P0133 means the front oxygen sensor is responding too slowly. This is a code about speed rather than accuracy: the sensor is still reporting roughly the right thing, it is simply taking too long to do it. Usually that means an ageing sensor. Diagnose soon and drive gently. Indicative UK repair cost is £60 to £300.
The front oxygen sensor is not a passive instrument, it is the feedback the fuelling runs on. The engine computer adds fuel, watches the sensor swing rich, takes fuel away, watches it swing lean, and repeats that several times a second. Everything depends on the sensor keeping up. A sensor that has gone lazy makes the computer chase a reading that is already out of date, so the mixture wanders either side of correct instead of sitting on it, and the result is worse economy and dirtier running without anything obviously breaking.
Can I drive with P0133?
Diagnose soon, drive gently. There is no sudden failure waiting at the end of this one. The car will drive normally, use a little more fuel than it should, and run a mixture that is less precise than the designers intended. The reason to deal with it is the thing sitting downstream: a converter spends its life being fed by this sensor's decisions, and months of imprecise fuelling is one of the ways a healthy converter gets tired early.
What P0133 means for your MOT
Three routes, and the middle one is specific to this sensor.
- The lamp. A major defect under section 8.2.1.2 of the DVSA MOT inspection manual for petrol cars first used on or after 1 July 2003.
- Lambda, because this sensor is what controls it. The lambda coefficient the tester measures is the direct product of the closed-loop fuelling this sensor drives. When the sensor cannot keep up, that figure wanders, and a coefficient outside the manufacturer's range or the default limits is a major defect. Of all the oxygen sensor codes, this is the one with the most direct line to the measured result.
- The sensor as a component. Section 8.2.1.1 names oxygen sensors among the emissions components a tester inspects, and one that is missing, obviously modified or obviously defective is a major defect on any vehicle first used on or after 1 September 2002.
What usually causes it
- A worn sensor. The ordinary answer and the likeliest one. These are consumable items with a service life, and what fails first is almost always their speed rather than their accuracy, which is precisely what this code measures.
- An exhaust leak near the sensor. A leak upstream of it lets fresh air into the stream, and a sensor sampling a mixture of exhaust and atmosphere behaves erratically. Section 8.2.1.1 treats an exhaust leak affecting emissions as a defect of its own, so finding one solves two problems.
- A wiring fault. A chafed lead or a corroded connector slows and distorts a signal that is only a fraction of a volt to begin with. The sensor's lead lives under the car in the weather, which is why this is worth looking at before buying the part.
There is a fourth possibility worth raising honestly, because it is the one that catches people out. Oxygen sensors are poisoned by things they should never be exposed to: engine oil burning past worn components, coolant from a failing head gasket, and silicone from the wrong sealant used on a previous repair. If a car is using oil or losing coolant and has gone through more than one of these sensors, the sensor is the messenger rather than the fault, and replacing it again will not be the end of it.
What it costs to fix in the UK
£60 to £300. A sensor on an engine where it can be reached from above is the bottom. The top is where the sensor has seized into the manifold, which is common on a car that has done big mileage, and where freeing it becomes most of the labour. A wiring repair can land anywhere in between depending on how much of the loom has to be unwrapped.
Before paying for a diagnosis, it is worth knowing that live data on this one takes a couple of minutes to read. What a car diagnostic costs in the UK gives you the going rate to weigh that against.
These are indicative UK ranges for guidance only. Actual causes and prices vary by vehicle, and a stored code points to a system, not always the exact part.
What to check first
- Watch the sensor switch, and count. This is the measurement the code is about. With the engine warm and at a steady raised idle, a healthy front sensor crosses between rich and lean briskly and repeatedly. A trace that drifts slowly up and down, or lingers at the top and bottom, is a slow sensor and that is your answer. Looking only at the voltage value, rather than how fast it changes, misses the entire point of this code.
- Listen for an exhaust leak before the sensor, cold. A manifold or front pipe leak ticks loudest on a cold start before the metal expands and closes it up. Finding one here saves you the price of a sensor.
- Check the lead and the connector. Follow it from the sensor to where it plugs in, looking for melted insulation where it has touched the exhaust, and for corrosion inside the plug.
- Ask what else the engine is doing. Oil consumption, coolant disappearing, or a previous repair with the wrong sealant. If any of those is true, deal with it alongside the sensor rather than after the second one fails.
Related codes
- P0420, catalyst efficiency below threshold. Often stored alongside this one, and worth reading because a lazy front sensor can both cause and imitate a catalyst fault.
- P0171, system too lean (Bank 1). The fuel trim consequence of a sensor the computer can no longer rely on.
- P0172, system too rich (Bank 1). The same relationship in the other direction, and the page that explains why a sensor fault can look like a fuelling fault.
Get it narrowed down before you spend
A stored code tells you which system is unhappy, not which part has failed. If you want it narrowed down before you spend anything, send the code and your symptoms and a mechanic will read them personally. Quick Diagnosis is £20, and if we can't help, you don't pay.
Written by the First Diagnostic mechanic. Over a decade in the trade: a VW-only workshop running diagnostics and electrical repair, then BMW-only through to full engine rebuilds. Now an independent all-makes workshop, plus building and wiring drift cars for competition, including trackside work as head mechanic. More about who reads your codes.