U059F
NetworkInvalid Data Received From NOX Sensor "B"
The ECU is getting readings from the second NOx sensor (sensor B, usually sitting after the SCR catalyst) that don't add up, so it flags the data as invalid. On most modern diesels this is the sensor that tells the system whether the AdBlue dosing is actually cleaning up the exhaust. When it goes faulty or drops out, the engine doesn't know if its emissions are in spec, and it tends to throw a light first then start counting down to limp mode if you ignore it. It's a common one on Euro 6 diesels, and the sensors themselves are not cheap.
ⓘ Information only. This page provides general educational information about fault code U059F. We do not assess the urgency or safety implications of any specific fault. That requires in-person diagnosis by a qualified mechanic. Full terms.
Recommended next steps
Whether a fault is urgent, drivable, or routine depends entirely on the cause on a specific vehicle, and that can only be determined by a qualified mechanic with diagnostic equipment. If a warning light is illuminated, the most reliable next step is professional diagnosis.
What does U059F mean?
U059F is a Network (CAN bus, module communications) fault code. It indicates: Invalid Data Received From NOX Sensor "B".
This is a standardised OBD-II code. The technical definition is the same regardless of the make or model of vehicle, although specific causes and symptoms can vary between vehicles.
Symptoms commonly associated with this code
Symptoms that drivers often report alongside this code. Not all may apply to every case:
- • Engine warning light on the dash, often the first and only sign for a while
- • An emissions or SCR/AdBlue warning message, sometimes with a 'engine will not restart in X miles' countdown
- • Reduced fuel economy and noticeably higher AdBlue usage
- • Limp mode if the fault sticks around, with power pulled right back
- • Occasional rough idle or a flat spot on acceleration
Possible causes
Causes commonly associated with U059F, listed in approximate order of typical investigation. The actual cause on a specific vehicle can only be confirmed by professional diagnosis.
- 1. The NOx sensor B itself has failed or is on its way out, by far the usual cause. These sensors live in a brutal environment and have a finite life, often packing up between 80,000 and 120,000 miles
- 2. Soot or carbon caked on the sensor probe, fooling it into sending nonsense values
- 3. Corroded, chafed or unplugged wiring and connectors at the sensor, water ingress into the plug is a classic
- 4. A CAN bus communication fault, since these sensors are smart and talk to the ECU over the network rather than a simple voltage signal
- 5. Contamination from oil burning, AdBlue crystallising in the exhaust, or thermal shock cracking the element
- 6. Out of date sensor or ECU software, where the module reports data the ECU doesn't accept
How mechanics typically diagnose
A typical diagnostic sequence used by mechanics, provided here for educational reference only. Diagnostic work should be performed by a qualified mechanic with the appropriate tools and training.
- 1. Read live NOx data for both sensors on a capable scan tool and compare them. If sensor B reads stuck, wildly out of range, or shows no value at all while sensor A looks sensible, you've found your problem area
- 2. Pull all stored codes, not just U059F. SCR efficiency codes and AdBlue dosing faults often sit alongside it and change the whole diagnosis
- 3. Inspect the sensor B connector and harness for green corrosion, moisture, melted insulation or a bent pin. Unplug it, check both ends, this catches more of these than people expect
- 4. Back-probe the sensor power supply and ground with a multimeter to confirm it's actually getting a clean feed before you condemn the sensor
- 5. Check the exhaust around the sensor for leaks, heavy soot or AdBlue crystal build-up that could be skewing the reading
- 6. Clear the codes and road test over a proper run including some motorway, then recheck whether U059F comes straight back
Common questions about U059F
Should I buy a budget NOx sensor or pay for the genuine part? +
This is one job where I'd lean towards the genuine sensor or a recognised brand like Bosch or Continental. NOx sensors are clever units with their own electronics, and the cheap pattern ones off auction sites are notorious for reading badly, dropping off the CAN bus, or just dying again within months. On a lot of Euro 6 diesels the ECU is fussy and won't accept a non-OE part properly. Spending £150 to save £100 only to do the job twice is a poor trade.
Is it safe to keep driving like this? +
You can usually carry on for now, but don't sit on it. The car will run dirtier and burn more AdBlue, and on most modern diesels the system gives you a warning and a mileage countdown before it refuses to restart. If it drops into limp mode you'll have very little power on the motorway, which isn't safe. Treat it as something to sort within a few weeks, not months.
Will this fail my MOT? +
The code on its own isn't an automatic fail, but if the engine warning light is on when the tester looks at the dash, that's an MOT failure under the current rules for an illuminated MIL. On a diesel with a knackered SCR system you can also fail the emissions side if the AdBlue treatment isn't working. Fix the fault, clear the light, and drive enough cycles for it to stay off before you book the test.
Roughly what does it cost to put right? +
If it's the sensor, the part alone is often £150 to £350 depending on the car, plus an hour or two of labour. At a good independent garage you're typically looking at £250 to £500 all in, and a main dealer can push that into four figures once you add their hourly rate and a genuine part. A simple wiring repair or software update is cheaper, sometimes just the diagnostic time of around £60 to £120.
Information only, not professional advice
The information on this page is provided for general guidance and educational purposes only. It is not a substitute for diagnosis or repair advice from a qualified mechanic. Always verify any fault before paying for repairs. carfaultcodes.co.uk accepts no liability for decisions made based on this information. Full terms →