Powertrain, generic (extended)

P24F5 NOx Sensor Delayed Response - High to Low (Bank 1 Sensor 2)

Check engine lightOn
EmissionsRelevant
Drive cycleRequired
Limp modeNo

P24F5 is the same delay check for NOx sensor 2 on bank 1, which sits behind the SCR catalyst and judges its conversion. If its value lags when NOx at the outlet falls, the efficiency estimate is distorted and the SCR monitor could pass or fail wrongly, so the sensor must react in time. A late reaction means the first visible change comes too long after the expected drop. No manufacturer publishes P24F5. CARB separates delayed from slow response in its sensor definitions; GM's 2025 diesel table shows a 2-second delay limit in its front-sensor test P22F9 as a model for such timing. Hella points to ammonia slip and crystal residue as threats to the element. P22FD is the slow falling case; P24F4 sensor 1.

Likely causes

DEF crystals or ammonia slip attacking the rear sensor element
Aged measuring cells of sensor 2 reacting late
Condensate in the cold tailpipe area affecting the sensor

Symptoms

  • Check engine light on
  • Possibly an SCR efficiency code alongside

Repair estimate

DifficultyMediumDIY possible
Parts and labour€150–900
Time0.5–2 h

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

Parts involved

nox sensor b1s2

Related codes

P24F4P22FDP22FCP23C2

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