P23B6: NOx sensor 3 supply voltage is below range

P23B6 is a supply-voltage diagnosis for the NOx sensor after the second catalyst. The source explicitly enables this monitor when the sensor is generation 3.0. That generation condition is a real applicability limit, not a detail to transfer automatically to other sensor hardware.

Two different voltage conditions

The failure criterion is sensor supply below 9.90 V, while the powertrain relay voltage must be above 11 V for the check to run. These refer to different monitored locations or reports. Healthy relay voltage therefore does not, by itself, establish healthy sensor supply.

The engine must be running, the NOx bus relay commanded on and dewpoint reached. CAN communication and data validity must remain acceptable. The counter is 125 failures out of 250 samples on a 25 ms task; reporting is Type B, two trips.

Diagnosis and confirmation

Confirm sensor generation and compare the sensor's own supply status with the qualifying relay-voltage information. Treat a missing CAN message as a communication problem rather than a measured low supply. The summary does not identify a fuse, terminal, wiring route or exact failed component. Verification should demonstrate that this generation-specific supply check actually runs and remains within range after the identified cause is addressed, rather than relying on battery voltage alone.

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 1461.