P24F4: 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. 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.

Bank 1 is the side of the engine with cylinder 1. Inline engines only have this bank.

Symptoms

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

Most 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

What to check, in order

Start with the simplest. You can tick off each step.

1. Read the codes and the NOx data before and after the SCR catalyst
2. With the connector unplugged, check whether the signal wire is making contact with ground.
3. Check the sensor's connector and wiring
4. Look for exhaust leaks ahead of the sensor
5. Confirm that the AdBlue/DEF is of good quality
6. Replace the sensor only after ruling out power, ground, and leaks

Good to know

• Repair difficulty: medium
• It affects emissions.

What each character of P24F4 means

• P: Powertrain: engine and transmission
• 2: Generic code: same on every make
• 4: Emissions
• F4: Specific fault in that system

Frequently asked questions

Can I keep driving with code P24F4?

You can usually keep driving in the short term, but get it repaired soon: on many diesels, the warning eventually limits starting or engine power.

What causes code P24F4?

The most common causes are: 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.

Related codes

• P24F5NOx Sensor Delayed Response - High to Low (Bank 1 Sensor 2)
• P22FANOx Sensor Performance - Slow Response High to Low (Bank 1 Sensor 1)
• P22F9NOx Sensor Performance - Slow Response Low to High (Bank 1 Sensor 1)
• P23C2NOx Sensor Delayed Response - High to Low (Bank 1 Sensor 3)

Sources

• law.cornell.edu: 13 CCR 1968.2
• gsi.ext.gm.com: 25OBDG04A
• hella.com: nitrogen oxide sensor nox sensor

Values and procedures vary by manufacturer. If it affects brakes, steering, fuel or high voltage, see a professional. Updated: 2026-11-04. How we build these pages.