Diagnostic article Generic OBD-II
P23C1: NOx sensor 3 sensing cavities fail an afterrun self-test
P23C1 compares the third NOx sensor internal self-test response with its stored reference.
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Documented application
2025 GM 25OBDG04B diagnostic parameter file; the cited monitor is documented, but individual vehicle, engine and calibration applicability has not been established.
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.
P23C1 concerns sensing-element integrity in the NOx sensor after the second catalyst. The source describes detecting restricted measurement cavities that could reduce sensitivity. It changes internal current setpoints, raises oxygen in the second cavity to a target and compares the resulting response with a stored new-sensor reference.
A filtered internal comparison
The response-to-reference ratio fails above 125% or below 75%, after exponentially weighted processing. The row lists Type A, one-trip reporting and a 500 ms task. It permits additional tests in specified filter-response modes, so the number of available tests is not fixed across every state.
Driving prerequisites must first be achieved, after which the ECM requests the test at key-off. Valid sensor communication, electrical health, heater state, exhaust conditions and supporting aftertreatment signals all matter.
Confirmation needs the afterrun event
An apparently accurate running NOx reading is not a completed cavity-integrity test. Check whether the afterrun request and result occurred before deciding that work has been verified. The continuation contains questionable temperature units and idle-speed values; this article does not turn those cells into driving instructions. Use the exact calibration's service procedure to qualify the test. The code identifies an abnormal self-test response, but the table alone cannot prove where a restriction is located or authorize a specific replacement.
Source and application limits
This explanation covers the cited 2025 GM monitor. Confirm the vehicle and calibration in its service information before applying these limits; inclusion in this file does not establish model compatibility. GM diagnostic parameters, PDF page 1464. GM diagnostic parameters, PDF page 1465. GM diagnostic parameters, PDF page 1466. GM diagnostic parameters, PDF page 1467. GM diagnostic parameters, PDF page 1468.
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
NOx Sensor Performance - Sensing Element Bank 1 Sensor 3
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