric catalyst heater, that circuit is checked too, because a heater that does not run changes the warm-up the efficiency monitor sees. Confirm the heater from the diagram before a heater part is ordered.
The efficiency result blocks an OBD inspection while it is stored. Power loss is a restricted substrate, checked separately. Clear misfires and fuel-trim faults, test the downstream sensor, inspect for exhaust leaks, and test the heater circuit only when the diagram shows one.
Common causes
- Slow or biased downstream oxygen sensor producing a false low-efficiency reading
- Exhaust leak ahead of the rear O2 sensor skewing the signal
- Upstream misfire sending raw fuel into the catalyst
- Rich-running condition overheating and damaging the converter
- Catalyst heater element or its circuit failed on vehicles using a heated catalyst
- Oil or coolant contamination poisoning the substrate
- Catalytic converter with low oxygen storage, after the sensor, the leak, and any heater circuit the diagram shows have been checked
Symptoms
- Check engine light with P0423 stored
- No fuel-control change from the efficiency result itself
- Failed emissions or smog inspection
- Higher cold-start emissions if a catalyst heater isn't working
- Possible reduced power if the converter substrate is breaking up
- May appear alongside P0420, P0422, or related codes
Diagnostic steps
- 1.Determine whether the vehicle actually uses an electrically heated catalyst — this decides whether the heater circuit is even part of the diagnosis.
- 2.Scan for and repair any misfire or fuel-trim codes first, since they damage the catalyst and must be fixed before judging it.
- 3.If equipped with a catalyst heater, check its fuse, relay, element, and wiring for a fault that delays light-off.
- 4.Inspect for exhaust leaks between the engine and the rear O2 sensor.
- 5.Compare upstream and downstream data only when the upstream sensor is a switching zirconia sensor. A wideband sensor does not switch. Use the oxygen-storage test in the service information for that calibration.
- 6.Confirm the downstream O2 sensor is responsive and not lazy before considering converter replacement.
Repair cost
$200 – $2,500
The low end of the printed bracket is the downstream oxygen sensor, an exhaust leak, or the heater fuse or relay when the diagram shows a heater. The high end is the catalyst. Read the bracket as a planning span, not a quoted price. Repair a misfire or rich condition that is still stored before the catalyst is installed.
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 intermediate DIY job. It usually involves diagnostic steps, specialty parts, and some careful work in tight spaces. If you have the tools and a service manual or trustworthy video for your specific vehicle, it is achievable in a weekend. Otherwise, a competent independent shop will be faster.