Powertrain, generic (extended)

P2C7A SCR NOx Catalyst Efficiency Below Threshold Bank 1 Catalyst 2

Check engine lightOn
EmissionsRelevant
Drive cycleRequired
Limp modePossible

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.

Likely causes

ECM software that flags the second SCR falsely (GM 3.0 l diesel bulletins)
Second SCR catalyst aged, poisoned or with reduced ammonia storage
Drifting NOx sensor in front of or behind the second SCR
Poor DEF quality or under-dosing by the reductant injector
Exhaust leak between the sensors that distorts the NOx readings

Symptoms

  • MIL on after one trip
  • Exhaust fluid or emissions warning in the instrument cluster
  • Usually no noticeable change in driving behaviour

Repair estimate

DifficultyHardWorkshop job
Parts and labour€100–3,000
Time0.5–4 h

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

Parts involved

scr catalyst b1 cat2reductant dosing unit

Related codes

P20EEP20EFP2BA6P2BA9P20F4

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