P2C7ASCR NOx Catalyst Efficiency Below Threshold Bank 1 Catalyst 2
- Generic OBD-II code · Powertrain
- Turns on the check-engine light
- Matters for the ITP
- Can trigger limp mode
- Needs a drive cycle to confirm a fix
- How the car detects it
- P2C7A concerns the second SCR catalyst on bank 1, typically an underfloor brick behind a first SCR stage. GM uses it on its 3.0-litre Duramax diesel: the 2020 table 20OBDG04 compares the efficiency of this catalyst, taken from the NOx sensors ahead of and behind it, with a reference built from the inlet NOx and a calibrated offset. A test is evaluated once 400 mg of NOx plus ammonia have entered the brick, results pass an EWMA filter, and the code is Type A with the MIL on after one trip; 25OBDG04B keeps this logic. GM bulletins PIP5682 (2020 LM2), PIP5888A (2022 LM2) and 23-NA-193 (2023 LZ0) cure false sets with new ECM software. P20EE covers the main SCR efficiency. Expect higher NOx and possibly a DEF warning.
- What you may notice
- MIL on after one trip
- Exhaust fluid or emissions warning in the instrument cluster
- Usually no noticeable change in driving behaviour
- Likely causes
- PossibleECM software that flags the second SCR falsely (GM 3.0 l diesel bulletins)
- PossibleSecond SCR catalyst aged, poisoned or with reduced ammonia storage
- PossibleDrifting NOx sensor in front of or behind the second SCR
- PossiblePoor DEF quality or under-dosing by the reductant injector
- RareExhaust leak between the sensors that distorts the NOx readings
- Inspection (ITP)
- Matters for the ITP. While the check-engine light is on, the car fails the emissions check.
- Repair
- Hard · Usually a job for a workshop.Typical cost: €100 – €3,000
- Related codes
Explanation from obdex (CC0), exactly as the app shows it. Guidance only; confirm with a workshop.