P21B0: channel-three circuit-high can mean open load or short to battery

P21B0 is the high-circuit diagnostic for reductant level channel 3 in this GM example. It can result from an open-load error or a short-to-battery error. Retaining that distinction is more useful than treating every circuit-high title as proof of the same physical defect.

The status separates two electrical paths

The source identifies status 3 as open load, associated with a voltage response between 3.56 and 4.74 V at 1.5 milliseconds after a test impulse. Status 2 identifies short to battery, associated with a response above 4.74 V at that same delay.

The ECM requires ignition on, battery voltage above 8 V and the appropriate general enabling conditions. It monitors the reported error at a one-second rate and requires persistence beyond three seconds. That is separate from the much shorter impulse-response timing inside the sensor test.

Verify the reporting path, not merely the tank level

A technician should preserve which channel-three error status accompanied the code. That information changes the circuit investigation and avoids combining an open-load diagnosis with a short-to-power assumption.

The table does not prescribe replacing a tank assembly or prove that reductant quantity is low. It gives no safe terminal-level impulse-test procedure. Use the correct vehicle wiring and module diagnostics to establish the fault location and valid status reporting. Adding fluid or clearing the code does not demonstrate that either electrical path has been repaired.

Source and applicability

GM 2015 15OBDG04 Engine Diagnostics, cited reductant level-channel-three 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 209 · General Motors source, physical page 210