P23B8 code meaning
NOx Sensor Heater Sense Circuit Low (Bank 1 Sensor 3)
Powertrain system Generic code P23xx codes Check engine light
What P23B8 means
Smart NOx sensors report heater temperature through a separate sense connection, which GM calls the HTemp pin. P23B8 means this line on sensor 3 of bank 1 is pulled to ground. GM's 2025 table 25OBDG04B looks for the ground short on the post-second-catalyst sensor once the powertrain relay is above 11 V, the relay feeding the sensor bus is switched on, the supply exceeds 9.90 V and the dew point is reached; 80 of 160 samples at 25 ms must fail, Type B with two trips. For Continental Gen 3.5 sensors GM adds that the sensor itself waits 1.5 s before passing on an electrical fault. Bulletin 21-NA-149 names P23B8 where the engine harness of 3.0L LM2 trucks has rubbed through. P23B9 is the short to power; P2210 and P22A8 cover sensors 1 and 2.
P23B8 at a glance
- Check engine lightYes, usually on
- Emissions testCan fail
- Limp modeNot expected
- Repair difficultyMedium, DIY possible
- Typical labor0.5 to 3 hours
- Clears after repairWhen cleared with a scanner
Symptoms of P23B8
- Check engine light after two trips
- Sensor 3 does not report valid NOx values
- Often stored together with other wiring-related codes
Common causes of P23B8
- Most likelySense wire chafed through and touching body, frame or exhaust ground
- PossibleWater or salt bridging the sense pin to ground in the connector
- PossibleInternal short in the sensor or its control unit
Check the most likely causes first. Replacing parts by guesswork is the most expensive way to clear a code.
Parts involved
Nox sensor b1s3 heater
What each part of P23B8 means
| P | Powertrain system: engine, fuel system, ignition, emissions and transmission. |
| 2 | Generic code (second SAE range): it means the same on every make. |
| 3 | Area: ignition system or misfire. |
| B8 | The exact fault in that area. |
What to do next
- Note P23B8 and any other codes stored with it: several codes together often point to one cause.
- Check for loose connectors and damaged wiring around the parts the code names.
- Check open recalls for your car: some faults are fixed for free by the dealer. Enter your VIN.
- Clear the code only after the repair. If it comes back, the fault is still there.
Buying a used car? Scan it for stored codes before you pay. A recently cleared memory can hide a problem.
Questions about P23B8
What does P23B8 mean?
P23B8 means "NOx Sensor Heater Sense Circuit Low (Bank 1 Sensor 3)". Smart NOx sensors report heater temperature through a separate sense connection, which GM calls the HTemp pin. It is a generic code, so it means the same on every make.
Is it safe to drive with P23B8?
Usually yes, for a short time, if the check engine light is steady and the car drives normally. If the light flashes or the car runs badly, stop and have it checked.
Will P23B8 fail an emissions test?
Yes, it can. P23B8 is emissions-related, so most inspection programs fail a car while it is stored or while the check engine light is on.
How hard is P23B8 to fix?
Difficulty: medium. A typical repair takes 0.5 to 3 hours of work. A careful DIY repair is possible with basic tools and a scanner.
Will P23B8 clear itself after the repair?
Clear the code with a scanner after the repair. If the fault is fixed, it does not come back.