P21AB: level-channel-two circuit-high covers two reported fault types

P21AB is the high-circuit record for reductant level channel 2 in this GM table. Its useful distinction is that two different communicated error statuses can produce the code. The title alone does not tell the technician which electrical condition was detected.

Read the reported error status

The open-load path uses status 3. The source associates it with a sensor response between 3.56 and 4.74 V measured 1.5 milliseconds after a test impulse. A separate short-to-battery path uses status 2 and a response above 4.74 V at that same specified delay.

Both paths require ignition on, battery voltage above 8 V and the general enabling conditions. The error must exist for more than three seconds, with the ECM's stated one-second monitoring rate. The impulse response and the ECM's monitoring interval describe different stages of the diagnosis.

Do not collapse open and short conditions

Retain the channel-two error status with the code. An open-load classification and a short-to-battery classification lead to different circuit questions even though they share P21AB. Neither is equivalent to a simple low reductant level.

The parameter table supplies no connector pinout or mandatory sensor replacement. The appropriate vehicle procedure must determine whether the fault lies in a connection, wiring or internal electronics. A steady meter reading at an unidentified point cannot recreate the specified impulse test. Verification should establish valid electrical status reporting, not merely a cleared record or a refilled tank.

Source and applicability

GM 2015 15OBDG04 Engine Diagnostics, cited reductant level-channel-two high circuit example only. Per-code vehicle, engine and calibration applicability has not been established; use the matching vehicle procedure.

General Motors source, physical page 208 · General Motors source, physical page 209