U05A8 code meaning
Invalid Data Received From NOx Sensor C
Network system Generic code U05xx codes Check engine light
What U05A8 means
A third NOx sensor appears on exhaust systems with two SCR stages, where sensor C watches the gas after the second catalyst. GM's 2025 ECM tables (25OBDG04B) are the public example: the engine controller checks alive rolling count and checksum of the post-second-catalyst NOx signals, eight failures in 18 samples or three in ten depending on the signal, with battery voltage of at least 11 V for 5 s, and logs U05A8 as Type B. The companion 25OBDG04A set has no such entry. U02A1 covers loss of sensor C messages, U05A9 the case where sensor C refuses engine data, U059E and U059F the first two sensors. The second SCR stage cannot be judged while the code is present.
U05A8 at a glance
- Check engine lightYes, usually on
- Emissions testCan fail
- Limp modePossible
- Repair difficultyMedium
- Typical labor0.5 to 2 hours
- Clears after repairAfter a drive cycle
Symptoms of U05A8
- Check engine light after two trips
- SCR monitor for the second catalyst incomplete
Common causes of U05A8
- Most likelyControl unit of NOx sensor C faulty
- PossibleHarness or connector of the rearmost sensor damaged
- Less likelySensors swapped between positions during repair
Check the most likely causes first. Replacing parts by guesswork is the most expensive way to clear a code.
Parts involved
Nox sensor c
What each part of U05A8 means
| U | Network system: communication between the control modules (CAN bus). |
| 0 | Generic code: it means the same on every make (SAE standard). |
| A8 | The exact fault in that area. |
What to do next
- Note U05A8 and any other codes stored with it: several codes together often point to one cause.
- Check for loose connectors and damaged wiring around the parts the code names.
- Check open recalls for your car: some faults are fixed for free by the dealer. Enter your VIN.
- Clear the code only after the repair. If it comes back, the fault is still there.
Buying a used car? Scan it for stored codes before you pay. A recently cleared memory can hide a problem.
Questions about U05A8
What does U05A8 mean?
U05A8 means "Invalid Data Received From NOx Sensor C". A third NOx sensor appears on exhaust systems with two SCR stages, where sensor C watches the gas after the second catalyst. It is a generic code, so it means the same on every make.
Is it safe to drive with U05A8?
Drive gently and only to a workshop. With U05A8 the car can go into limp mode (reduced power) to protect itself.
Will U05A8 fail an emissions test?
Yes, it can. U05A8 is emissions-related, so most inspection programs fail a car while it is stored or while the check engine light is on.
How hard is U05A8 to fix?
Difficulty: medium. A typical repair takes 0.5 to 2 hours of work. This is a job for a workshop.
Will U05A8 clear itself after the repair?
After the repair the car has to complete a drive cycle (a mix of city and highway driving) before the test runs again. Until then the readiness monitor shows "not ready", which can fail an inspection.