U02A1: Lost Communication With NOx Sensor C

What it means

The vehicle bus reports loss of communication with NOx sensor C (smart sensor with own ECU). The SCR catalyst control loop loses its actual NOx feedback and may switch to substitute values.

At a glance

• Usually lights the warning light of the system that lost communication
• Affects emissions; the car can fail an emissions test
• The engine or gearbox may go into limp mode

How it works

NOx sensor C is a smart exhaust sensor with its own control electronics that reports nitrogen-oxide readings to the emissions system. U02A1 sets when its network messages are lost, so the SCR system lacks sensor C's actual NOx feedback and may use substitute values.

Common causes

Most likely first.

1. Bus wiring open or shorted
2. Module without power or ground
3. Module failed

Symptoms

• Multiple warning lights may illuminate
• Affected modules may go into limp mode or shut down
• Specific functions inoperative depending on module
• Diagnostic tools may show communication faults

How to diagnose it

1. Scan all modules and note freeze-frame data and SCR or sensor communication codes to establish whether NOx sensor C alone is offline.
2. Check whether sensor C appears in scan-tool data and inspect its accessible connector and harness for exhaust heat damage, corrosion, or looseness.
3. Check the sensor's power and ground circuits using vehicle wiring information and the manufacturer's specification.
4. Test the network wiring to sensor C for opens, shorts, and poor connections, paying attention to harness sections near hot exhaust parts.
5. If power, ground, and bus wiring pass, evaluate NOx sensor C and its integrated electronics using the vehicle's diagnostic procedure.

Common fixes

• Repair damaged or heat-affected wiring or a poor connector at NOx sensor C.
• Restore a failed power or ground connection to sensor C.
• Repair an open or short in the communication wiring serving sensor C.
• Replace NOx sensor C if its circuits are sound and the sensor still fails to communicate.

Can I keep driving?

You can generally move the vehicle if it runs normally, but avoid prolonged driving because loss of NOx sensor C feedback can impair SCR emissions control and may trigger reduced-power operation. Repair the fault promptly, especially if an emissions or warning countdown appears.