P23B1: open heater-control circuit at NOx sensor 3

P23B1 is an open-load diagnosis for the heater-control connection of NOx sensor 3. The source describes this sensor as post-second-catalyst. Heater-control integrity is the monitored function; a low exhaust temperature or weak NOx conversion is not the setting criterion.

What the controller needs

The NOx bus relay must be commanded on, powertrain relay voltage must exceed 11 V and sensor supply must exceed 9.90 V. Communication-loss and invalid-data conditions must be absent. Under those conditions, the controller uses 80 failures out of 160 samples on a 25 ms task. Reporting is Type A, one trip.

This open-circuit row does not list the dewpoint requirement that appears in the adjacent short-circuit entries. Fault timing should therefore be interpreted against this entry's own conditions.

Where the distinction leads

Identify heater control separately from the HTemp resistance-sense connection covered by P23B4. A disconnected sense path and an open heater-control path have different diagnostic reports even though they belong to one sensor system. Use the exact circuit information to determine what is serviceable and how to test it. For verification, establish valid sensor communication and completion of the heater-control check without the open-load report; observing a heater command alone does not prove that the monitored circuit conducts.

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 1456.