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

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.

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

Symptoms

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

Most 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

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 P24F5 means

• P: Powertrain: engine and transmission
• 2: Generic code: same on every make
• 4: Emissions
• F5: Specific fault in that system

Frequently asked questions

Can I keep driving with code P24F5?

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 P24F5?

The most common causes are: 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.

Related codes

• P24F4NOx Sensor Delayed Response - High to Low (Bank 1 Sensor 1)
• P22FDNOx Sensor Performance - Slow Response High to Low (Bank 1 Sensor 2)
• P22FCNOx Sensor Performance - Slow Response Low to High (Bank 1 Sensor 2)
• 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.