P2B0B: Cylinder 4’s cold-start injection pulse lacks opening evidence

P2B0B concerns an expected cylinder 4 injection pulse during catalyst warm-up for which the controller does not obtain the expected injector-opening evidence. GM detects the armature or pintle movement indirectly through voltage-flux feedback from the injector coil. This is different from inferring combustion quality solely from crankshaft motion.

A command is not proof that fuel was delivered

The monitor distinguishes a commanded pulse from the feedback associated with movement. The source describes both an absent opening magnitude and evaluation of a converted opening-magnitude metric against supporting calibration data. Sending a pulse command therefore does not demonstrate that the injector responded as expected.

The supporting threshold is indexed by measured fuel-rail pressure. That controller metric should not be turned into a universal connector-voltage test or transferred from a different calibration file without checking its definitions.

Verify the warm-up opportunity

Small-pulse diagnostics, catalyst warm-up and voltage-feedback data readiness must be enabled. The table uses cylinder-event sampling and lists a 50–100 sample monitoring span. A normal result after warm-up has ended does not reproduce that cold-start diagnostic opportunity.

Useful evidence includes the affected cylinder, the warm-up state and whether the feedback data was ready for the commanded pulses. The monitor does not directly measure injected fuel mass or identify why movement evidence was missing. Fuel-system diagnosis requires the applicable service information and appropriate handling of the pressurized system.

Source and applicability

2025 GM 25OBDG03B documented monitor only. Vehicle, engine and calibration applicability must be confirmed from the matching service information; no model fitment is inferred. These criteria explain controller logic, not a complete repair procedure. 25OBDG03B, PDF page 867. 25OBDG03B, PDF page 1372.