Powertrain, generic (extended)

P22FD NOx Sensor Performance - Slow Response High to Low (Bank 1 Sensor 2)

Check engine lightOn
EmissionsRelevant
Drive cycleRequired
Limp modeNo

P22FD stands for the falling edge at the rear NOx sensor on bank 1: after a NOx peak downstream of the SCR catalyst, the signal decays more slowly than permitted. No manufacturer documents this code publicly, and no bulletin in the NHTSA index names it. How such a check works is visible in GM's front-sensor test P22FA, which times the drop between two fractions of the starting value (on the 6.6L LML 2.3 s from 70 to 40 %), while GM itself puts rear-sensor dynamics into P229F. Engines with a third NOx sensor have P23C0 for the same fault at sensor 3. Section (f)(5.2.2)(A) of CARB's OBD II rule names response rate among the sensor faults to detect. A sluggish outlet sensor makes the SCR look weaker than it is and can lead to efficiency codes such as P20EE.

Likely causes

Rear NOx sensor aged or coated, signal falls too slowly
Urea crystals around the sensor tip after dosing faults
Exhaust leak close to the outlet sensor

Symptoms

  • Check engine light on
  • SCR efficiency code may appear as well

Repair estimate

DifficultyMediumWorkshop job
Parts and labour€100–1,100
Time0.3–2 h

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

Parts involved

nox sensorscr catalyst

Related codes

P22FCP22FEP22FAP229FP23C0P20EE

References: law.cornell.edu, gsi.ext.gm.com, gsi.ext.gm.com.