P23AB: NOx Sensor Processor Performance (Bank 1 Sensor 3)

A NOx sensor is a smart part: the probe in the exhaust is wired to a small control unit with its own processor, which runs the heater, evaluates the cell currents and reports over CAN. P23AB means that this electronics in sensor 3 of bank 1 – usually the one behind a second SCR catalyst – reports an internal error or fails its self-checks. No manufacturer documents P23AB publicly. Ford's 2023 diesel OBD summary shows what such checks contain for its front sensor: RAM failure and ROM or EEPROM checksum errors, reported there under P2200. CARB requires NOx sensors to be monitored for faults that make them unusable for the SCR checks. P06EA and P06EB are the processor codes of sensors 1 and 2. Repair normally means a new sensor.

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

Symptoms

• Check engine light on
• Sensor 3 values missing or frozen in live data
• SCR or emissions warning on the display

Most likely causes

• Internal electronics of the sensor 3 control unit defective
• Supply voltage dips or poor ground resetting the sensor electronics
• Sensor control unit damaged by heat or water ingress

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. Compare the reading with other engine data while varying the engine speed.
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 P23AB means

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

Frequently asked questions

Can I keep driving with code P23AB?

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 P23AB?

The most common causes are: internal electronics of the sensor 3 control unit defective; supply voltage dips or poor ground resetting the sensor electronics; sensor control unit damaged by heat or water ingress.

Related codes

• P06EANOx Sensor Processor Performance Bank 1 Sensor 1
• P06EBNOx Sensor Processor Performance Bank 1 Sensor 2
• P23ACNOx Sensor Circuit (Bank 1 Sensor 3)
• P23B6NOx Sensor Supply Voltage Circuit (Bank 1 Sensor 3)

Sources

• law.cornell.edu: 13 CCR 1968.2
• fordservicecontent.com: obd summary diesel 2023
• 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.