P23C0 NOx Sensor Performance - Slow Response High to Low (Bank 1 Sensor 3)

With P23C0 the third NOx sensor on bank 1 is too sluggish when NOx falls, for instance when the driver lifts off and the engine goes into fuel cut-off. The module measures how long the reading takes to come down between two levels. No manufacturer documents P23C0 publicly. GM's 2016 diesel table judged the front-sensor counterpart P22FA during a load-to-overrun manoeuvre using a short and a long timer, failing it beyond 4 s on the first or 8 s on the second, Type B. Deposits from condensate, fuel or oil on the sensor element slow the gas exchange and age it faster, as HELLA notes. CARB expects such response-rate losses to be detected. A sensor that lags on the way down keeps reporting NOx that is already gone, which can make the SCR system look less effective than it is. P22FA and P22FD are sensors 1 and 2.

Likely causes

Symptoms

• Check engine light on
• Sensor 3 value stays raised for seconds after lifting off
• Driveability unaffected

Repair estimate

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

Parts involved

Related codes

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