P0423: Heated Catalyst Efficiency Below Threshold (Bank 1)
Quick Answer
Safe to drive (short-term)
P0423 means the heated catalyst on Bank 1 is not converting emissions as efficiently as the ECU expects. A heated catalyst has an electric heating element that brings it to working temperature within seconds of a cold start. Such catalysts are still uncommon on production cars, so first check the manufacturer's definition of the code for your vehicle. On cars that do have one, the usual causes are a failed heater element, a fault in the heater's wiring or control, or a worn catalyst substrate.
- Severity
- Moderate · Emissions
- Estimated repair cost
- From €105 to €2320+, varies by vehicle
€105 or €2320, depending on your car.
- Fix exhaust leak (gasket or flex pipe)€105–€340
- Replace heated catalytic converter (Bank 1)€720–€2320
A scan shows whether P0423 is really stored on your car, which codes sit next to it and how urgent each one is.
A scan needs a Bluetooth OBD2 adapter (Bluetooth LE on iPhone), about €25-60; the built-in demo runs without one. See which adapter to buy
What does P0423 mean?
P0423 is a emissions-related diagnostic trouble code classified as moderate severity.
Where it is on the car
- Engine
- Oxygen sensor before the converter (sensor 1)
- Catalytic converter
- Oxygen sensor after the converter (sensor 2, or 3 on some cars)
- Silencer and tailpipe
What are the symptoms of P0423?
- Check engine light illuminated, often shortly after a cold start
- Higher exhaust emissions during warm-up (not usually noticeable from the driver's seat)
- Possible faint sulfur smell during warm-up
- Failed emissions test or periodic technical inspection
- No change in how the car drives in most cases
What causes P0423?
Failed e-cat heater element or substrate degradation
: high
Faulty downstream O2 sensor (Bank 1, Sensor 2)
: medium
Wiring or connector fault in the heater circuit, or a failed heater relay or control module
: medium
Prior misfire history damaging the heated brick
: low
Oil consumption fouling the catalyst substrate
: low
Exhaust leak ahead of the downstream O2 sensor skewing its reading
: low
Is it safe to drive with P0423?
Generally yes, for short-term driving. Safe for short-term driving. The code points to the catalyst's cold-start performance, so emissions are higher than they should be during warm-up, and a stored catalyst code with the check engine light on is the kind of defect that gets a car rejected at an emissions check or periodic technical inspection.
How do you diagnose P0423?
- Read freeze frame data and confirm the code triggered shortly after cold start, which is typical of heated catalyst faults
- Check for companion codes such as P0424 (heated catalyst temperature below threshold) or manufacturer-specific heater circuit codes, and read the manufacturer's definition of P0423 for your vehicle
- Measure the resistance of the catalyst heater element and compare it with the manufacturer specification
- Inspect the wiring harness and connector to the heater control module for corrosion, broken pins, or water ingress
- Graph the Bank 1 downstream O2 sensor after a cold start and compare its behaviour with the manufacturer's data: a working catalyst settles the downstream signal quickly, while a failing one lets it follow the upstream sensor
- Inspect the exhaust ahead of the downstream O2 sensor for leaks, which can skew its reading
How much does P0423 cost to fix?
| Repair | Parts Cost | Labor Cost | Total Estimate | DIY Difficulty |
|---|---|---|---|---|
| Replace heated catalytic converter (Bank 1) | €600–€2000 | €120–€320 | €720–€2320 | Professional |
| Replace downstream O2 sensor (Bank 1) | €60–€180 | €60–€130 | €120–€310 | Moderate |
| Fix exhaust leak (gasket or flex pipe) | €25–€120 | €80–€220 | €105–€340 | Moderate |
Prices estimated as of October 2026. Costs vary by region, vehicle, and shop.
Related diagnostic codes
Common questions about P0423
- What is a heated catalyst (e-cat) and why does my car have one?
- An electrically heated catalyst contains a resistive element that warms the catalyst to its working (light-off) temperature within seconds of a cold start, before exhaust heat alone could do it, which cuts emissions in the first minutes of driving. Suppliers offer them mainly for hybrid and 48 V mild-hybrid systems, where the heater can draw on a larger electrical supply, but they are still uncommon on production cars.
- Why is replacing a heated catalyst so much more expensive than a regular one?
- Heated catalysts contain an integrated electrical heater element, a high-current connector, and often a dedicated control module or relay. The assembly is more complex than a passive cat and is often available only as an original part, which is why parts alone range from 600 to 2,000 EUR depending on the vehicle.
- Can a wiring problem trigger P0423 instead of the catalyst itself?
- Yes. A broken heater wire, corroded pin, or failed heater control module can cause the e-cat to never reach light-off temperature, which the ECU then interprets as catalyst inefficiency. Always perform a heater-circuit resistance check before condemning the converter.
- Will an aftermarket non-heated catalyst work as a replacement?
- No. The ECU monitors the heater circuit. Replacing a heated cat with a passive cat will trigger both P0423 and heater-circuit codes immediately, and the car will not present clean at an emissions check. A replacement has to be a heated catalyst with the same heater circuit, or the ECU will keep flagging it.