Besides its drive line, the heater of a NOx sensor has a sense connection through which its resistance, and with it the element temperature, is measured. P23B4 reports a fault in that sense path for the third sensor on bank 1. GM's 2025 diesel table 25OBDG04B checks the HTemp pin of the post-second-catalyst sensor for an open circuit: 80 of 160 samples, Type A, MIL after one trip. A note in the table says that the Continental Gen 3.5 sensors debounce electrical faults internally for 1.5 s before reporting them, which lets the affected wire be pinpointed. Bulletin 21-NA-149 adds P23B4 to the harness-chafe list of LM2-powered 3.0L trucks from 2021–2022. Heater circuit monitoring is a CARB requirement. P23B5 is the plausibility check, P2208 and P22A6 the other sensors.
Medium severity. Can I keep driving?You can usually keep driving in the short term, but get it repaired soon: on many diesels, the warning eventually limits starting or engine power.
Bank 1 is the side of the engine with cylinder 1. Inline engines only have this bank.
Symptoms
- Check engine light after one trip
- Sensor 3 temperature status invalid
Most likely causes
- Heater sense lead broken between probe and module
- Backed-out or corroded terminal at the probe connector
- Internal break in the sensor 3 probe
What to check, in order
Start with the simplest. You can tick off each step.
Good to know
- Repair difficulty: medium
- It affects emissions.
What each character of P23B4 means
- P: Powertrain: engine and transmission
- 2: Generic code: same on every make
- 3: Ignition and misfires
- B4: Specific fault in that system
Frequently asked questions
Can I keep driving with code P23B4?
You can usually keep driving in the short term, but get it repaired soon: on many diesels, the warning eventually limits starting or engine power.
What causes code P23B4?
The most common causes are: heater sense lead broken between probe and module; backed-out or corroded terminal at the probe connector; internal break in the sensor 3 probe.
Related codes
- P23B5NOx Sensor Heater Sense Circuit Range/Performance (Bank 1 Sensor 3)
- P23B8NOx Sensor Heater Sense Circuit Low (Bank 1 Sensor 3)
- P23B9NOx Sensor Heater Sense Circuit High (Bank 1 Sensor 3)
- P23BANOx Sensor Heater Sense Circuit Intermittent (Bank 1 Sensor 3)
- P2208NOx Sensor Heater Sense Circuit (Bank 1 Sensor 1)
- P22A6NOx Sensor Heater Sense Circuit (Bank 1 Sensor 2)
Sources
Values and procedures vary by manufacturer. If it affects brakes, steering, fuel or high voltage, see a professional. Updated: 2026-11-04. How we build these pages.