OBD-II trouble code
U029D: Lost Communication With NOx Sensor "A"
Application matters: A is the manufacturer-assigned NOx sensor identifier. It does not universally specify a bank, upstream/downstream location or separate replaceable module. Use the complete scan and vehicle information to identify the participant.
A control module did not receive the expected communication from NOx sensor A. Identify the reporting module and actual sensor assignment before replacing a sensor or treating it as a concentration fault.
Quick facts
- System
- Network
- Category
- NOx Sensor Network Communication
- Severity
- Medium severity
- Driving guidance
- Depends on symptoms and vehicle guidance
- Repair cost range
- Get a vehicle-specific estimate after diagnosis.
- DIY difficulty
- Shop recommended
A code alone cannot establish whether driving is safe. Stop safely for a red brake or oil-pressure warning, overheating, severe misfire, stalling, or impaired braking or steering, and follow the owner's manual. Restraints faults can reduce crash protection even when the vehicle runs normally.
Part of a family with 2 codes
U029D is one of 2 codes covering lost communication with nox sensor. The identifier must be matched to the exact vehicle. Related codes can share a diagnostic approach, but their circuit, failure condition, and required tests can differ.
Read the full guide for this family →Browse every code in Network, or start from the full code library.
What does U029D mean?
U029D reports lost communication with NOx sensor A. The reported condition is missing communication, not a direct measurement of excessive nitrogen oxides or proof that the sensing element has failed. A complete scan is needed to identify who reported the loss and which expected participant is involved.
Ford’s 2023–2024 diesel summary gives a specific CAN-monitor example: the expected NOx sensor A master message is not received. The same section separately names other missing-message and invalid-data results. A missing message and received-but-invalid data therefore should not be collapsed into one sensor-performance diagnosis.
The A designation is an identifier to match to the actual vehicle. It cannot locate a sensor, establish a common power feed or tell you how many independently replaceable electronic units are installed. U029E names the other A/B communication subject; its presence or absence alone does not establish a shared or separate cause.
Qualified diagnosis should assess current communication, relevant power and connection information, companion faults and the correct network procedure. Do not disconnect participants, swap sensors or probe a bus using generic resistance values. A sensor, wiring or controller repair requires its own findings, and any required configuration or setup belongs to the confirmed repair.
Possible causes
- A supply or connection problem affecting communication with the identified A participant
- A network or interface fault established by the matched diagnostic checks
- A sensor-associated electronic unit or controller problem confirmed after the relevant communication assessment
Symptoms
- U029D recorded with an emissions or malfunction warning
- A scan that cannot obtain the expected information from the assigned participant
- Other communication or power-related codes, if the actual fault affects more than one function
Diagnostic steps
Save the full scan report, code status, and available freeze-frame data before clearing anything. Confirm the reporting module, any failure-type suffix, and the procedure for the exact vehicle. The steps below are an overview; use manufacturer specifications and safety procedures for testing.
- 1.Save the complete multi-module scan, including U029D, any available suffix, current/history status and event information.
- 2.Have a technician identify the reporting controller and the vehicle’s assigned NOx sensor A.
- 3.Assess current communication and relevant supply, connection and network conditions under the actual vehicle procedure. Do not use one missing message as a parts verdict.
- 4.Leave bus measurements, protected connector access and any required programming to qualified personnel. Avoid generic pinouts, resistance recipes or trial sensor swaps.
- 5.Repair the established cause and verify the required communication and original complaint. Follow actual vehicle warnings; a cleared code or one successful response alone does not establish completion.
Repair cost
Get a vehicle-specific estimate after diagnosis.
A trouble code alone does not price the repair. Ask the shop to identify the confirmed fault and itemize diagnosis, parts, labor, programming or calibration, and taxes.
Estimate your repair
Run the numbers for your vehicle
Open the Repair Cost Estimator with module communication diagnosis preselected. Adjust labor rate and vehicle category to fit your situation.
Related repairs
These links are planning examples; the code does not confirm that any listed part has failed.
DIY vs shop
A qualified shop is recommended because this can involve network diagnosis and, when required, module configuration or programming with vehicle-compatible tooling. Ask the technician what testing confirmed the repair.