Powertrain, generic (extended)

P23BE NOx Sensor Performance - Signal Biased/Stuck Low (Bank 1 Sensor 3)

Check engine lightOn
EmissionsRelevant
Drive cycleRequired
Limp modeNo

P23BE is the low-side mirror of P23BD: NOx sensor 3 on bank 1 shows less NOx than is plausible, or it stays near zero while the engine is clearly producing some. Behind a working SCR system the true value is small anyway, which makes a low-stuck sensor the more dangerous error for monitoring, because it can hide a failing catalyst. No manufacturer documents P23BE publicly. Ford's 2023 diesel summary sets a negative-offset limit of about 20 ppm for the front sensor under P2201. Volvo's troubleshooting guide treats the low-stuck counterparts P225D and P225F as a probe measuring outside air, so an exhaust leak or air entry is the likelier cause than the sensor. CARB lists gain-low failure modes for NOx sensors. P225D and P225F are sensors 1 and 2.

Likely causes

Exhaust leak or air drawn in ahead of sensor 3, diluting the gas
Measuring cell lost sensitivity and reads too low
Sensor not seated correctly so it samples too little exhaust

Symptoms

  • Check engine light on
  • Sensor 3 reads near zero under load
  • Possibly an audible exhaust leak at the rear section

Repair estimate

DifficultyMediumDIY possible
Parts and labour€50–900
Time0.5–2.5 h

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

Parts involved

nox sensor b1s3

Related codes

P23BDP23ADP225DP225F

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