Failed or degraded NOx sensor C, including internal electronics failure that stops bus communication.; Damaged, chafed, or corroded wiring in the sensor harness or connector pins.; Open or shorted controller area network data line between the sensor and the module.; Blown fuse or poor ground supplying power to the NOx sensor control module.; Water intrusion into the connector or sensor housing causing intermittent communication loss.; Software or calibration issue in the powertrain control module affecting bus polling of the sensor.; A separate module on the same network segment pulling the bus down and disrupting sensor messages.

DTC codes / U02xx · Lost communication with a control module / U02A1

U02A1Lost Communication With NOx Sensor C

Q: What does code U02A1 mean?

Lost Communication With NOx Sensor C. U02A1 is a generic communication fault indicating that the engine control module has lost the controller area network signal from the nitrogen oxide sensor designated as Sensor C. The module detects this when the sensor stops broadcasting its periodic status and measurement messages on the bus for a calibrated period, not when the sensor simply reports an implausible reading.

U02A1 is a generic communication fault indicating that the engine control module has lost the controller area network signal from the nitrogen oxide sensor designated as Sensor C. The module detects this when the sensor stops broadcasting its periodic status and measurement messages on the bus for a calibrated period, not when the sensor simply reports an implausible reading.

SystemNetwork

Code typeGeneric (all makes)

SeverityMedium

Fix difficultyIntermediate

AreaLost communication with a control module

Symptoms you may notice

Illumination of the check engine light, often with no other noticeable change in vehicle behavior.
A stored U02A1 code accompanied by other communication or sensor codes in the same module.
Reduced engine power or limp-home mode on vehicles that rely on NOx sensing for emissions compliance.
Poor fuel economy or altered regeneration strategy if the powertrain control module defaults to a conservative operating map.
Intermittent warning light operation that correlates with vibration, moisture, or temperature extremes.
No drivability complaint at all in many cases, with the fault discovered only during a scan tool check.

Common causes, most likely first

Failed or degraded NOx sensor C, including internal electronics failure that stops bus communication.
Damaged, chafed, or corroded wiring in the sensor harness or connector pins.
Open or shorted controller area network data line between the sensor and the module.
Blown fuse or poor ground supplying power to the NOx sensor control module.
Water intrusion into the connector or sensor housing causing intermittent communication loss.
Software or calibration issue in the powertrain control module affecting bus polling of the sensor.
A separate module on the same network segment pulling the bus down and disrupting sensor messages.

How to diagnose and fix it

Scan all modules for communication codes and record freeze-frame data to confirm the fault is current rather than history.
Inspect the NOx sensor C connector and harness for corrosion, chafing, and pushed-back pins before any parts are ordered.
Check the sensor supply voltage and ground at the connector with a multimeter and verify the key-on power feed.
Measure controller area network bus resistance and voltage at the sensor connector to rule out a shorted or open data line.
Substitute a known-good NOx sensor of the same part type and clear codes to see whether communication returns.
If wiring and sensor check out, perform a module software update or reflash per the manufacturer's service information.
Clear all codes after repair and complete a drive cycle that includes the conditions needed for the monitor to run.

Why NOx sensor C goes silent

A NOx sensor that communicates perfectly but reports an implausible value will not set U02A1; that fault belongs to a different code family, so replacing the sensor for a reading problem is a misdiagnosis.
The sensor's internal control module shares power and ground with other emissions components, so a blown fuse or corroded ground stud can take out communication without any damage to the sensor itself.
On many vehicles the NOx sensor sits on a dedicated sub-bus, and a single failed sensor can disrupt messages from other nodes on that segment, producing a cluster of unrelated communication codes.
Intermittent U02A1 faults that appear only in wet weather usually trace to a connector that is not sealed properly rather than to the sensor element.
Technicians who replace the sensor before checking bus voltage often find the new part sets the same code because the fault lies in the harness or module.

Can I keep driving with U02A1?

The vehicle usually remains drivable because the fault is a communication loss rather than a mechanical failure, but continued driving may prevent emissions monitors from completing and can trigger reduced power on some models. Avoid long trips and heavy loads until the circuit is diagnosed, and do not clear the code repeatedly in an attempt to make the light stay off.

Tools that help

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Guidance is general and written for all OBD-II vehicles. Exact tests and values vary by model, so confirm the fault before replacing parts.