P24F4: NOx Sensor Delayed Response - High to Low Bank 1 Sensor 1
The pre-catalyst NOx sensor on bank 1 (sensor 1) reacts too slowly when NOx in the exhaust drops from a high to a low level — for example after a fuel cut or transition into low-load. Aged ceramic cells, soot deposits or condensation inside the sensor protective tube slow the diffusion of NOx to the measuring element.
What P24F4 means
P24F4 is a generic OBD-II powertrain code in the emissions group. In plain English: the sensor is reacting too slowly to changes in the exhaust, so the computer can’t trust its readings. Bank 1 is the side of the engine with cylinder 1 (inline engines only have bank 1).
- Check engine light
- Usually on
- Emissions-related
- Yes — can fail inspection
- Limp mode possible
- Not typical
- Drive cycle to clear
- Not required
Get it fixed
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Symptoms
- Check engine light or NOx warning on
- AdBlue/DEF level or quality warning
- Possible no-start countdown after ignore period
- Failed emissions inspection (NOx)
Common causes
- 01Aged sensor element with reduced response speedMost likely
- 02Soot clogging the protective tubePossible
Can I drive with P24F4?
Usually yes, for a short time.
The car will often drive normally, but get it diagnosed soon. Left alone it can raise fuel use and emissions and will fail an OBD inspection. If you notice rough running, warning lights flashing, overheating or a burning smell, stop driving.
Why we rate it moderate
- Turns on the check engine light
- Emissions-related: will usually fail an OBD inspection
Severity is our rule-based rating from the code’s flags in OBDex and the system it affects, not an inspection of your car. If the check engine light is flashing, or you notice overheating, low oil pressure, or brake or steering problems, stop driving and get help.
How to diagnose P24F4
- 1
Confirm the code and save freeze-frame data
Read the codes with an OBD-II scan tool and note every code, not just P24F4. Save the freeze-frame data (speed, load, temperature when the fault set) before clearing anything; it tells you when the fault happens.
- 2
Check for related codes first
Look for P24F5. Related codes often share a cause; a circuit or sensor code usually needs fixing before a performance code like this one.
- 3
Test the circuit
Inspect the wiring and connector at the nox sensor (bank 1, sensor 1) for damage, corrosion or chafing. With a multimeter, check supply voltage, ground and signal against the specifications in the service manual for your vehicle.
- 4
Test the likely causes in order
Start with the most likely cause and work down: Aged sensor element with reduced response speed; soot clogging the protective tube. Test before replacing parts.
- 5
Check for recalls and service bulletins
Look up recalls for your exact vehicle with its VIN, and ask a dealer about technical service bulletins (TSBs). Manufacturers sometimes issue software updates or extended warranties for known faults.
- 6
Clear the code and verify the repair
After the repair, clear the code and drive the car under the conditions in the freeze-frame data to confirm it does not return.
P24F4 repair cost estimate
Estimate, not a quote. Labor times are typical ranges for this code from OBDex, and $125/hr is an assumed rate. Shop rates and labor times vary by vehicle and region. Diagnosis time may be billed separately.
- DIY difficulty
- Moderate
- Good DIY job?
- Usually a shop job
Needs a scan tool and some experience with testing.
Parts and systems involved
- NOx sensor (Bank 1, sensor 1)
P24F4 FAQ
What does P24F4 mean?
P24F4 means “NOx Sensor Delayed Response - High to Low Bank 1 Sensor 1”. The pre-catalyst NOx sensor on bank 1 (sensor 1) reacts too slowly when NOx in the exhaust drops from a high to a low level — for example after a fuel cut or transition into low-load. Aged ceramic cells, soot deposits or condensation inside the sensor protective tube slow the diffusion of NOx to the measuring element.
Is P24F4 serious? Can I keep driving?
We rate P24F4 as moderate severity. Usually yes, for a short time. The car will often drive normally, but get it diagnosed soon. Left alone it can raise fuel use and emissions and will fail an OBD inspection. If you notice rough running, warning lights flashing, overheating or a burning smell, stop driving.
What causes P24F4?
The most common causes, from most to least likely: Aged sensor element with reduced response speed; Soot clogging the protective tube. Test before replacing parts, because several causes can set the same code.
How much does it cost to fix P24F4?
Typical labor is about $75–$250 at an assumed $125/hour shop rate, plus parts. That is an estimate, not a quote: labor times and rates vary by vehicle and region, and diagnosis may be billed separately.
Can I fix P24F4 myself?
DIY difficulty is moderate. Most owners leave this to a shop because it needs involved testing or specialist tools. Always confirm the cause before buying parts.
Will P24F4 clear itself?
If the fault was intermittent, the warning may go off once the computer stops seeing it, but the code stays in memory. Fix the cause; clearing the code alone doesn’t repair anything.