P0428
PowertrainCatalyst Temperature Sensor Circuit High (Bank 1, Sensor 1)
This one is usually down to a corroded or broken wire at the catalyst temperature sensor, or the sensor itself reporting a duff signal. The sensor sits in the hot end of the exhaust near the cat, and the ECU watches its voltage to keep an eye on cat temperature. When that voltage reads higher than the sensor could realistically produce, the ECU flags an open circuit or a high reading and logs P0428. It's an electrical fault on bank 1, sensor 1, not a sign your cat has actually cooked itself.
ⓘ Information only. This page provides general educational information about fault code P0428. We do not assess the urgency or safety implications of any specific fault. That requires in-person diagnosis by a qualified mechanic. Full terms.
Recommended next steps
Whether a fault is urgent, drivable, or routine depends entirely on the cause on a specific vehicle, and that can only be determined by a qualified mechanic with diagnostic equipment. If a warning light is illuminated, the most reliable next step is professional diagnosis.
What does P0428 mean?
P0428 is a Powertrain (engine, transmission, fuel system) fault code. It indicates: Catalyst Temperature Sensor Circuit High (Bank 1, Sensor 1).
This is a standardised OBD-II code. The technical definition is the same regardless of the make or model of vehicle, although specific causes and symptoms can vary between vehicles.
Symptoms commonly associated with this code
Symptoms that drivers often report alongside this code. Not all may apply to every case:
- • Engine warning light on, frequently the only thing you'll notice day to day
- • On some vehicles the engine drops into limp mode and pulls power back as a precaution
- • Slightly worse fuel economy, more obvious on a steady motorway run
- • Occasional rough running or a brief hesitation off the throttle
- • No symptoms at all in many cases, the light is the only clue
Possible causes
Causes commonly associated with P0428, listed in approximate order of typical investigation. The actual cause on a specific vehicle can only be confirmed by professional diagnosis.
- 1. Damaged or corroded wiring to the sensor, the connectors live in a brutal environment of heat, road salt and vibration, so this is where the fault usually hides
- 2. Faulty catalyst temperature sensor pushing out a voltage the ECU treats as impossible
- 3. Loose or dirty connector pins at the sensor plug, a classic cause of an intermittent high reading that comes and goes
- 4. Open circuit or high resistance somewhere in the sensor harness or its ground path
- 5. Exhaust leak near the sensor mounting throwing the readings off
- 6. Failed PCM, but this is rare and only worth considering once everything upstream checks out
How mechanics typically diagnose
A typical diagnostic sequence used by mechanics, provided here for educational reference only. Diagnostic work should be performed by a qualified mechanic with the appropriate tools and training.
- 1. Read the freeze frame data first so you know exactly when the code set, a cold-start trigger points one way and a fault under load points another
- 2. Get under the car and inspect the sensor wiring and connector. Look for melted insulation, green corrosion on the pins, and a plug that isn't fully seated
- 3. Back-probe the connector with a multimeter and check the signal voltage against the workshop figure for that engine. A reading pinned at supply voltage usually means an open circuit
- 4. Check continuity through the harness between the sensor and the ECU, and confirm the ground is clean. High resistance here will mimic a failed sensor
- 5. Inspect for an exhaust leak around the sensor boss, because air getting in skews the temperature reading
- 6. If the wiring and sensor both test good, only then look at the PCM or check for any service bulletins on that model
Common questions about P0428
How long should this take to sort out? +
A connector clean or a wiring repair is usually an hour or two of labour once the fault is located, and a fair chunk of that is just getting the car up and finding the damaged bit. Swapping the sensor itself is typically under an hour if it isn't seized solid. The diagnosis is what eats the time, so a garage that charges a diagnostic fee will spend that finding which of the three usual suspects it actually is.
Is a cheap aftermarket sensor any good, or do I stick with the genuine part? +
A quality aftermarket sensor from a known brand is fine for most cars and costs a good bit less than dealer price. Steer clear of the bargain-bin no-name parts off the auction sites, because a temperature sensor that reads slightly off will just set the same code again and you'll be back where you started. Spend a little more on a reputable brand and you'll only do the job once.
Can I keep driving with it showing? +
For short local trips it's generally alright, since this is an electrical fault and not the cat actually overheating. The risk is that some engines pull power or go into limp mode while the code is active, which makes the car unpleasant and slow. Get it looked at before any long motorway journey, and don't ignore it for weeks on end.
Will it cause an MOT failure? +
The code on its own isn't tested at MOT, but the engine warning light is. If the MIL is lit when the car goes in, that's a major defect and a fail. Fix the underlying wiring or sensor fault, clear the light, and run the car for a few drive cycles to make sure it stays off before the test.
Information only, not professional advice
The information on this page is provided for general guidance and educational purposes only. It is not a substitute for diagnosis or repair advice from a qualified mechanic. Always verify any fault before paying for repairs. carfaultcodes.co.uk accepts no liability for decisions made based on this information. Full terms →