P249D: closed-loop reductant estimate indicates insufficient dosing

P249D is labeled flow too low, but the cited PCS strategy infers the condition through SCR coverage estimates. It compares an extended-Kalman-filter estimate with a chemical model; no direct flow-meter reading is specified as the failure criterion.

Two conditions per catalyst or brick

For SCR1, the estimated coverage ratio must be below 0.010 and the model-minus-estimate difference above 0.400. SCR2 uses a lower coverage boundary of 0.005 with the same initial difference threshold. Active-fault repass limits differ. The controller requires a result sustained for at least 60 seconds and uses Type B, two-trip reporting.

Both relevant substrate temperatures must be within 210°C to 300°C, dosing and estimator correction must be active, and chemical-model, electrical and component-management faults must be absent. Continuation rules omit nonexistent SCR2 or DEF2 hardware and explain dual-brick modeling.

What to investigate

Identify the actual catalyst and dosing architecture before treating every condition as a second physical system. Review the estimate/model disagreement together with dosing eligibility. A low estimate with invalid model inputs is not a qualified version of this test.

Verification requires the sustained, enabled comparison. The code is evidence of insufficient dosing effect in this strategy, but does not by itself prove a clogged injector or justify replacing the pump.

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 834. GM diagnostic parameters, PDF page 835.