Powertrain, generic (extended)

P24F4 NOx Sensor Delayed Response - High to Low (Bank 1 Sensor 1)

Check engine lightOn
EmissionsRelevant
Drive cycleRequired
Limp modeNo

P24F4 concerns the dead time of the upstream NOx sensor on bank 1 when engine-out NOx falls, typically on lift-off into overrun: the reading stays put too long before it starts dropping, even if the later descent is normal. CARB defines this split for exhaust sensors, delayed response being the wait before the first reaction and slow response a sluggish transition. No manufacturer documents P24F4 publicly. GM's 2025 diesel table shows the method for the rising edge: inside the front-sensor test P22F9 a timer runs from the NOx model reaching 30 ppm until the sensor does, and over 2 s is a fault. Hella names deposits and ageing as reasons for late NOx readings. P22FA is the slow falling case, P24F5 sensor 2.

Likely causes

Soot or oil ash on the diffusion barrier of the front sensor
Aged measuring cells reacting late to falling NOx
Exhaust leak ahead of the sensor distorting the transient

Symptoms

  • Check engine light on
  • Front NOx value lingers on lift-off
  • Engine runs normally

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 b1s1

Related codes

P24F5P22FAP22F9P23C2

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