What it means
NOx sensor A (smart sensor with own ECU) received implausible data from the ECM/PCM. Engine load, fuel rate or temperature data needed for the sensor's internal compensation fails plausibility checks; the sensor reports degraded readings.
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 A has its own electronics and relies on engine data from the ECM/PCM, such as load, fuel rate, and temperature, to compensate its readings. U05A1 is set when sensor A judges the engine data it receives from the ECM/PCM implausible and reports degraded NOx readings.
Common causes
Most likely first.
- ECM/PCM internal fault or sensor input drift
- Software version mismatch after replacement
- Gateway module routing CAN messages incorrectly
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
- Scan all modules and save freeze-frame data to identify related ECM/PCM, sensor A, exhaust-temperature, SCR, and communication faults.
- Review live data received by NOx sensor A and compare engine load, fuel-rate, and temperature values with the ECM/PCM's corresponding data for gaps or contradictions.
- Inspect the ECM/PCM and sensor A connectors and harnesses for damage, corrosion, moisture, or recent disturbance.
- Check the communication path and module power and grounds using the manufacturer's procedure, focusing on data sent from the ECM/PCM to sensor A.
- If the ECM/PCM, sensor A, or gateway was recently replaced or updated, verify software compatibility and configuration.
- If the ECM/PCM's reported engine values are themselves implausible, diagnose their source before replacing sensor A; otherwise follow the manufacturer's procedure to isolate the communication or receiving-sensor fault.
Common fixes
- Repair damaged wiring or secure the ECM/PCM and sensor A connectors.
- Resolve the underlying engine sensor or ECM/PCM data fault if the transmitted values are incorrect.
- Correct module software or configuration after a replacement or update.
- Replace NOx sensor A only if the ECM/PCM data and communication path are valid but sensor A still rejects them.
Can I keep driving?
Drive only as needed if the vehicle runs normally, and arrange prompt diagnosis because sensor A may report degraded emissions data. Avoid heavy loads if power is limited, and stop if a warning indicates the vehicle should not continue.
A code shows where the control module detected a fault, not always which part failed. Check the causes in order and confirm with a test before replacing parts. Have several codes? Check them together
This is a generic code: it means the same on every make. The car’s maker may add more detail in its own service information.