U029D code meaning
Lost Communication With NOx Sensor A
Network system Generic code U02xx codes Check engine light
What U029D means
NOx sensor A is normally the engine-out or SCR inlet sensor; its control unit puts NOx and oxygen values on CAN. U029D means these frames have stopped. GM treats it as Type A: its 2016 diesel tables wait 10 s, the 2025 tables (25OBDG04A/B) about 1 s for most frames, at 11 to 18 V with the bus otherwise healthy; on the 2014 6.6 L LML the code also counted checksum errors. Ford's 2023 diesel summary allows just over 1 s. Ford found water in the sensor module plug of 6.7 L F-Super Duty trucks from 2017-2019 and replaced module and harness 14406 (TSB 18-2365). Volvo and Mack name fuses, supply, J1939 wiring and terminating resistors, and on 2023 trucks a start-up timing bug. Deutz uses U029D for the upstream SCR sensor. MIL, open SCR checks and derate are possible.
U029D at a glance
- Check engine lightYes, usually on
- Emissions testCan fail
- Limp modePossible
- Repair difficultyMedium
- Typical labor0.5 to 3 hours
- Clears after repairAfter a drive cycle
Symptoms of U029D
- MIL on
- SCR or NOx monitor not completed
- Engine derate on some trucks
Common causes of U029D
- Most likelyWater or corrosion in the NOx sensor module connector or harness
- PossibleBlown fuse, open supply or ground of the sensor module
- PossibleSensor CAN wiring damaged or terminating resistor faulty
- PossibleEngine control software with start-up timing fault
- Less likelyNOx sensor control unit internally failed
Check the most likely causes first. Replacing parts by guesswork is the most expensive way to clear a code.
Parts involved
Nox sensor aVehicle bus wiring
What each part of U029D means
| U | Network system: communication between the control modules (CAN bus). |
| 0 | Generic code: it means the same on every make (SAE standard). |
| 9D | The exact fault in that area. |
What to do next
- Note U029D and any other codes stored with it: several codes together often point to one cause.
- Check for loose connectors and damaged wiring around the parts the code names.
- Check open recalls for your car: some faults are fixed for free by the dealer. Enter your VIN.
- Clear the code only after the repair. If it comes back, the fault is still there.
Buying a used car? Scan it for stored codes before you pay. A recently cleared memory can hide a problem.
Questions about U029D
What does U029D mean?
U029D means "Lost Communication With NOx Sensor A". NOx sensor A is normally the engine-out or SCR inlet sensor; its control unit puts NOx and oxygen values on CAN. It is a generic code, so it means the same on every make.
Is it safe to drive with U029D?
Drive gently and only to a workshop. With U029D the car can go into limp mode (reduced power) to protect itself.
Will U029D fail an emissions test?
Yes, it can. U029D is emissions-related, so most inspection programs fail a car while it is stored or while the check engine light is on.
How hard is U029D to fix?
Difficulty: medium. A typical repair takes 0.5 to 3 hours of work. This is a job for a workshop.
Will U029D clear itself after the repair?
After the repair the car has to complete a drive cycle (a mix of city and highway driving) before the test runs again. Until then the readiness monitor shows "not ready", which can fail an inspection.