P23BD: NOx Sensor Performance - Signal Biased/Stuck High (Bank 1 Sensor 3)

P23BD means NOx sensor 3 on bank 1, the last probe downstream of the second SCR catalyst, reports more NOx than the operating state allows, or its value no longer comes down from a high level. A common test is engine overrun without fuel, when the exhaust contains no NOx at all: Volvo, for its inlet-sensor counterpart P225C, expects the reading to fall below 50 ppm within roughly 10 s. Ford's 2023 diesel summary allows the front sensor a positive offset of about 40 ppm under P2201. No manufacturer publishes P23BD itself. CARB names the offset-high failure mode for NOx sensors explicitly. A falsely high value behind the SCR system can make a healthy catalyst look worn out and lead to needless parts. P225C and P225E are sensors 1 and 2.

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

Symptoms

• Check engine light on
• SCR efficiency code may follow although the catalyst is fine
• Sensor 3 reading stays high in overrun

Most likely causes

• Measuring cell drifted upward with age or contamination
• Sensor element damaged by thermal shock or poisoning
• Monitor calibration too strict, fixed by an engine software update

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. Check the sensor's ground and make sure the signal wire is not receiving voltage from another circuit.
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 P23BD means

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

Frequently asked questions

Can I keep driving with code P23BD?

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 P23BD?

The most common causes are: measuring cell drifted upward with age or contamination; sensor element damaged by thermal shock or poisoning; monitor calibration too strict, fixed by an engine software update.

Related codes

• P23BENOx Sensor Performance - Signal Biased/Stuck Low (Bank 1 Sensor 3)
• P23ADNOx Sensor Circuit Range/Performance (Bank 1 Sensor 3)
• P225CNOx Sensor Performance - Signal Biased/Stuck High (Bank 1 Sensor 1)
• P225ENOx Sensor Performance - Signal Biased/Stuck High (Bank 1 Sensor 2)

Sources

• law.cornell.edu: 13 CCR 1968.2
• static.nhtsa.gov: MC 10181863 0001
• fordservicecontent.com: obd summary diesel 2023
• static.nhtsa.gov: MC 10215279 0001

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.