Diagnostic article Generic OBD-II
P2C7A: underfloor SCR efficiency evaluated with filtered evidence
P2C7A evaluates underfloor SCR ammonia storage and NOx conversion using qualified sensor data and an EWMA filter.
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Documented application
Documented 2025 GM gm-2025-04b monitor only; vehicle, engine and controller applicability must be established separately.
The code alone does not identify a failed part or establish that a vehicle is safe to drive. Confirm the vehicle, reporting module and applicable service procedure.
P2C7A concerns the second, underfloor SCR catalyst in this GM strategy. Its diagnostic efficiency index considers both ammonia storage capability and NOx conversion. The source identifies NOx sensors 2 and 3 around SCR2 and notes that their measurements respond to both NOx and ammonia.
More than a single upstream/downstream ratio
The controller compares a measured efficiency index with a calibrated reference and applies an exponentially weighted moving average, or EWMA, to their difference. Testing occurs after a specified upstream NOx-plus-ammonia integral reaches 300 mg. The listed zero comparison is a filtered difference boundary, not a claim that zero-percent catalyst efficiency is acceptable.
The continuation imposes extensive qualifications: reliable sensor measurements and slip detection, suitable temperatures and exhaust flow, usable reductant delivery, appropriate ammonia storage and allowed dosing states. Rapid-response and initial-response filter modes also affect how often tests can execute.
Diagnostic implications
A raw downstream NOx reading alone cannot reproduce the monitor. Ammonia response, dosing state and the validity of the upstream reference affect interpretation. Similarly, an incomplete monitor after a repair may reflect unmet thermal, storage or flow conditions rather than a new catalyst failure.
Review related sensor and reductant faults before treating P2C7A as proof that catalyst replacement is required. Verification needs the applicable efficiency test to execute with reliable inputs and qualified catalyst conditions. The source supplies the monitor's reasoning, not a universal road-test recipe or a direct pass/fail limit for a handheld emissions reading.
Source and applicability
Documented 2025 GM gm-2025-04b monitor only; vehicle, engine and controller applicability must be established separately. GM monitor table, PDF page 875; GM monitor table, PDF page 876; GM monitor table, PDF page 877; GM monitor table, PDF page 878; GM monitor table, PDF page 879.
Article source documents
Source checks describe how the text was prepared; they do not certify a diagnosis or confirm compatibility with every vehicle using this code.
Original code reference
Dictionary definition
SCR NOx Catalyst Efficiency Below Threshold Bank 1 Catalyst 2
This definition retains its original catalog scope. The article above is limited to its explicitly documented application.
Reference scope
Generic OBD-II definition.
A manufacturer label does not establish the model, year, engine or module to which a diagnosis applies. Read the safety and diagnostic limits.
Definition source
- DTC Database: pinned community code definitions
Source checked · MIT