P20B9: How the first DEF tank heater is checked for an open circuit

P20B9 is the open-circuit diagnosis for reductant heater 1, identified in this GM table as the first tank heater. Both the high-side and low-side control paths are within the scope of the monitor; the label does not isolate one connector or conductor.

Shutdown allows the fault to be classified

An abnormal driver observation while the heater is commanded on initiates the diagnostic response. The table explains that the precise circuit fault can only be identified with the heater off, so the controller removes the command before completing the classification.

The off-state open-circuit test uses a particular combination of high-side and low-side ADC readings. These are internal diagnostic measurements, not published voltages for arbitrary external test points. The documented accumulation is ten samples at 0.2-second intervals, giving a two-second failure period.

What an absent heater command means

A zero command after the fault should not automatically be interpreted as a controller that never tried to heat the fluid. It may reflect the intentional transition needed to identify the electrical problem. Reviewing the command history and driver status together is therefore more informative than viewing either alone.

This monitor is different from a resistance or delivered-power performance check. Its result supports an open-path investigation using the correct heater circuit diagram; it does not prove that the tank must be replaced. The source excludes specified internal controller faults and places recovery on the next driving cycle, which must be considered when judging a retest.

Source and scope

2025 GM 25OBDG04B DEFC monitor calibration only; this document does not establish applicability to every model, engine or reductant system. These are documented monitor criteria, not a universal repair procedure. GM PDF, physical page 194.