Fix U029D: Mechanic’s solutions that WORKED

The U029D: NOX Sensor A Communication Error means that there’s an issue with the communication between the vehicle’s control modules and NOx Sensor A. This sensor plays a vital role in monitoring exhaust gases, particularly nitrogen oxides, which are crucial for meeting emission standards and ensuring optimal engine performance.

U029D quick overview

Meaning	U029D: NOX Sensor A Communication Error
Is it serious?	This code can be serious as it affects the vehicle’s emission control system and can lead to performance issues.
Possible causes	– Faulty NOx Sensor A – Damaged or disconnected wiring to the NOx Sensor A – Controller Area Network (CAN) bus communication problems
How to diagnose?	Inspect NOx Sensor A: Check the sensor for damage or corrosion. Check Wiring and Connectors: Examine wiring and connectors for damage or poor connections. Test CAN Bus Signals: Ensure proper signal transmission in the CAN bus system. Evaluate Engine Control Software: Check for any required software updates.

How to fix U029D – learn from mechanics

I looked into U029D service cases at iatn.com website where in the private forums the mechanics seek advice from other mechanics. I looked up a bunch of cases to help you decide how to fix your issue and see what would a real mechanic do to solve it.

Case #1: Ram

Vehicle Information: 2012 Ram 3500 Laramie 6.7L

Issue + Repair History:

• Check engine light on and transmission in limp mode.
• Codes: P0717 (Input speed sensor 1 circuit no signal), P0657 (Solenoid supply voltage circuit), P0710 (Transmission temperature sensor circuit).
• Replaced input speed sensor and checked wiring.

Computer Codes & Descriptions:

• U010c – Turbo module communication loss
• U010e – Reductant module lost communication
• U029d – NOx module A lost communication
• U029e – NOx module B lost communication

Suggestions to Fix Issue:

• Check for damaged wiring harness, especially under the front driveshaft.
• Investigate potential shorts or opens in the J1939 data link.
• Examine the wire harness for damage at the transmission and transfer case.
• Consider checking all fuses for any issues.
• Scope the CAN lines at the module setting the codes.

Final Fix:

• Harness damage was found at the driveshaft.
• Two green/white bus wires were twisted together but crossed, causing communication issues.
• After correctly connecting the wires, cleared codes, and no issues reappeared.

Case #2: Mercedes

Vehicle Information: 2014 Mercedes-Benz Sprinter 2500 3.0L

Issue + Repair History:

• After DPF filter and upstream NOx sensor replacement and repair of a burnt NOx harness, CAN bus codes appeared.
• Codes: U029E, U0100, U0046 for downstream NOx sensor.
• Harness to rear NOx sensor ohmed out with no issues found.

Related Repair History:

• Vehicle initially came with DPF filter clogged, overheating, and melted NOx harness upstream.
• Replaced DPF filter, DEF injector, upstream NOx sensor, and repaired NOx harness.

Computer Codes & Descriptions:

• U029E – CAN bus downstream NOx
• U0100 – CAN bus downstream NOx
• U0046 – CAN bus downstream NOx

Suggestions to Fix Issue:

• Check power, ground, and CAN bus voltage at the NOx sensor.
• Drive the vehicle at highway speeds for a long distance for a system reset.
• Replace the rear NOx sensor.
• Perform teach-in procedure after NOx sensor replacement.
• Inspect the entire wiring harness for shorts, not just ohm testing.
• Check fuses in the left kick panel, especially #23, which controls NOx sensors.

Final Fix:

• Found a 10 amp fuse (#23) to be the culprit, controlling NOx sensors up and downstream.
• The issue was traced back to diesel particle filter flex damaging the NOx wires and AdBlue injector line.
• Replaced the fuse and resolved the issue.

Case #3: Ford

Vehicle Information: 2017 Ford Focus 2.0L Diesel

Issue + Repair History:

• Check engine light on with code U029D, suggesting NOx Sensor A communication problem.
• Sensor and wiring visually intact upon initial inspection.

Related Repair History:

• Recently serviced for routine maintenance, no history of exhaust or sensor-related issues.

Computer Codes & Descriptions:

• U029D – Fault in communication with NOx Sensor A.

Suggestions to Fix Issue:

• Thorough electrical testing of NOx Sensor A circuit for continuity and shorts.
• Evaluate health of the NOx sensor using specialized diagnostic tools.

Final Fix:

• Discovered a wiring issue near the exhaust system, leading to intermittent sensor signal loss.
• Repaired wiring and secured it away from high-heat areas.
• Reset the code, and the vehicle performed well in follow-up tests.

Case #4: Volkswagen

Vehicle Information: 2018 Volkswagen Jetta 2.0L TDI

Issue + Repair History:

• U029D error code indicating a problem with NOx Sensor A’s communication.
• Visual inspection showed no apparent wiring damage.

Related Repair History:

• No previous repairs or issues related to the exhaust system or sensors.

Computer Codes & Descriptions:

• U029D – Communication error with NOx Sensor A.

Suggestions to Fix Issue:

• Check for software updates for the engine control unit that might address sensor communication.
• Evaluate the NOx Sensor A for possible failure.

Final Fix:

• Updated the engine control software which included an improvement in sensor communication protocols.
• After update, the error code did not reappear, confirming a successful fix.

Case #5: Audi

Vehicle Information: 2015 Audi A4 2.0L TDI

Issue + Repair History:

• Displayed U029D code, signaling a communication issue with NOx Sensor A.
• No visible defects found in the initial check of sensor and wiring.

Related Repair History:

• Vehicle had regular maintenance, but no history of exhaust system or sensor repairs.

Computer Codes & Descriptions:

• U029D – Communication error with NOx Sensor A.

Suggestions to Fix Issue:

• Inspect NOx Sensor A and its wiring for subtle damage or wear.
• Perform electrical tests for shorts or interruptions in the sensor circuit.

Final Fix:

• Identified and repaired a slightly damaged wire in the NOx Sensor A circuit.
• After repairing and securing the wiring, cleared the code, and the vehicle passed all diagnostic tests without issue.

Advice	Brief Description of Diagnostic Step
Inspect NOx Sensor and Wiring	Check the NOx Sensor A and its wiring for damage or corrosion.
Perform Electrical Tests	Conduct continuity and short circuit tests on the sensor circuit.
Check for Software Updates	Verify if there are any updates for the engine control unit.
Evaluate CAN Bus Signal Integrity	Test the Controller Area Network bus signals at the sensor.
Inspect for Wiring Damage	Look for damage or wear in the wiring, especially near high-heat areas.
Repair and Secure Wiring	Repair any identified wiring issues and secure wires to prevent future damage.