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

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.

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

Symptoms

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

Most 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

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 P23BE means

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

Frequently asked questions

Can I keep driving with code P23BE?

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 P23BE?

The most common causes are: 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.

Related codes

• P23BDNOx Sensor Performance - Signal Biased/Stuck High (Bank 1 Sensor 3)
• P23ADNOx Sensor Circuit Range/Performance (Bank 1 Sensor 3)
• P225DNOx Sensor Performance - Signal Biased/Stuck Low (Bank 1 Sensor 1)
• P225FNOx Sensor Performance - Signal Biased/Stuck Low (Bank 1 Sensor 2)

Sources

• law.cornell.edu: 13 CCR 1968.2
• 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.