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U02A1 Lost Communication With NOx Sensor C

Check engine lightOn
EmissionsRelevant
Drive cycleRequired
Limp modePossible

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.

Likely causes

Engine harness chafed on a hot component such as the EGR cooler
Blown fuse feeding the NOx sensor modules
Water or corrosion at the sensor C connector
NOx sensor C control unit internally failed

Symptoms

  • MIL on after two drives
  • Further NOx or PM sensor codes stored at the same time
  • SCR monitor for the second stage incomplete

Repair estimate

DifficultyMediumWorkshop job
Parts and labour€20–900
Time0.5–3 h

Typical range across vehicles; the real cost depends on what the testing finds.

Parts involved

nox sensor cvehicle bus wiring

Related codes

U029DU029EU05A8U05A9U0352

References: gsi.ext.gm.com, static.nhtsa.gov, static.nhtsa.gov.