P24ED: NH3 Sensor Calibration Memory

What it means

The control module detected a fault involving NH3 sensor calibration memory. The code does not specify the memory technology or whether calibration is required after replacement.

At a glance

• Usually lights the check-engine light
• Affects emissions; the car can fail an emissions test

How it works

The NH3 sensor measures ammonia in the exhaust so the emissions system can assess reductant dosing and aftertreatment performance. The control module relies on calibration data stored for that sensor. P24ED sets when it detects a fault involving that calibration memory, such as data that is missing or cannot be used.

Common causes

Most likely first.

1. New sensor installed but not coded/learned to the ECM
2. Corrupted calibration memory after voltage drop
3. Module software outdated, reflash may resolve

Symptoms

• Check-engine light on
• Aftertreatment or emissions warning
• Reduced engine power
• Emissions inspection failure

How to diagnose it

1. Read all stored and pending aftertreatment codes and freeze-frame data to see whether this fault followed a sensor replacement, low-voltage event, or another emissions fault.
2. Check the NH3 sensor connector and harness for loose connections, corrosion, damage, or signs the sensor was recently replaced.
3. Use a scan tool to check whether the module reports the NH3 sensor as recognized and whether calibration or learned-status data is available.
4. Verify battery and charging-system condition and check for related low-voltage codes that could have interrupted calibration data.
5. Follow the manufacturer's procedure to perform any required NH3 sensor coding or relearn, then clear the code and confirm whether it returns.
6. If calibration data remains invalid, check for applicable module software updates and test the sensor or control module according to the manufacturer's diagnostic procedure.

Common fixes

• Complete the required NH3 sensor coding or calibration learn after replacement.
• Repair a loose, corroded, or damaged NH3 sensor connector or harness.
• Correct an underlying battery or charging-system problem that caused a voltage drop.
• Update the control module software if an applicable manufacturer update addresses calibration memory.
• Replace the NH3 sensor or repair the control module only after the manufacturer's tests identify the faulty component.

Can I keep driving?

You can often drive cautiously if the vehicle runs normally, but arrange prompt diagnosis because the emissions system may warn, limit power, or fail inspection. Do not ignore any reductant or no-start countdown warning.