U02A1 code meaning
Lost Communication With NOx Sensor C
Network system Generic code U02xx codes Check engine light
What U02A1 means
A third NOx sensor, C, is fitted where the aftertreatment needs one more measuring point, typically behind a second SCR stage. U02A1 is set when its CAN messages no longer reach the engine controller. GM's 2025 tables (25OBDG04B) wait longer than 10 s without any of its five messages, with 11 to 18 V and the bus up, and rate it Type B, so the MIL comes on after the second failed trip; the sibling codes for sensors A and B are Type A there. GM also lists U02A1 in bulletin 21-NA-149 among codes caused by chafed engine harnesses on 3.0 L LM2 diesel trucks and SUVs. Ford's SSM 51750 links it on 2023 F-Super Duty 6.7 L to a harness 14406 rubbing on the EGR cooler near C146; first check 20 A fuse F18. The second SCR stage then goes unchecked.
U02A1 at a glance
- Check engine lightYes, usually on
- Emissions testCan fail
- Limp modePossible
- Repair difficultyMedium
- Typical labor0.5 to 3 hours
- Clears after repairAfter a drive cycle
Symptoms of U02A1
- MIL on after two drives
- Further NOx or PM sensor codes stored at the same time
- SCR monitor for the second stage incomplete
Common causes of U02A1
- Most likelyEngine harness chafed on a hot component such as the EGR cooler
- PossibleBlown fuse feeding the NOx sensor modules
- PossibleWater or corrosion at the sensor C connector
- Less likelyNOx sensor C control unit internally failed
Check the most likely causes first. Replacing parts by guesswork is the most expensive way to clear a code.
Parts involved
Nox sensor cVehicle bus wiring
What each part of U02A1 means
| U | Network system: communication between the control modules (CAN bus). |
| 0 | Generic code: it means the same on every make (SAE standard). |
| A1 | The exact fault in that area. |
What to do next
- Note U02A1 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 U02A1
What does U02A1 mean?
U02A1 means "Lost Communication With NOx Sensor C". A third NOx sensor, C, is fitted where the aftertreatment needs one more measuring point, typically behind a second SCR stage. It is a generic code, so it means the same on every make.
Is it safe to drive with U02A1?
Drive gently and only to a workshop. With U02A1 the car can go into limp mode (reduced power) to protect itself.
Will U02A1 fail an emissions test?
Yes, it can. U02A1 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 U02A1 to fix?
Difficulty: medium. A typical repair takes 0.5 to 3 hours of work. This is a job for a workshop.
Will U02A1 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.