U059E
NetworkInvalid Data Received From NOX Sensor "A"
The ECU is constantly checking the data stream coming from the NOx sensor against what it expects to see, and with U059E it's decided the numbers coming back are nonsense. Either they're wildly out of range, jumping around, or just not adding up against the rest of the SCR system. For you the owner, this means the car can no longer trust one of the sensors that polices its AdBlue dosing, so it throws a warning and often starts dialling back power to protect its emissions compliance. It's almost always a diesel with AdBlue, and it's almost always either the sensor itself dying or water in the connector.
ⓘ Information only. This page provides general educational information about fault code U059E. 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 U059E mean?
U059E is a Network (CAN bus, module communications) fault code. It indicates: Invalid Data Received From NOX Sensor "A".
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 management light on, sometimes with an AdBlue or 'emissions fault' message in the driver display
- • Power noticeably pulled back, especially when you ask for a hard pull on a slip road or motorway
- • Limp mode kicking in after a few drive cycles if the fault stays present
- • AdBlue running down quicker than normal as the system overdoses to compensate
- • Fuel economy creeping up on the trip computer
- • A faint ammonia or sharp chemical smell from the tailpipe on some cars
Possible causes
Causes commonly associated with U059E, listed in approximate order of typical investigation. The actual cause on a specific vehicle can only be confirmed by professional diagnosis.
- 1. NOx sensor itself worn out or sooted up. These sit in a brutal spot in the hot exhaust and rarely last the life of the car, with many failing somewhere past 60,000 to 90,000 miles
- 2. Water sitting in the sensor connector plug. Common on cars where the plug is mounted low or exposed, and it corrupts the signal long before it kills the sensor outright
- 3. Corroded or chafed wiring between the sensor and ECU, often where the loom passes near heat or moving suspension components
- 4. Cracked sensor ceramic from thermal shock, usually after a cold start with the tip already damaged, or from a stone strike
- 5. CAN bus communication fault interrupting the data the sensor sends back
- 6. Diesel control unit fault, uncommon but it does happen and gets misdiagnosed as a sensor
- 7. Oil or fuel contamination reaching the exhaust and coating the sensor element
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. Pull all codes and read the freeze frame data so you can see what the engine was doing the moment the fault logged. A NOx fault that only appears under load tells you something different to one stuck on at idle
- 2. Unplug the NOx sensor connector and look inside. Green crust, damp, or water sitting in the plug is your answer more often than the sensor itself, so dry it out, treat it, and retest before spending money
- 3. Follow the loom from the sensor and check for rubbed-through insulation, melted sections near the exhaust, and any joints that have gone brittle
- 4. Watch the live NOx reading at idle, part throttle and under load. Implausible flatlines or readings that don't move with engine demand point at the sensor
- 5. Back-probe the wiring for continuity, shorts to earth, and a clean power and ground supply at the sensor
- 6. If the wiring is sound and the readings are still rubbish, replace the sensor with a quality part and clear the adaptations
Common questions about U059E
What actually happens if I just keep driving with it on? +
The car carries on but it stops trusting its NOx readings, so it tends to overdose AdBlue and runs the engine more conservatively. Expect creeping fuel use, faster AdBlue consumption, and a fair chance of limp mode within a handful of drive cycles. You won't damage the engine in the short term, but you're driving a car that's flagging an emissions fault, and on many AdBlue systems repeated faults lead to a no-restart countdown that locks you out until it's fixed.
How quickly do I need to sort this? +
Not an emergency, but don't sit on it for weeks either. If it's water in the connector you want to catch that before the corrosion spreads up the loom and turns a £20 clean into a wiring repair. If the car has already dropped into limp mode or is showing a restart warning, treat it as urgent and get it scanned within a few days rather than risk being stranded.
Is it the sensor that's gone or just the wiring and plug? +
Both are common, and the only way to know is to open the connector and check the wiring before you condemn the sensor. Plenty of U059E codes turn out to be water in the plug or a chafed wire, and people waste £300 on a sensor that was never faulty. Rule out the cheap stuff first. If the loom is clean, the power and ground are good, and the live data is still implausible, then it's the sensor.
How long does the repair usually take? +
Cleaning and drying a connector is a quick job, half an hour at a garage. A wiring repair depends on where the damage is and can run an hour or two. Replacing the sensor is usually under an hour of labour once they reach it, though seized fittings on a high-mileage exhaust can drag that out, plus a few minutes to reset the adaptations and clear the codes.
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 →