Why Chrysler Removed the Ammonia Sensor and What U12A4 Means for Your ECM

In recent years, many Dodge and RAM 3500/4500/5500 trucks equipped with the 6.7L Cummins diesel engine have experienced emissions-related recalls—one of the most notable being Recall S86 and fault code U12A4.

If you've seen this code or had your ammonia sensor removed by a dealership, you're not alone. In this blog, we explain why Chrysler removed the ammonia sensor on certain vehicles, what code U12A4 indicates, and how to resolve issues caused by outdated calibrations.

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🔧 Why the Ammonia Sensor Was Removed

Between 2013–2015, RAM Cab Chassis trucks were equipped with an ammonia sensor module as part of the SCR (Selective Catalytic Reduction) system. This sensor helped detect NOx emissions and regulated diesel exhaust fluid (DEF) dosing.

However, many of these sensors failed prematurely or delivered unreliable data, leading to excessive warranty claims and inconsistent drivability. Chrysler responded with Emissions Recall U84 and later Recall S86, which included:

• Permanent removal of the ammonia sensor and its associated module
• Replacement of the SCR catalyst
• ECM (or PCM) reprogramming with updated calibration that no longer expects input from the ammonia sensor

In short, the ammonia sensor system was eliminated from the emissions control strategy, and the engine computer was updated to run without it.

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📍 Where Is the Ammonia Sensor Located?

On affected Dodge and RAM 3500/4500/5500 trucks, the ammonia sensor is mounted on top of the SCR (Selective Catalytic Reduction) catalyst, which is part of the vehicle’s exhaust aftertreatment system. It connects via a short harness to the ammonia sensor module, which is secured to the frame rail nearby. This setup was originally designed to monitor emissions performance but has since been phased out on many vehicles following emissions-related recalls.

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⚠️ What Is DTC U12A4?

If your vehicle was not properly reprogrammed or the wrong calibration was flashed, the Powertrain Control Module may still expect communication from the now-removed ammonia sensor. This results in the following DTC:

U12A4 – Lost Communication With Ammonia Sensor

Even if the sensor was physically removed as part of the recall, failing to install the correct calibration will cause this fault to persist, and may affect DEF dosing or emissions readiness.

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✅ The Fix: Reprogramming the ECM With the Correct Calibration

To resolve U12A4 and ensure proper emissions operation:

Verify whether your truck previously had the ammonia sensor removed.
(This can be confirmed through recall history or visual inspection.)

If U12A4 is active, it likely means the ECM was not flashed with the correct non-ammonia calibration.

Reprogram the ECM with the correct software version for vehicles without the ammonia sensor.
This will remove the dependency on ammonia sensor data and prevent U12A4 from setting.

At ARES Technologies, we supply rebuilt and pre-programmed ECMs that include the latest Chrysler calibrations—configured for both ammonia-sensor and sensor-delete configurations. If you provide us with the VIN and emission setup, we’ll ensure you receive the correct software for your application.

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ARES Technologies Can Help

If you're dealing with:

DTC U12A4 after a recall

A replaced SCR system with no ammonia sensor

A mismatched ECM calibration

We can program a replacement ECM or flash your current unit with the correct software to clear the fault and restore emissions compliance.

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