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.

Likely causes

Symptoms

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

Repair estimate

Typical range across vehicles; the real cost depends on what the testing finds.

Parts involved

Related codes

References: law.cornell.edu, static.nhtsa.gov, fordservicecontent.com, static.nhtsa.gov.