P249E: closed-loop reductant estimate indicates excessive dosing

P249E is the excessive-flow side of the documented PCS reductant-control diagnosis. The controller uses estimated SCR coverage and its disagreement with the chemical model, rather than a stated direct volumetric flow measurement.

Direction of disagreement matters

For either applicable SCR unit, estimated coverage above 0.850 together with estimate-minus-model difference above 0.400 meets the initial failure criteria. This direction is the reverse of P249D's low-delivery comparison. When a fault is already active, different thresholds and repass logic apply. A result must persist for at least 60 seconds; reporting is Type B, two trips.

Qualification includes substrate temperatures between 210°C and 300°C, active dosing and estimator correction, and absence of relevant model or electrical faults. Architecture notes distinguish a second SCR unit from a second modeled brick and omit conditions for absent DEF hardware.

Useful diagnostic decisions

First establish which hardware and modeled bricks are represented in the data. Then examine both the high estimated coverage and its difference from the model; one elevated value alone is not the complete initial criterion. Do not infer a leaking injector solely from the generic “flow too high” wording.

Confirmation requires the qualified comparison to run long enough and satisfy its appropriate repass logic. A code clear or momentary normal estimate does not reproduce that result.

Source and application limits

This explanation covers the cited 2025 GM monitor. Confirm the vehicle and calibration in its service information before applying these limits; inclusion in this file does not establish model compatibility. GM diagnostic parameters, PDF page 836. GM diagnostic parameters, PDF page 837.