What does the sensor after the catalytic converter do?

It monitors the converter rather than the mixture. The sensor in front of the converter is the one that controls fuelling, switching rapidly as the control unit hunts around the correct ratio. The rear sensor should see a much smoother, steadier exhaust because a healthy converter stores and releases oxygen. Comparing the two is how the car judges whether the converter is still working, which is why a fault in the rear sensor usually changes nothing about how the car drives but can produce a catalyst efficiency code that is not genuine.

Will P0136 fail an MOT?

Yes. The illuminated engine management lamp is a major defect on petrol cars first used on or after 1 July 2003. There is a second and less obvious risk: the catalyst efficiency readiness monitor depends on this sensor, so it cannot complete while the sensor is faulty. If codes have been cleared before the test, that monitor will show as not ready, and on many cars the emissions check cannot be finished until the monitors have run, which is a separate major failure.

Can a faulty rear oxygen sensor cause a P0420 code?

Yes, and it is one of the main reasons catalytic converters get replaced unnecessarily. The catalyst efficiency check works by comparing the front and rear sensor signals, so if the rear sensor is stuck, slow or corroded, the comparison is meaningless and the converter can be condemned when it is perfectly serviceable. If both codes are stored together, resolve the sensor fault first and see whether the catalyst code returns before authorising any converter work.

Why do rear oxygen sensors fail more often than front ones?

Position. The rear sensor and its connector hang beneath the car behind the catalytic converter, directly exposed to road spray, salt, grit and stones, which in a British winter is about as hostile as it gets for an electrical connection. Water in the plug is a very common cause of a stuck signal, and corrosion of the wiring along the underbody run is at least as likely as the sensor element itself failing. The sensor also tends to seize in its boss, so removal can be awkward.

Powertrain code · standard

P0136Rear oxygen sensor after the catalytic converter reading incorrectly

Standard definition: O2 Sensor Circuit Malfunction (Bank 1, Sensor 2)

Last updated 27 July 2026

Quick answer

This is the sensor behind the catalytic converter, and its job is entirely different from the one in front. It does not control fuelling on most cars — it grades the converter. That is why a fault here usually changes nothing about how the car drives, and why it can make a catalyst code appear that is not real.

How the car works this out

The rear sensor sits downstream of the catalytic converter, where a working converter has evened out the swings in exhaust chemistry. Its signal should therefore be comparatively flat and steady, in contrast to the rapidly switching front sensor. The control unit sets this code when the rear signal is implausible: parked at a fixed voltage, failing to respond when the mixture is deliberately forced rich or lean during a monitor, or reading in a way that cannot be reconciled with the front sensor and the fuel trims. A signal stuck high — persistently reporting no oxygen — is a particularly common form of it, and points strongly at water in the connector or at an engine that genuinely is running rich.

What you might notice

Normally nothing at all, since this sensor does not control fuelling on most vehicles
Occasionally a small drift in fuel trims on cars that use the rear sensor to fine-tune the front one
A catalyst efficiency code appearing alongside it
Fuel consumption very slightly up on some vehicles

What causes it

Most likely first. A code names the check that failed, not the part that failed it, so more than one of these can produce the same number.

1Water and road salt getting into the sensor connector, which hangs under the car behind the converter in the most exposed position of any sensor on the vehicleCommon
How to check: Unplug and look for moisture, green corrosion or a plug full of grit. A signal parked high is very often a wet connector rather than anything to do with the exhaust.
2An aged sensor, contaminated or worn out after a long life in a hostile placeCommon
3Chafed, corroded or road-damaged wiring on the underbody runCommon
How to check: Follow the loom along the floor. Wiring here takes stones, salt and heat, and a break is at least as likely as a sensor failure.
4An exhaust leak between the converter and the sensor, drawing air in and making the reading meaninglessPossible
How to check: Check the joint behind the converter and any flexi section. This is also the mechanism that produces a false catalyst efficiency code.
5The engine running genuinely rich, holding the downstream signal highPossible
How to check: Look at the fuel trims and any fuelling codes stored. A rich engine explains a stuck-high rear sensor without anything being wrong with the sensor.
6A genuinely failed catalytic converter allowing raw exhaust chemistry straight throughLess common

What to do about it

1Understand what this sensor is for before spending anything. On most cars it does not adjust fuelling — it exists to referee the catalytic converter. That is why nothing feels wrong and why the code is easy to dismiss.
2Check the connector and the underbody wiring first. This sensor lives in the worst environment on the car and its plug fills with water more readily than anything under the bonnet.
3If a catalyst efficiency code is stored alongside this one, deal with this one first. A rear sensor that cannot be trusted makes the converter's report card meaningless, and buying a converter on that evidence is an expensive way to find out.
4Look for exhaust leaks between the converter and the sensor, which produce misleading readings from perfectly good components.
5Allow for a seized sensor when planning the job. This one has spent its life under the car and removing it without damaging the boss is not always straightforward — a garage will warm the pipe and use penetrating oil rather than simply pulling on a spanner.

Can you keep driving?

Yes

On the great majority of cars this sensor plays no part in how much fuel the engine receives, so the car drives, performs and consumes fuel exactly as before. The consequence is entirely about what the car can and cannot verify about itself, which turns into an MOT problem rather than a mechanical one.

Will it fail an MOT?

The lamp fails the car as a major defect, but the more troublesome consequence of this code is what it does to the readiness monitors. The catalyst efficiency monitor works by comparing this sensor against the front one, so with the rear sensor faulty that monitor generally cannot run to completion. If the codes have been cleared at any point before the test, the tester will find the catalyst monitor sitting permanently not ready, and on many cars the emissions check cannot be completed until it has — which is a major failure in its own right, on top of the lamp. There is a practical warning attached as well: because this sensor is what produces the evidence behind a catalyst efficiency code, never authorise a catalytic converter while this code is also stored.

The light this turns on

Engine management lightThe engine management system has found a fault it cannot correct and has stored a code for it. The car will usually still drive, but an illuminated engine light is an automatic MOT failure on petrol cars first used from 1 July 2003 and diesels from 1 July 2008.

Common questions

What does the sensor after the catalytic converter do?: It monitors the converter rather than the mixture. The sensor in front of the converter is the one that controls fuelling, switching rapidly as the control unit hunts around the correct ratio. The rear sensor should see a much smoother, steadier exhaust because a healthy converter stores and releases oxygen. Comparing the two is how the car judges whether the converter is still working, which is why a fault in the rear sensor usually changes nothing about how the car drives but can produce a catalyst efficiency code that is not genuine.
Will P0136 fail an MOT?: Yes. The illuminated engine management lamp is a major defect on petrol cars first used on or after 1 July 2003. There is a second and less obvious risk: the catalyst efficiency readiness monitor depends on this sensor, so it cannot complete while the sensor is faulty. If codes have been cleared before the test, that monitor will show as not ready, and on many cars the emissions check cannot be finished until the monitors have run, which is a separate major failure.
Can a faulty rear oxygen sensor cause a P0420 code?: Yes, and it is one of the main reasons catalytic converters get replaced unnecessarily. The catalyst efficiency check works by comparing the front and rear sensor signals, so if the rear sensor is stuck, slow or corroded, the comparison is meaningless and the converter can be condemned when it is perfectly serviceable. If both codes are stored together, resolve the sensor fault first and see whether the catalyst code returns before authorising any converter work.
Why do rear oxygen sensors fail more often than front ones?: Position. The rear sensor and its connector hang beneath the car behind the catalytic converter, directly exposed to road spray, salt, grit and stones, which in a British winter is about as hostile as it gets for an electrical connection. Water in the plug is a very common cause of a stuck signal, and corrosion of the wiring along the underbody run is at least as likely as the sensor element itself failing. The sensor also tends to seize in its boss, so removal can be awkward.

Check any vehicle's MOT history

Free, instant, and straight from DVSA records. No sign-up.