P24F4 code meaning
NOx Sensor Delayed Response - High to Low (Bank 1 Sensor 1)
Powertrain system Generic code P24xx codes Check engine light
What P24F4 means
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.
P24F4 at a glance
- Check engine lightYes, usually on
- Emissions testCan fail
- Limp modeNot expected
- Repair difficultyMedium, DIY possible
- Typical labor0.5 to 2 hours
- Clears after repairAfter a drive cycle
Symptoms of P24F4
- Check engine light on
- Front NOx value lingers on lift-off
- Engine runs normally
Common causes of P24F4
- Most likelySoot or oil ash on the diffusion barrier of the front sensor
- PossibleAged measuring cells reacting late to falling NOx
- Less likelyExhaust leak ahead of the sensor distorting the transient
Check the most likely causes first. Replacing parts by guesswork is the most expensive way to clear a code.
Parts involved
Nox sensor b1s1
What each part of P24F4 means
| P | Powertrain system: engine, fuel system, ignition, emissions and transmission. |
| 2 | Generic code (second SAE range): it means the same on every make. |
| 4 | Area: auxiliary emission controls (catalytic converter, EGR, EVAP). |
| F4 | The exact fault in that area. |
What to do next
- Note P24F4 and any other codes stored with it: several codes together often point to one cause.
- Check for loose connectors and damaged wiring around the parts the code names.
- Check open recalls for your car: some faults are fixed for free by the dealer. Enter your VIN.
- Clear the code only after the repair. If it comes back, the fault is still there.
Buying a used car? Scan it for stored codes before you pay. A recently cleared memory can hide a problem.
Questions about P24F4
What does P24F4 mean?
P24F4 means "NOx Sensor Delayed Response - High to Low (Bank 1 Sensor 1)". 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. It is a generic code, so it means the same on every make.
Is it safe to drive with P24F4?
Usually yes, for a short time, if the check engine light is steady and the car drives normally. If the light flashes or the car runs badly, stop and have it checked.
Will P24F4 fail an emissions test?
Yes, it can. P24F4 is emissions-related, so most inspection programs fail a car while it is stored or while the check engine light is on.
How hard is P24F4 to fix?
Difficulty: medium. A typical repair takes 0.5 to 2 hours of work. A careful DIY repair is possible with basic tools and a scanner.
Will P24F4 clear itself after the repair?
After the repair the car has to complete a drive cycle (a mix of city and highway driving) before the test runs again. Until then the readiness monitor shows "not ready", which can fail an inspection.