U029D00 Code: Lost Communication With NOX Sensor A – Diagnosis and Repair Guide

What Does the Error Code U029D00 Really Mean?

The diagnostic trouble code (DTC) U029D00 indicates a “Lost Communication With NOX Sensor A”. This is a generic OBD-II code, meaning it can occur in most modern vehicles equipped with this system. Essentially, it signals a failure in communication between the NOX Sensor A (NOXS-A) and other control modules within your vehicle’s network.

Modern vehicles utilize complex communication networks, and the most prevalent of these is the Controller Area Network (CAN) bus. Think of it as the central nervous system of your car, allowing various electronic control units (ECUs) to exchange data. When the U029D00 code appears, it means this vital communication pathway to the NOXS-A has been disrupted.

Prior to the widespread adoption of CAN bus around 2004, older systems like Serial Communications Interface (SCI), SAE J1850 (PCI bus), and Chrysler Collision Detection (CCD) were used. Regardless of the specific system, the core principle remains: modules need to communicate to function correctly. Without proper CAN bus operation, modules can’t share information, and even your diagnostic scan tool might struggle to retrieve data, depending on the affected circuit.

The NOX Sensor A is crucial for your vehicle’s emission control system. Typically, it’s screwed into the exhaust pipe, usually downstream of the Selective Catalytic Reduction (SCR) catalytic converter. This sensor monitors nitrogen oxides (NOx) levels in the exhaust. It receives data from various sensors, either directly wired or transmitted via the CAN bus from the Powertrain Control Module (PCM). This data exchange is essential for the sensor to verify the catalytic converter’s effectiveness in reducing NOx emissions. To pinpoint which module is designated as “A” in your specific vehicle, always consult your vehicle’s service manual.

Code Severity and Associated Symptoms

The severity of a U029D00 code is generally considered high. This is because the inability of the PCM to receive data from the NOX sensor directly impacts the vehicle’s ability to accurately monitor and control emissions. While manufacturers might have built-in fail-safe modes for complete communication network failures, the vehicle’s operation is definitely compromised when the NOXS-A is offline.

Here are common symptoms associated with a U029D00 DTC:

• Malfunction Indicator Light (MIL) illuminated: The check engine light will likely be on.
• NOXS-A malfunction: The sensor may be completely inactive, failing to power up or produce any voltage readings.

Potential Causes of a U029D00 Error

Several factors can lead to a U029D00 code. The most frequent causes include:

• Open circuit in the CAN bus + or – circuit: A break or disconnection in either of the CAN bus communication wires.
• Short circuit to power or ground in either CAN bus circuit: One of the CAN bus wires might be inadvertently connected to a power source or ground.
• Lack of power or ground to the NOXS-A module: The NOX sensor module itself isn’t receiving the necessary electrical supply to operate.
• Rarely – a faulty control module: In less common cases, the NOXS-A module itself might be defective, but this is less likely than wiring or power issues.

Step-by-Step Diagnostic and Repair Procedures for U029D00

Preliminary Checks:

Before diving into complex diagnostics, always start with the basics.

1. Check for Technical Service Bulletins (TSBs): For any electrical issue, your first step should be to check for TSBs related to your vehicle model and year. Manufacturers often issue TSBs for known problems, which can provide valuable shortcuts and manufacturer-recommended solutions, saving you time and effort.

2. Scan for Other Diagnostic Trouble Codes: Use your OBD-II scanner to check for any other DTCs, especially those related to bus communication or battery/ignition issues. Address these codes first. Incorrectly diagnosing a U029D00 code is common if fundamental power or communication issues are present but overlooked.

Accessing the NOX Sensor Module:

1. Attempt to Communicate with the NOXS-A: After clearing any other codes, specifically try to communicate with the NOXS-A module using your scan tool.
   • If communication is possible: The U029D00 code might be intermittent or stored in memory. Further investigation might be needed, but the immediate issue is not present.
   • If communication fails: The U029D00 code is active, and the problem is currently occurring.

Inspecting Power and Ground Circuits:

1. Fuse Inspection: Locate the fuses that supply power to the NOXS-A module. Your vehicle’s wiring diagram (available in the repair manual) will be essential here. Check each fuse for continuity. Replace any blown fuses.

2. Ground Inspection: Identify the ground points for the NOXS-A module. These are typically bolted to the vehicle’s chassis or engine block. Ensure these connections are clean and tight. Corrosion or loose grounds are common culprits in electrical problems. For thorough cleaning:

   • Disconnect the ground connections.
   • Use a wire brush and a baking soda/water solution to clean both the connector and the grounding point on the vehicle body.
   • Re-tighten the connections securely.

3. Re-test Communication: After addressing fuses and grounds, clear the U029D00 code and re-attempt to communicate with the NOXS-A module. If communication is restored, the problem was likely a fuse or a ground connection.

CAN Bus Circuit Inspection:

1. Visual Inspection of Connectors and Wiring: If communication is still lost, focus on the CAN bus circuit and the NOXS-A connector itself.

   • Disconnect the Negative Battery Terminal: Always disconnect the negative battery terminal before working on electrical connectors to prevent accidental shorts.
   • Locate the NOXS-A Connector: It’s usually found on the exhaust pipe, after the SCR catalytic converter.
   • Inspect Wiring and Connectors: Carefully examine the wiring harness and connectors leading to the NOXS-A. Look for:
     • Scraping or rubbing damage
     • Exposed or bare wires
     • Burn marks or melted plastic
   • Connector Terminal Inspection: Disconnect the NOXS-A connector and inspect the terminals (metal pins inside). Check for:
     • Bent or broken pins
     • Corrosion (greenish tint)
     • Burn marks

2. Clean Connectors: If corrosion is present, use electrical contact cleaner and a plastic bristle brush to gently clean the terminals. Allow them to dry completely and apply dielectric grease to the terminals before reconnecting to prevent future corrosion.

Voltage Checks:

You’ll need a Digital Volt-Ohmmeter (DVOM) for these steps.

1. Verify Power and Ground at the NOXS-A Connector:

   • Wiring Diagram: Obtain the wiring diagram for your specific vehicle to identify the power and ground wires at the NOXS-A connector.
   • Reconnect Battery (NOXS-A Disconnected): Reconnect the negative battery cable, but leave the NOXS-A connector disconnected.
   • Power Check: Set your DVOM to DC Voltage. Connect the red lead to each B+ (battery voltage) supply terminal at the NOXS-A connector. Connect the black lead to a known good ground (battery negative terminal if unsure). You should read battery voltage at each power supply terminal.
   • Ground Check: Move the red lead of your DVOM to the positive battery terminal (B+). Keep the black lead on each ground terminal at the NOXS-A connector. You should again read battery voltage, indicating a good ground circuit. If voltage is absent for either power or ground, trace and repair the respective circuit.

2. CAN Bus Voltage Checks:

   • Identify CAN C+ and CAN C- Circuits: Refer to your wiring diagram to locate the CAN C+ (also sometimes called HSCAN +) and CAN C- (HSCAN -) circuits at the NOXS-A connector.
   • Key On, Engine Off (KOEO): Turn the ignition key to the “ON” position, but do not start the engine.
   • CAN C+ Voltage: Connect the black lead of your DVOM to a good ground. Connect the red lead to the CAN C+ terminal. You should typically see approximately 2.5-2.7 volts, with slight fluctuations.
   • CAN C- Voltage: Move the red lead to the CAN C- terminal. You should see approximately 2.3-2.5 volts, also with slight fluctuations. Note: Some manufacturers might specify a lower voltage for CAN C-, around 0.5V. Always consult the specifications for your vehicle manufacturer.

Seeking Professional Help:

If all the above tests pass, and you still cannot communicate with the NOXS-A module or clear the U029D00 code, it’s highly likely that the NOXS-A module itself is faulty. Replacement will likely be necessary.

Important Note on NOX Sensor Replacement: Many NOX sensors require programming or calibration to the vehicle’s ECU after replacement. This often involves using a professional-grade scan tool and following specific procedures. Consult your vehicle’s repair manual and consider seeking assistance from a qualified automotive technician for NOX sensor replacement and programming.

This diagnostic guide is for informational purposes only. Always consult your vehicle-specific repair manual and follow safety precautions when performing automotive repairs. We are not responsible for any outcomes resulting from actions taken based on this information.