Diagnostic article Generic OBD-II
P23B9: NOx sensor 3 heater-sense path shorted to power
P23B9 checks for a power-short indication at the third NOx sensor HTemp connection.
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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.
In this monitor, P23B9 means a short to power on NOx sensor 3's HTemp heater-sense connection. The diagnosis concerns resistance feedback from the heater, rather than the heater-control output or a maximum temperature threshold.
What permits detection
The sensor needs a commanded-on bus relay, supply above 9.90 V and powertrain relay voltage above 11 V. It must have reached dewpoint and provide valid CAN data without a lost-communication condition. The failure counter is 80 out of 160 samples on a 25 ms task. The reporting designation is Type B, two trips.
Continental generation 3.5 units are additionally described as applying a 1.5-second internal debounce to electrical fault identification. That statement is qualified by sensor generation.
Using the circuit distinction
A high fault in heater control would be P23B3 in this group; P23B9 points to the sensing path. Establishing that distinction before testing avoids treating both connections as one circuit. Check the exact service procedure for safe electrical assessment and the sensor's repairable boundaries. After correction, a completed, qualified HTemp check is the relevant evidence. Clearing the code before the sensor reaches dewpoint or observing an active heater command cannot alone demonstrate that this high sense-circuit fault has been resolved.
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 1463.
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 Heater Sense Circuit High 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