P23BF: NOx Sensor Performance - Slow Response Low to High (Bank 1 Sensor 3)

P23BF concerns how quickly NOx sensor 3 on bank 1 follows a rise in NOx. The control module waits for an increasing transient and times the climb of the sensor signal against what a model predicts. No manufacturer publishes P23BF. GM's 2025 table demonstrates the method on the front sensor under P22F9: timing runs from 30 ppm up to 220 ppm, and the sensor fails when it trails by over 2 s or by more than a tenth of the model's time. For a probe behind two SCR catalysts this is harder, since little NOx is left to reach it. A Continental smart sensor is specified with a 33-66 % response time of 1300 ms when fresh and 1650 ms when aged. CARB counts response rate among NOx sensor performance faults. P22F9 and P22FC are sensors 1 and 2; P23C0 is the falling edge.

Bank 1 is the side of the engine with cylinder 1. Inline engines only have this bank.

Symptoms

• Check engine light on
• Sensor 3 signal trails the front sensor during acceleration
• No change in how the engine runs

Most likely causes

• Soot, oil ash or urea residue blocking the gas inlet of the sensor
• Measuring cells aged beyond their response specification
• Element running too cold because the heater is weak

What to check, in order

Start with the simplest. You can tick off each step.

1. Read the codes and the NOx data before and after the SCR catalyst
2. With the connector unplugged, check whether the signal wire is making contact with ground.
3. Check the sensor's connector and wiring
4. Look for exhaust leaks ahead of the sensor
5. Confirm that the AdBlue/DEF is of good quality
6. Replace the sensor only after ruling out power, ground, and leaks

Good to know

• Repair difficulty: medium
• It affects emissions.

What each character of P23BF means

• P: Powertrain: engine and transmission
• 2: Generic code: same on every make
• 3: Ignition and misfires
• BF: Specific fault in that system

Frequently asked questions

Can I keep driving with code P23BF?

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 P23BF?

The most common causes are: soot, oil ash or urea residue blocking the gas inlet of the sensor; measuring cells aged beyond their response specification; element running too cold because the heater is weak.

Related codes

• P23C0NOx Sensor Performance - Slow Response High to Low (Bank 1 Sensor 3)
• P23C2NOx Sensor Delayed Response - High to Low (Bank 1 Sensor 3)
• P22F9NOx Sensor Performance - Slow Response Low to High (Bank 1 Sensor 1)
• P22FCNOx Sensor Performance - Slow Response Low to High (Bank 1 Sensor 2)

Sources

• law.cornell.edu: 13 CCR 1968.2
• gsi.ext.gm.com: 25OBDG04A
• ni.com: NOx Sensor USB UM RevA

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.