Diagnostic article Generic OBD-II
P21AF: level-channel-three low circuit is a discrete error report
GM P21AF evaluates a short-to-ground status for reductant level channel three, separately from the actual fluid-level interpretation.
Read before applying this article
Documented application
GM 2015 15OBDG04 Engine Diagnostics, cited reductant level-channel-three low circuit example only. Per-code vehicle, engine and calibration applicability has not been established; use the matching vehicle procedure.
The code alone does not identify a failed part or establish that a vehicle is safe to drive. Confirm the vehicle, reporting module and applicable service procedure.
P21AF addresses reductant level channel 3 in the cited GM diagnostic table. The ECM monitors a CAN-reported discrete sensor status. Status 1 is identified as the channel-three short-to-ground error; it is not a direct numerical measurement of the tank's remaining fluid.
Keep the channel and test response together
The source associates the low-circuit status with a sensor voltage below 0.17 V directly after a test impulse. The ECM evaluation requires ignition on, battery voltage above 8 V and the stated general enabling conditions. The error must last more than three seconds while the monitor runs at a 0.01-second rate.
The same electrical error class exists for other level channels, but that does not make their data interchangeable. The actual channel-three status must remain attached to the diagnostic record and be mapped through the vehicle's service information.
Electrical validity comes before level conclusions
A technician should preserve the error status, power conditions and any related reporting faults. A changed fluid-level display after refilling does not, by itself, verify the channel-three circuit or the impulse-response test.
The document does not identify the faulty wire, connector or internal sensor element, and it contains no component replacement rule. Use the matching circuit and communication procedure to locate the cause. The printed impulse-response voltage is a controller diagnostic criterion, not permission to apply a test voltage or probe an unspecified reductant sensor connection.
Source and applicability
GM 2015 15OBDG04 Engine Diagnostics, cited reductant level-channel-three low circuit example only. Per-code vehicle, engine and calibration applicability has not been established; use the matching vehicle procedure.
Article source documents
Source checks describe how the text was prepared; they do not certify a diagnosis or confirm compatibility with every vehicle using this code.
Original code reference
Dictionary definition
Reductant Level Sensor C Circuit Low
This definition retains its original catalog scope. The article above is limited to its explicitly documented application.
Reference scope
Generic OBD-II definition.
A manufacturer label does not establish the model, year, engine or module to which a diagnosis applies. Read the safety and diagnostic limits.
Definition source
- DTC Database: pinned community code definitions
Source checked · MIT