P20EE: SCR efficiency is a filtered model-based judgment

P20EE is a selective catalytic reduction efficiency code. The reviewed GM SCR1 strategy estimates maximum ammonia-storage capacity using a model, then evaluates deviations from a nominal value. It does not simply compare one upstream NOx reading with one downstream reading at an arbitrary moment.

Filtering changes the meaning of a snapshot

The diagnostic averages 300 valid samples taken at 250-millisecond intervals and then applies an exponentially weighted moving-average filter. The printed fail and repass thresholds for the resulting parameter are both 0.35000. Fast-initial, rapid-response and standard modes use different filter gains and permitted test counts.

Accordingly, a single favorable NOx ratio or a momentary poor ratio is not the same evidence as the completed filtered result. The calibration also evaluates the model's supporting conditions rather than accepting every operating sample.

What must be credible first

The source requires reliable upstream and downstream NOx measurements, a ready DEF system and absence of relevant chemical-model, thermal-model, flow, temperature and dosing faults. It also conditions evaluation on DEF availability, concentration when the sensor is fitted, and appropriate operating states. The continuation includes additional thermal and model-confidence gates.

Verification should establish that those prerequisites permit an actual SCR efficiency test and review the resulting filtered parameter. A code-free interval while dosing or sensing faults inhibit evaluation is inconclusive. The table lists Type A, one-trip reporting but does not prove the catalyst needs replacement or identify a specific contamination cause. This explanation is bounded to the documented 2025 GM SCR1 monitor and requires vehicle-specific applicability confirmation.

Source: GM monitor, PDF page 780; GM monitor, PDF page 781; GM monitor, PDF page 782; GM monitor, PDF page 783.