Mercedes NOx sensor replacement in Accrington

What replacement and verification looks like

When the evidence points at the sensor, we fit the correct part for the position and model and complete the reset the vehicle specifies. We take care with the exhaust thread, then run the car so the control unit sees plausible readings, and confirm the message and any countdown have cleared properly rather than simply been erased.

When the evidence points somewhere else, we say so before a part is ordered. Replacing a sensor that was reporting the exhaust accurately leaves the real fault in place, and the same code returns once the vehicle has run the test again.

Which Mercedes models and engines use NOx sensors?

NOx sensors are fitted to Mercedes diesels that use SCR and AdBlue: most Euro 6 cars and vans, plus the earlier BlueTEC models with an AdBlue tank. The OM651 2.1 litre four cylinder, the OM654 2.0 litre that replaced it and the OM642 3.0 litre V6 run right across the range, so a C-Class, E-Class, GLC, A-Class or B-Class can share a sensor arrangement with a van. Euro 5 diesels without SCR usually have no NOx sensor at all, and some Euro 6 models control NOx with a storage catalyst rather than an AdBlue tank.

On the commercial side, Euro 6 versions of the Sprinter 906, the current Sprinter 907 and the Vito 447 work on the same principle. A Sprinter NOx sensor fault behaves much like the same code on a C-Class, although access, part numbers and countdown behaviour differ. Common Mercedes Sprinter AdBlue problems covers the van side in more detail.

Where are NOx sensor 1 and NOx sensor 2 bank 1 on a Mercedes?

Sensor 1, called bank 1 sensor 1 or the upstream sensor, sits ahead of the SCR catalyst; sensor 2, bank 1 sensor 2, sits after it. Upstream means before the catalyst, reporting what the engine produced. Downstream means after it, reporting what is left once the AdBlue has reacted, which is why the downstream sensor is usually the one behind an SCR efficiency fault.

Each sensor is a probe screwed into the exhaust with its own small control module on the loom, usually clipped to the underbody or a bracket near the SCR unit. Probe and module are one part with a fixed harness length, so the sensor is renewed complete rather than repaired. Exact positions vary: on many layouts the upstream sensor sits after the DPF, some later models carry a third NOx sensor, and on Sprinter and Vito the downstream sensor often sits in or just after the SCR unit under the vehicle, where access and seized threads affect the labour.

What do P229F and P13DF mean on a Mercedes?

P229F is a NOx sensor circuit range or performance fault on bank 1 sensor 2, the downstream sensor, and it is one of the codes Mercedes owners see most often. A P-code is the standardised fault code the vehicle stores, and it names what was observed rather than the part that failed. P229F appears with a dead sensor, but equally with an AdBlue injector that is not dosing, low system pressure, poor-quality fluid, an exhaust leak near the probe or a tired SCR catalyst.

P13DF is a Mercedes-specific code owners typically meet when an SCR or AdBlue warning will not reset. In practice it points at an emissions fault the control unit still treats as unresolved, or at a repair where the specified reset was not completed, so clearing codes changes nothing until the cause is fixed and the procedure is carried out properly. What the P229F code means on a Mercedes works through the diagnosis in detail.

Mercedes NOx sensor recall and software update: what is covered

Mercedes-Benz has run diesel emissions software updates and NOx sensor service measures on some models, but coverage is decided model by model and vehicle by vehicle. Whether your car qualifies has to be checked against its VIN by a franchised Mercedes dealer. We cannot confirm eligibility, extend coverage or carry out a manufacturer measure, so ask the dealer first.

That call is worth making before you pay for a repair, particularly on a younger car or one that has already had an emissions software update applied. If the dealer says the work is not covered, the sensor and the wider SCR system can be tested and repaired here. If an update was applied and the warning appeared afterwards, tell us, because it changes how the stored data reads.

Mercedes NOx sensor replacement cost: what changes the price

The part is the largest variable: a genuine Mercedes NOx sensor commonly runs to several hundred pounds before fitting, and the upstream and downstream sensors are priced differently. Only one of them may be at fault, so the position that has failed changes the figure.

Labour depends on access, on whether the probe has seized into the exhaust, and on the reset the vehicle needs afterwards. A quotation should separate testing, the part, fitting and the reset so you can see what you are paying for. We do not publish a single figure, because an A-Class and a Sprinter are different jobs. NOx sensor replacement cost in the UK explains the range and what to ask for in a quote.

• Which sensor has actually failed, upstream or downstream, and whether both are genuinely faulty
• Genuine Mercedes, original equipment supplier or aftermarket part, and the specification the vehicle expects
• Access on the vehicle and whether the probe has seized into the exhaust
• The reset or relearn procedure, and clearing a countdown where one has already started
• Any wiring, connector, dosing injector or exhaust leak repair found during testing

Resetting the AdBlue countdown after a Mercedes NOx sensor is fitted

Fitting the sensor is often only half the repair, because the warning and any countdown usually clear only after the specified reset and a drive cycle. A countdown is the remaining mileage or number of restarts a Mercedes displays once an emissions fault has been stored, and it does not disappear simply because a new part is in place.

The vehicle normally has to see plausible readings from the new sensor before it accepts the repair, and the reset needs the manufacturer's procedure and the right diagnostic access. That is the usual reason a home-fitted or fast-fit sensor leaves the message on the dashboard. NOx sensor testing and replacement covers the same ground on other makes, and AdBlue countdown and no-start warnings explains what the numbers mean.

Why do cheap Mercedes NOx sensors come back with the same code?

Cheap sensors come back because a NOx sensor is a networked module, not a simple probe, and the part number and calibration have to match what the vehicle expects. A sensor with the wrong number or an older specification can fit physically, then report values the control unit will not accept and store the same code within a few hundred miles.

We also see sensors that were the wrong answer from the start. If the downstream reading was out of range because the AdBlue injector was not dosing, a new sensor changes nothing and the money is gone. Testing first is what stops the same repair being paid for twice, and it is why sensor work sits inside our AdBlue and SCR system repair service. We repair and replace: we do not delete or bypass emissions systems, we do not fit emulators, and we do not arrange for anyone else to do it.