P24ED: NH3 Sensor Calibration Memory
The NH3 slip sensor stores its individual calibration coefficients in non-volatile memory (EEPROM/flash). The ECM cannot read valid calibration values — either checksum fails, the memory is empty after replacement, or communication with the sensor electronics is broken. Without the trim values the raw ammonia reading is unreliable.
What P24ED means
P24ED is a generic OBD-II powertrain code in the emissions group.
- 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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Common causes
- 01New sensor installed but not coded/learned to the ECMMost likely
- 02Corrupted calibration memory after voltage dropPossible
- 03Module software outdated, reflash may resolveLess likely
Can I drive with P24ED?
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 P24ED
- 1
Confirm the code and save freeze-frame data
Read the codes with an OBD-II scan tool and note every code, not just P24ED. Save the freeze-frame data (speed, load, temperature when the fault set) before clearing anything; it tells you when the fault happens.
- 2
Fix other faults first
If other codes are stored, start with power-supply, sensor-circuit and communication codes. They can cause this code as a side effect.
- 3
Check live data
Watch the related sensor and actuator readings in live data (NH3 sensor, Sensor eeprom) and compare them with expected values in the service information for your vehicle.
- 4
Test the likely causes in order
Start with the most likely cause and work down: New sensor installed but not coded/learned to the ECM; corrupted calibration memory after voltage drop; module software outdated, reflash may resolve. 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.
P24ED 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
- NH3 sensor
- Sensor eeprom
P24ED FAQ
What does P24ED mean?
P24ED means “NH3 Sensor Calibration Memory”. The NH3 slip sensor stores its individual calibration coefficients in non-volatile memory (EEPROM/flash). The ECM cannot read valid calibration values — either checksum fails, the memory is empty after replacement, or communication with the sensor electronics is broken. Without the trim values the raw ammonia reading is unreliable.
Is P24ED serious? Can I keep driving?
We rate P24ED 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 P24ED?
The most common causes, from most to least likely: New sensor installed but not coded/learned to the ECM; Corrupted calibration memory after voltage drop; Module software outdated, reflash may resolve. Test before replacing parts, because several causes can set the same code.
How much does it cost to fix P24ED?
Typical labor is about $40–$190 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 P24ED 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 P24ED 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.