U029D and U029E Fault Codes: Lost Communication With a NOx Sensor

Quick answer: U029D is a generic diesel fault code meaning “lost communication with NOx sensor A”, and U029E is the same for NOx sensor B. A NOx sensor has its own small control module that sends readings to the engine control unit over the vehicle’s data network (the CAN bus). These codes mean those messages have stopped arriving. That’s a different fault from a sensor giving a bad reading: the cause can be the sensor’s module, but it can just as easily be its power feed, a fuse, an earth, a damaged wire or a wet, corroded connector. If both codes show together, look at what the two sensors share before blaming either one. Get every stored code read with live data before you buy a sensor. Our mobile diagnostic visit is £185, included in the price if the full fix goes ahead on the day. For eligible off-road, export or motorsport vehicles only (it is not lawful on vehicles used on public roads, and we check eligibility first), our software fix starts from £249 with a lifetime warranty.

Last updated: 9 November 2026

U029D usually shows up with an engine management light and, on many vehicles, an AdBlue or emissions warning that can turn into a countdown to a no-start. Scan tools word it in different ways, such as “lost communication with NOx sensor A”, “NOx sensor 1 no communication” or “NOx inlet sensor missing signal”, and some add a suffix such as U029D-00. This guide explains what the code is actually telling you, why it isn’t the same as a failed sensor, what you can check yourself and how to get it sorted without buying a part you didn’t need. If your dash shows a message rather than a code, start with AdBlue system fault messages explained.

What does U029D mean?

Codes beginning with U are network codes. They’re about modules talking to each other, not about a measurement being out of range. A modern NOx sensor isn’t just a probe in the exhaust: the probe is wired to a small control module, usually a box on the sensor’s own loom, which works out the NOx level and sends it to the engine control unit as a digital message over the CAN bus.

U029D means the engine control unit (or another module that listens for that sensor) has stopped receiving those messages from NOx sensor A for long enough to log a fault. Without the reading, the system can’t check how well the SCR catalyst and AdBlue dosing are cleaning the exhaust, so it records an emissions fault and, depending on the vehicle, starts the warning and countdown sequence.

What’s the difference between U029D and U029E?

U029D refers to NOx sensor A and U029E to NOx sensor B. On many SCR diesels sensor A is the upstream (inlet) sensor, before the SCR catalyst, and sensor B is the downstream (outlet) sensor after it, but manufacturers label them in their own way, so the letter should be checked against the vehicle’s own data rather than assumed. Our guide to P229F, P2200 and P2201 explains what the two sensors do.

How is U029D different from P2200 or P229F?

With a P2200-family or P229F code, the engine control unit is getting a reading from the sensor but doesn’t trust it: it’s out of range, slow or implausible. With U029D or U029E it isn’t getting a message at all. That matters because a “no message” fault has more possible causes outside the sensor. A sensor with no power, no earth or a broken data wire looks exactly the same to the engine control unit as a sensor that has died. Fitting a new sensor to a circuit that can’t power it or carry its signal leaves the code exactly where it was.

What can cause a U029D code?

These are the areas a proper diagnosis works through. They’re in the order we’d check them, not by how often they happen, because that varies from vehicle to vehicle:

• Battery and charging voltage. A weak battery, a recent flat battery or jump start, or a charging fault can make modules drop off the network, especially on cold starts. Communication codes logged during a voltage dip can stay stored afterwards.
• The sensor’s power feed. The module needs a fused supply, often through a relay it shares with other emissions parts. A blown fuse or failed feed silences the sensor completely.
• The earth. A poor earth connection has the same effect as a missing feed, and can come and go with heat and vibration.
• The CAN wires. The two data wires run along the underside of the vehicle near a hot exhaust. Heat, chafing or damage from earlier work can break a wire or short the two together.
• Connectors. Water getting into the sensor or module plug, corrosion on the pins, or a plug that wasn’t fully clipped back after other work.
• The sensor’s control module. The module itself can fail, in which case it stops talking even though everything feeding it is fine.
• A sensor that was recently changed. On some vehicles a new sensor has to be registered or adapted with a diagnostic tool, and the wrong part number may not be recognised.
• Software. Some manufacturers have issued software updates for communication faults, so it’s worth checking for recalls or updates.

What does it mean if U029D and U029E show together?

If the system has lost contact with both NOx sensors at once, start with what they have in common: on many vehicles that’s the same fuse or relay, the same earth point and the same stretch of data wiring. Two sensors failing at the same moment is possible but less likely than one shared fault taking both off the network. If other modules have logged communication or low-voltage codes at the same time, the battery, charging system or main network wiring move to the top of the list.

If U029D is on its own, the fault is more likely to be in that one sensor’s module, connector or its own branch of wiring. Either way, it’s worth reading the codes from every module, not just the engine, because the pattern of what’s missing tells you where to look.

What should I check before booking?

A few minutes of checks can save you a visit, and they make any diagnosis quicker:

1. Write down the exact dash message and any miles left on a countdown. A photo of the dash is ideal.
2. Note every code, not just U029D, including whether U029E or any battery-voltage codes are stored with it, and don’t let anyone clear them before they’ve been looked at.
3. Think about the battery. Has the vehicle had a flat battery, a jump start or slow cranking recently? If so, say so. A battery test is a cheap first step.
4. Think about recent work. Exhaust, DPF or sensor work, a new sensor, underbody damage, or driving through deep water shortly before the fault appeared are all worth mentioning.
5. Look at the wiring only when the exhaust is cold. If you can see the NOx sensor loom, check for a plug hanging loose, melted insulation or wires rubbing on the exhaust or a bracket. Don’t pull plugs apart or probe pins yourself.
6. Check the fuses listed for the emissions or NOx sensors in your handbook’s fuse chart. If one has blown, a fuse that blows again straight away points to a short that needs finding, not another fuse.
7. Check for recalls. Use the GOV.UK vehicle recall checker or ask a franchised dealer whether any emissions software update applies.

If you’re not sure whether the problem is the sensor or something around it, our guide to NOx sensor faults explained covers the symptoms, and what a NOx sensor fault costs to sort explains the cost side.

Will a new NOx sensor fix U029D?

Only if the sensor’s own module has actually failed. If the real problem is the power feed, a fuse, the earth, the data wiring or a wet connector, a new sensor will sit on the same circuit and stay just as silent, and you’ll have paid for a part that didn’t need changing. That’s why the circuit should be checked before anyone orders a sensor. Where a new sensor genuinely is needed, some vehicles also need it registering with a diagnostic tool afterwards, or the code can stay.

Will clearing the code fix U029D?

Not if the fault is still there. Clearing turns the light off until the engine control unit next listens for the sensor, and the code comes straight back if the messages still aren’t arriving. If the code was logged during a one-off voltage dip and the circuit is healthy, it may stay away, but you can only know that once it’s been tested. Clearing also wipes the freeze-frame data that shows when the fault happened, and on many vehicles it won’t stop a countdown that has already started. Our guide to resetting an AdBlue warning explains when a reset holds and when it can’t.

Can I keep driving with U029D?

Usually for a while, but plan to get it sorted. On many vehicles a NOx sensor that isn’t reporting is treated as an emissions fault, which can start the AdBlue countdown, and once that reaches zero the engine may not restart after you switch off. It won’t cut out mid-journey because of the countdown. Our guide to the AdBlue no-start countdown explains how it works. If you’ve seen burning, melted wiring or smoke near the exhaust, stop and get it checked before driving further.

Is U029D different on a Transit, Sprinter, VW or Range Rover?

The code means the same thing on each: a NOx sensor has stopped talking to the network. What changes is where the sensors and their modules sit, how the wiring is routed, which fuse feeds them, how the manufacturer labels sensor A and B, and whether a new sensor needs registering. That’s why we diagnose on the vehicle rather than going by the code alone. For model-specific advice, see our pages on the Ford Transit, Mercedes Sprinter, Volkswagen and Range Rover and Land Rover.

How do you fix a U029D code?

We start with a mobile diagnostic visit, £185. We come to your home, workplace or depot, read the codes stored across the vehicle’s control modules with their freeze-frame data, and check whether each NOx sensor is responding on the network. Then we work through the circuit: battery and supply voltage, the sensor’s feed and earth at the connector, the data wiring and the connectors themselves, along with the rest of the SCR system on live data. That tells us whether the sensor’s module, its wiring or something else is behind the code. You get written findings and control module resets, and the £185 is included in the price if the full fix goes ahead on the day.

What happens next depends on what we find:

• If it’s the battery, a fuse, wiring, a connector or a part that needs fitting, we tell you. We don’t fit, clean or replace NOx sensors or other parts, but the written findings tell your garage exactly where to look, which can save paying for a sensor that didn’t need changing.
• If the vehicle is eligible, we offer a software solution, such as a NOx sensor delete or a full AdBlue delete, from £249. We save your original ECU file first, the fix is permanent unless you ask us to reverse it, and it carries our lifetime warranty.

There’s no deposit: you pay after the job, and if we can’t fix it, you don’t pay for the fix. Our AdBlue delete cost page explains what moves a quote within our range.

Removing or disabling emissions equipment, including NOx sensors and the AdBlue/SCR system, is not permitted on vehicles used on public roads and can affect your MOT, insurance and compliance. Deletes are carried out for off-road, motorsport, competition and export vehicles, and we check vehicle and use eligibility before any software work. Read is an AdBlue delete legal in the UK? before deciding.

Do you come out to U029D faults in London?

Yes. We cover all of London, with no extra charge for London jobs, plus Surrey, Sussex, Kent and the surrounding areas, from our base in Copthorne near Crawley. We work 8am to 8pm, 7 days a week, at homes, workplaces and depots, and same-day visits are usually possible. Jamie has 15 years in the trade, and we carry public liability insurance. Send your registration and postcode and we’ll confirm when we can attend. The full list of towns is on our areas covered page.

Got a U029D or U029E code? Get a fixed quote with your registration, the codes and the exact dash message. Or call 0207 046 1823 (London and Kent), 01483 963253 (Surrey), 01293 366085 (West Sussex), 01273 950316 (Brighton and East Sussex), or WhatsApp 07403 415966.

U029D fault code: frequently asked questions

About the author. Written by Mobile AdBlue Fix, a mobile AdBlue, NOx and SCR specialist based near Gatwick and covering London, Surrey, Sussex and Kent. We diagnose with manufacturer-level equipment before recommending anything, check vehicle and use eligibility before any software work, save your original ECU file, and back our work with a lifetime warranty. How our warranty works.