A NOx sensor only measures once its built-in heater has brought the ceramic element up to temperature. P23B1 reports that the heater control side of the third sensor on bank 1 is open. GM's 2025 diesel calibration (table 25OBDG04B) looks for a missing load on the heater control pin of the post-second-catalyst sensor and fails after 80 of 160 samples at a 25 ms rate, with powertrain relay voltage above 11 V and the sensor bus relay switched on. GM rates it Type A, so the MIL comes on after a single trip. Bulletin 21-NA-149 includes P23B1 in its chafed-harness list for 2021–2022 3.0L LM2 trucks. CARB requires heater circuits to be checked for opens and shorts. P23B2 and P23B3 are shorts; P2205 and P22A3 cover sensors 1 and 2.
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 never reports operating temperature
- SCR warning on the display
Most likely causes
- Heater element in the sensor 3 probe burnt through
- Broken heater conductor between probe and sensor module
- Probe connector not fully engaged after a repair
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 P23B1 means
- P: Powertrain: engine and transmission
- 2: Generic code: same on every make
- 3: Ignition and misfires
- B1: Specific fault in that system
Frequently asked questions
Can I keep driving with code P23B1?
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 P23B1?
The most common causes are: heater element in the sensor 3 probe burnt through; broken heater conductor between probe and sensor module; probe connector not fully engaged after a repair.
Related codes
- P23B2NOx Sensor Heater Control Circuit Low (Bank 1 Sensor 3)
- P23B3NOx Sensor Heater Control Circuit High (Bank 1 Sensor 3)
- P23B4NOx Sensor Heater Sense Circuit (Bank 1 Sensor 3)
- P23B7NOx Sensor Heater Control Circuit Range/Performance (Bank 1 Sensor 3)
- P2205NOx Sensor Heater Control Circuit/Open (Bank 1 Sensor 1)
- P22A3NOx Sensor Heater Control Circuit/Open (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.