What is the difference between P0135 and P0141?

They describe the same electrical fault in different sensors. P0135 is the heater in the sensor ahead of the catalytic converter, which affects how quickly the engine can start controlling its mixture after a cold start. P0141 is the heater in the sensor behind the converter, which affects how quickly the car can assess whether the converter is working. The practical consequences differ: the front one costs a little fuel on short journeys, while the rear one can stop the catalyst readiness monitor completing altogether, which has direct implications at MOT time.

Will P0141 fail an MOT?

Yes, and it carries an additional complication. The illuminated engine management lamp is a major defect on petrol cars first used on or after 1 July 2003, which fails the car by itself. Beyond that, the catalyst efficiency monitor cannot complete while this sensor's heater is faulty, so if codes have been cleared the monitor will show as permanently not ready, and on many cars the emissions test cannot be completed until the monitors have run. Driving further will not resolve it, because the monitor depends on the very component that has failed.

Does a rear oxygen sensor heater affect fuel economy?

Barely, if at all. On most vehicles the sensor behind the catalytic converter plays no part in deciding how much fuel the engine receives — it is there to assess the converter. A failed heater in it therefore has essentially no effect on consumption or performance, which is why the fault produces no symptoms and is usually discovered only when the codes are read.

Why is the rear oxygen sensor so hard to remove?

Because of where it lives. It threads into the exhaust behind the catalytic converter, underneath the car, where it spends years exposed to road salt, spray and heat cycling. The threads corrode into the boss and the sensor body itself can crumble when a spanner is put on it. Garages generally warm the pipe, use penetrating oil and a proper slotted sensor socket rather than forcing it, because stripping the threaded boss turns an inexpensive sensor replacement into replacing a section of exhaust.

Powertrain code · standard

P0141Heater in the rear oxygen sensor has failed

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

Last updated 27 July 2026

Quick answer

The heating element in the sensor behind the catalytic converter is not working. The knock-on effect is more significant than it first appears, because the catalyst monitor cannot run until this sensor is up to temperature — which is exactly the check a tester may need to have completed before the emissions test can be finished.

How the car works this out

Like the front sensor, the rear one contains its own resistive heating element on a circuit entirely separate from its signal wires. The control unit watches the current the heater draws or the resistance of its circuit and compares it with the expected range, setting this code when the circuit reads open, shorted or out of range. The rear sensor's heater actually works harder than the front one's in ordinary use, because gas reaching it has already given up heat to the converter and it sits further from the engine in the airflow under the car.

What you might notice

Nothing detectable while driving
No change in performance or economy that you would notice
Often found only when the codes are read after the lamp appears
Frequently stored alongside a catalyst efficiency code that never resolves

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.

1The heater element inside the sensor has gone open circuit after a long life in a cold, wet positionCommon
How to check: Unplug the sensor and measure the resistance across its heater terminals. A working element reads a low resistance of a few ohms; an open circuit confirms the sensor.
2A corroded connector under the car, or one that has filled with waterCommon
How to check: This connector is the most exposed on the car. Open it and look before assuming the sensor has failed.
3A blown fuse or failed relay on the heater supply, which is often shared with the front sensorPossible
How to check: If a front heater code is stored at the same time, suspect the shared supply rather than two sensors failing simultaneously.
4Chafed or broken heater wiring along the underbody, from stones, corrosion or a previous exhaust repairPossible
5A failed driver circuit in the control unitLess common

What to do about it

1Check the shared supply first if a front sensor heater code is also stored. Two heaters failing at once is far less likely than one fuse.
2Inspect the connector before condemning the sensor. Water ingress under the car is a leading cause and it is a much smaller repair.
3Plan for a seized sensor. This one sits under the floor behind the converter and has spent years being salted, so it often needs warming and penetrating oil rather than brute force. Damaging the threaded boss turns a sensor job into an exhaust section.
4Fix this before an MOT rather than after, because of what it does to the readiness monitors.
5Use a sensor of the correct specification, since heater resistance outside the expected range will set the same code on a physically sound part.

Can you keep driving?

Yes

There is no mechanical or safety consequence and no perceptible change in how the car behaves. On most vehicles this sensor does not influence fuelling at all, so economy and performance are unaffected. The entire practical impact is on what the car can verify about itself, which becomes an MOT problem rather than a driving one.

Will it fail an MOT?

This is the code where the readiness monitor rule bites hardest, and it is worth spelling out why it differs from the front sensor's heater code. The catalyst efficiency monitor cannot run until the rear sensor is at operating temperature and switching properly, so with its heater dead that monitor may never complete at all — not merely take longer. A tester can see the monitor status, and on many cars the emissions check cannot be finished while monitors are incomplete, so the car can fail on an incomplete emissions test as well as on the lamp, which is itself a major defect on petrol cars first used on or after 1 July 2003. It also means the usual advice about driving a hundred miles to reset the monitors will not work here: no amount of driving will complete a monitor that depends on a component still faulty.

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 is the difference between P0135 and P0141?: They describe the same electrical fault in different sensors. P0135 is the heater in the sensor ahead of the catalytic converter, which affects how quickly the engine can start controlling its mixture after a cold start. P0141 is the heater in the sensor behind the converter, which affects how quickly the car can assess whether the converter is working. The practical consequences differ: the front one costs a little fuel on short journeys, while the rear one can stop the catalyst readiness monitor completing altogether, which has direct implications at MOT time.
Will P0141 fail an MOT?: Yes, and it carries an additional complication. The illuminated engine management lamp is a major defect on petrol cars first used on or after 1 July 2003, which fails the car by itself. Beyond that, the catalyst efficiency monitor cannot complete while this sensor's heater is faulty, so if codes have been cleared the monitor will show as permanently not ready, and on many cars the emissions test cannot be completed until the monitors have run. Driving further will not resolve it, because the monitor depends on the very component that has failed.
Does a rear oxygen sensor heater affect fuel economy?: Barely, if at all. On most vehicles the sensor behind the catalytic converter plays no part in deciding how much fuel the engine receives — it is there to assess the converter. A failed heater in it therefore has essentially no effect on consumption or performance, which is why the fault produces no symptoms and is usually discovered only when the codes are read.
Why is the rear oxygen sensor so hard to remove?: Because of where it lives. It threads into the exhaust behind the catalytic converter, underneath the car, where it spends years exposed to road salt, spray and heat cycling. The threads corrode into the boss and the sensor body itself can crumble when a spanner is put on it. Garages generally warm the pipe, use penetrating oil and a proper slotted sensor socket rather than forcing it, because stripping the threaded boss turns an inexpensive sensor replacement into replacing a section of exhaust.

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