P226D: low DPF flow resistance can also implicate pressure-sensing pipes

P226D is labeled particulate filter deteriorated or missing substrate. The reviewed GM monitor detects low flow resistance, but its stated detection targets also include disconnected, clogged or blocked DPF pressure-sensor piping. The code alone therefore cannot prove that the filter has been removed or physically broken.

Pressure must be interpreted with flow

The monitored quantity is filtered flow resistance, with a low boundary of 0.01 kPa per liter per second. This is a normalized pressure-versus-flow quantity, not a raw differential-pressure limit that can be applied regardless of exhaust flow.

The monitor requires exhaust volume flow above 45 liters per second for more than one second and engine speed above 500 rpm. It runs during soot loading, with no regeneration in progress. A low pressure reading at idle or during a different operating phase does not automatically reproduce this fault.

Valid inputs and confirmation

Relevant DPF pressure-sensor electrical, rationality and offset faults must be absent. The upstream temperature model, air-flow measurement, atmospheric-pressure information and exhaust-flow estimate must also be usable; the continuation adds thermal and operating gates. Qualification is 20 failures within 30 samples at 100-millisecond intervals, with Type A, one-trip reporting.

Verification should establish credible pressure and flow evidence during an enabled loading-phase check, including the sensing-pipe path in the applicable diagnosis. The table does not prescribe filter replacement or a regeneration attempt. Exact vehicle and filter-system applicability remain to be confirmed, and physical inspection must follow the relevant service procedure.

Source: GM monitor, PDF page 1419; GM monitor, PDF page 1420; GM monitor, PDF page 1421.