P0426
PowertrainCatalyst Temperature Sensor Circuit Range/Performance (Bank 1, Sensor 1)
For most people the first sign is just the engine light coming on, with the car driving exactly as it did the day before. Some notice a slight drop in fuel economy or a touch less response when they put their foot down, but plenty of drivers feel nothing at all. What's happened is that the catalyst temperature sensor on Bank 1 (the side with cylinder 1) is sending the ECU a reading that falls outside the range or response it expects, so the module flags it as a circuit performance fault rather than a flat failure.
ⓘ Information only. This page provides general educational information about fault code P0426. 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 P0426 mean?
P0426 is a Powertrain (engine, transmission, fuel system) fault code. It indicates: Catalyst Temperature Sensor Circuit Range/Performance (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 spot
- • Fuel economy down a little, commonly around 5-10% in many cases
- • A mild hesitation or flat spot when accelerating
- • Higher exhaust emissions that show up on a gas analyser even when the car feels fine
- • On a fair number of cars, no drivability change whatsoever, just the light
Possible causes
Causes commonly associated with P0426, listed in approximate order of typical investigation. The actual cause on a specific vehicle can only be confirmed by professional diagnosis.
- 1. The catalyst temperature sensor itself worn or degraded, often the upstream sensor on Bank 1. This is where most P0426 cases end up
- 2. Wiring or connector trouble in the sensor circuit. These sit in a hot, dirty area under the car, so corroded pins and chafed looms are common
- 3. An exhaust leak near the sensor pulling in outside air, which throws the readings out and can mimic a sensor fault
- 4. A tired or partially blocked catalytic converter altering the temperatures the sensor sees
- 5. A PCM calibration glitch or internal module fault, which is rare and only worth considering once everything else 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. Plug in a scanner and pull P0426 along with any companion codes, then read the freeze-frame data to see the conditions when it set
- 2. Get under the car and inspect the Bank 1 sensor and its wiring for melted insulation, green corrosion on the pins, or a connector that's worked loose
- 3. Listen and look for exhaust leaks around the sensor boss and the joints ahead of it, since a small leak there is a classic cause of false readings
- 4. Back-probe the sensor signal with a multimeter once the exhaust is hot and compare it against the workshop manual values for that engine
- 5. Check continuity and the ground path back to the PCM, because a bad earth will upset the signal long before the sensor fails outright
Common questions about P0426
Can I keep driving it like this for a bit? +
Most cars will drive fine with P0426 and you can carry on for the short term, especially if you've noticed nothing beyond the light. The risk is if a duff temperature reading lets the engine run badly or hides damage building up in the cat, which is an expensive part to lose. Get it looked at within a week or two rather than ignoring it for months, and book it sooner if you also see a misfire or fuel trim code stored alongside.
Is this going to be an MOT fail? +
Not directly because of the code, but if the engine warning light is lit when the tester checks the dashboard, that's a fail on its own under current MOT rules. On top of that, a genuine cat or sensor problem can push the emissions reading over the limit during the gas test. Fix the underlying fault, clear the light, and run the car for a few drive cycles before you take it in so the light stays off.
What's it likely to cost to sort out? +
If it's just the sensor, expect somewhere in the low three figures fitted at an independent garage, with main dealers charging noticeably more for labour. A wiring or connector repair can be cheaper, often two figures to low three figures depending on how buried the damage is. The painful outcome is needing a catalytic converter, which can run from the high hundreds into four figures, so it pays to confirm the cat is actually the problem before committing to that spend.
Could an exhaust leak be behind this? +
Yes, and it's worth ruling out early. A leak upstream of or right next to the sensor lets fresh air in, which skews the temperature readings and can set P0426 even though the sensor is doing its job perfectly. A small blowing joint or a corroded section ahead of the sensor is often cheaper to fix than the sensor itself.
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 →