P203C: reductant level low-circuit code comes from smart-sensor status

P203C sounds like a simple low level-sensor voltage fault. In the reviewed GM reductant-controller table, the computation actually occurs inside a smart UTLC sensor. The sensor sends a diagnostic status over SENT when its piezo excitation or piezo circuit falls outside permitted limits.

“Low” includes more than one internal fault

The listed conditions include excitation voltage above 5.5 volts or below 4.5 volts, and piezo voltage above 2.0 volts or below 0.125 volt. These internal conditions are represented by a single diagnosis status bit. The low-circuit code therefore does not establish which one occurred, and it is not evidence that the tank is nearly empty.

Those voltages describe internal sensor criteria. The row does not identify an externally accessible terminal at which each value can safely be measured, so they should not be turned into improvised pin-probing instructions.

Prerequisites and confirmation

The vehicle must be in an active power mode, which can include accessory, run, start request or propulsion. Level-sensor power-supply faults, SENT communication faults and specified controller-internal faults must be absent. Qualification is four failing half-second samples, described as two seconds, with Type B reporting over two trips.

Verification should establish valid power and communication before judging the transmitted internal status. An absent fault while communication is inhibited does not demonstrate sensor recovery. The source explains the reporting path but provides no replacement verdict or exact vehicle coverage. Apply these facts only after confirming the installed UTLC and reductant-control system.

Source: GM monitor, PDF page 187.