● SENSOR DIAGNOSTICS
● DEALER-LEVEL KIT
• LIFETIME WARRANTY
Most vehicles carry two NOx sensors one before the SCR catalyst, one after and a fault with either can trigger the same warning light. Rather than replacing both on a guess, we test each one individually against live data, and check whether it’s genuinely the sensor or the cheaper-to-fix wiring and heater circuit around it, before recommending a NOx sensor replacement.
A classic sign of a NOx sensor fault the ECU's flagging an inconsistent reading between the two sensors, often before you'd notice anything different about how the car drives.
Once the ECU can't trust the NOx readings it's getting, some manufacturers' strategies pull power back as a precaution, particularly if the fault's been present a while.
A sensor feeding bad data can cause the engine to run a less efficient fuelling strategy than it needs to, which often shows up as a quiet drop in MPG before anything else does.
Since NOx sensors and the SCR/AdBlue system work together, a failing sensor can also throw off dosing accuracy sometimes read as an AdBlue fault when the sensor's actually the root cause.
Most Euro 6 diesels use two NOx sensors either side of the SCR catalyst one checking exhaust gas going in, one checking what comes out the other side. The ECU compares the two readings to work out whether the catalyst’s actually doing its job. When either sensor drifts, ages, or develops a wiring or heater fault, the comparison stops making sense, and the ECU logs a fault but the fault code alone won’t tell you which of the two sensors is actually at fault, or whether it’s the sensor at all rather than the circuit feeding it.
This is the step a lot of quick fixes skip. NOx sensors are complex, expensive components, and replacing one on the strength of a generic fault code is a common way to spend money without fixing anything because the actual problem was the other sensor, or a wiring fault, or a failing heater circuit rather than the sensor element itself. We test both sensors individually against live data before recommending a NOx sensor replacement, so you’re only paying for the part that’s actually failed.
You've had a check engine or AdBlue warning linked to NOx efficiency, a sensor's already been quoted without testing, or you want to know which of the two sensors is actually at fault before booking a replacement.
We test upstream and downstream NOx sensor readings against each other and against expected values, check the wiring and heater circuit for each, and confirm the specific fault before recommending replacement. Once replaced, the ECU's updated to recognise the new sensor, the fault's cleared, and live readings are verified before we leave. ✓ Both sensors tested individually, not replaced on a guess ✓ Wiring and heater faults ruled in or out before recommending a new sensor ✓ One visit, with a lifetime warranty on the work carried out
Tell us your registration, what's showing on the dashboard, and where you're based fixed price back within minutes.
Home, work, or roadside across London, the South East, Midlands or North Yorkshire we bring the diagnostic kit to wherever the vehicle actually is.
Both sensors tested, the actual fault confirmed and repaired or replaced, and a lifetime warranty on the work before we leave.
Our technicians carry make-specific NOx sensor diagnostic tools for diesel cars, vans, light commercial vehicles and fleet vehicles across the following manufacturers:
We're based in South East England, covering Greater London and the surrounding areas as our primary region, with consultations also available across the Midlands and North Yorkshire.
AdBlue Repair · DPF Cleaning · NOx Sensor Replacement · EGR Valve Cleaning · Diesel Engine Diagnostics · Emissions System Repair
Remove soot and ash build-up from your diesel particulate filter to restore performance and improve exhaust flow.
Diagnose and repair AdBlue system faults, replacing faulty components where needed to restore normal system operation.
Diagnose and resolve NOx sensor faults that can trigger warning lights and AdBlue-related system errors.
Optimise your ECU calibration to improve fuel efficiency while maintaining smooth everyday performance.
Increase engine power, torque and throttle response with a professionally calibrated ECU performance remap.
Complete a forced DPF regeneration to clear soot build-up and restore normal filter operation without replacement.
Most SCR-equipped diesels use one sensor before the catalyst and one after, comparing the two readings to check the system’s working properly. A fault usually points to one specific sensor rather than both we test each individually so you’re not paying to replace a sensor that’s actually fine.
Not always a NOx sensor fault code can also be caused by a wiring issue or a failing heater circuit around the sensor, which is often cheaper to fix than a full sensor replacement. We check this before recommending which route applies to you.
In most cases, replacement is the more reliable fix NOx sensors are complex components, and a genuinely failed sensor element isn’t something that holds up well to repair. Where the fault turns out to be wiring or a connector rather than the sensor itself, that can often be repaired instead.
It can a NOx sensor fault is part of the emissions monitoring system, and depending on how it’s presenting, it may show up as a relevant fault during testing. Getting it diagnosed and resolved beforehand avoids that becoming an issue.
Often yes in the short term, though some vehicles will reduce power or enter limp mode if the fault’s been present for a while. It’s best not to leave it, since a sensor fault can also affect AdBlue dosing accuracy over time.
Yes, it can the systems work closely together, and a NOx sensor feeding bad data can interfere with regeneration cycles completing properly. Worth mentioning if you’re also seeing DPF-related warnings alongside the NOx fault.
Typically 60-90 minutes for cars and vans, depending on sensor access and whether one or both need attention. Larger vans and HGVs can take longer.
We’re based in South East England, covering Greater London and the surrounding areas as standard, with mobile NOx sensor replacement also available across the Midlands and North Yorkshire.
Don't risk immobilisation or another dealer bill. Book a same-day mobile visit and drive away with a permanent fix.
