P22FC: NOx Sensor Performance - Slow Response Low to High (Bank 1 Sensor 2)

P22FC is reserved for the rear NOx sensor on bank 1, behind the SCR catalyst, when its signal climbs too slowly after NOx at that point increases. No manufacturer documents this code publicly, and the NHTSA bulletin index has no entry for it. GM, whose tables describe the front-sensor version P22F9, files the dynamic check of the rear sensor under P229F instead (2018 diesel fault bundle for the NOx2 dynamic test). CARB requires every NOx sensor to be watched for a loss of response rate before emissions pass the OBD limits (13 CCR 1968.2 (f)(5.2.2)(A)). Since a working SCR leaves little NOx downstream, a rising edge only shows during a dosing pause or a NOx burst. A slow outlet sensor can hide SCR faults; expect the MIL.

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

Symptoms

• Check engine light on
• No change in driving behaviour

Most likely causes

• Rear NOx sensor aged or contaminated by urea deposits, oil ash or cleaning agents
• Exhaust leak between SCR catalyst and sensor
• Calibration of the test itself, check for software updates

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 P22FC means

• P: Powertrain: engine and transmission
• 2: Generic code: same on every make
• 2: Fuel injection
• FC: Specific fault in that system

Frequently asked questions

Can I keep driving with code P22FC?

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 P22FC?

The most common causes are: rear NOx sensor aged or contaminated by urea deposits, oil ash or cleaning agents; exhaust leak between SCR catalyst and sensor; calibration of the test itself, check for software updates.

Related codes

• P22FDNOx Sensor Performance - Slow Response High to Low (Bank 1 Sensor 2)
• P22FENOx Sensor Performance - Sensing Element (Bank 1 Sensor 2)
• P22F9NOx Sensor Performance - Slow Response Low to High (Bank 1 Sensor 1)
• P229FNOx Sensor Circuit Range/Performance (Bank 1 Sensor 2)
• P20EESCR NOx Catalyst Efficiency Below Threshold (Bank 1)

Sources

• law.cornell.edu: 13 CCR 1968.2
• gsi.ext.gm.com: 18OBDG04 Engine Diagnostics

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.