P23C1 asks whether the measuring cells of NOx sensor 3 on bank 1 have lost sensitivity, for example because deposits clog the internal cavities. GM's 2025 table 25OBDG04B runs the test on the post-second-catalyst sensor during afterrun: the sensor shifts its internal pump currents so that oxygen in the second cavity rises to a target, and the outcome is set against a value the supplier recorded on the new part and wrote into the sensor EEPROM. A ratio above 125 % or below 75 % fails; results are smoothed with EWMA logic, the check runs once per trip at engine speeds under 1,500 rpm and exhaust temperatures below 400 °C, with Type A rating and a one-trip MIL. GM bulletin 21-NA-149 also lists P23C1 on 3.0L LM2 trucks. P22FB and P22FE are sensors 1 and 2.
Medium severity. Can I keep driving?You can usually keep driving in the short term, but get it repaired soon: on many diesels, the warning eventually limits starting or engine power.
Bank 1 is the side of the engine with cylinder 1. Inline engines only have this bank.
Symptoms
- Check engine light after one trip
- SCR monitoring results become unreliable
- No noticeable change in engine behaviour
Most likely causes
- Internal measuring cavities clogged by ash or other deposits
- Sensing cells worn out after high mileage
- Wiring fault upsetting the sensor supply during the test (GM 3.0L LM2)
What to check, in order
Start with the simplest. You can tick off each step.
Good to know
- Repair difficulty: medium
- It affects emissions.
What each character of P23C1 means
- P: Powertrain: engine and transmission
- 2: Generic code: same on every make
- 3: Ignition and misfires
- C1: Specific fault in that system
Frequently asked questions
Can I keep driving with code P23C1?
You can usually keep driving in the short term, but get it repaired soon: on many diesels, the warning eventually limits starting or engine power.
What causes code P23C1?
The most common causes are: internal measuring cavities clogged by ash or other deposits; sensing cells worn out after high mileage; wiring fault upsetting the sensor supply during the test (GM 3.0L LM2).
Related codes
- P23BCNOx Sensor Calibration Memory (Bank 1 Sensor 3)
- P23BDNOx Sensor Performance - Signal Biased/Stuck High (Bank 1 Sensor 3)
- P23BENOx Sensor Performance - Signal Biased/Stuck Low (Bank 1 Sensor 3)
- P22FBNOx Sensor Performance - Sensing Element (Bank 1 Sensor 1)
- P22FENOx Sensor Performance - Sensing Element (Bank 1 Sensor 2)
Sources
Values and procedures vary by manufacturer. If it affects brakes, steering, fuel or high voltage, see a professional. Updated: 2026-11-04. How we build these pages.