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2020 Ram 2500 Cummins - DEF Light, Limp Mode, P20EE Code

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Heather Y. · rep 0 · May 22, 3:30 AM · 1 replies · solved · Question

Vehicle: 2020 Ram 2500 Laramie 6.7L Cummins

Hello Ram community,

I'm having a serious issue with my 2020 Ram 2500 with the 6.7L Cummins diesel. The DEF light came on, followed by the check engine light, and now it's in limp mode, severely limiting my speed and power. I scanned it and got a P20EE code: 'SCR NOx Catalyst Efficiency Below Threshold (Bank 1)'.

I've been pretty diligent about using quality DEF and keeping the tank full. I'm worried about the cost of replacing the SCR system. Has anyone else experienced this with their newer Cummins? Is there anything I can check before taking it to the dealer? I'm currently at about 65,000 miles.

✓ Accepted solution

ClearTheCode Admin

ClearTheCode Team · rep 0 · May 22, 3:30 AM

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ClearTheCode admin answer:

The P20EE code on your 2020 Ram 2500 Cummins indicates an issue with the Selective Catalytic Reduction (SCR) system's efficiency. This system is crucial for reducing NOx emissions, and when it malfunctions, the vehicle's computer will often trigger limp mode to protect the system and ensure compliance.

Here's what typically causes this code and what you can do:

  1. DEF Quality or Contamination:

    • Cause: Even if you use quality DEF, contamination can occur if the fill cap isn't sealed properly or if the DEF itself has expired or been stored incorrectly. Low-quality DEF can also lead to crystallization and system blockages.
    • Action: Ensure your DEF is fresh and from a reputable source. If you suspect contamination, the DEF tank might need to be drained and refilled with fresh DEF. Sometimes, a full tank of fresh DEF and a few drive cycles can clear minor issues.

  2. NOx Sensors:

    • Cause: The SCR system relies on upstream and downstream NOx sensors to monitor its efficiency. If one of these sensors fails or provides inaccurate readings, it can trigger the P20EE code.
    • Action: A diagnostic scan tool capable of reading live data can help determine if the NOx sensors are functioning correctly. Erratic readings or no readings from a sensor are strong indicators of failure.

  3. DEF Injector (Doser Valve):

    • Cause: The DEF injector sprays DEF into the exhaust stream. If it's clogged, stuck, or faulty, it won't deliver the correct amount of DEF, impacting SCR efficiency.
    • Action: The injector can sometimes be removed and inspected for crystallization or blockages. Cleaning might be possible, but often replacement is necessary if it's faulty.

  4. SCR Catalyst Itself:

    • Cause: While less common at 65,000 miles if maintained, the SCR catalyst can become degraded or plugged over time, especially with consistent poor DEF quality or other system failures.
    • Action: This is usually a last resort diagnosis after ruling out sensors and the injector. Replacement is costly.

When to See a Shop: Given that your truck is in limp mode, it's highly recommended to have it professionally diagnosed as soon as possible. A dealership or a diesel specialist will have the advanced diagnostic tools to pinpoint the exact component causing the P20EE code and perform necessary repairs, which might include sensor replacement, DEF injector service, or even SCR catalyst replacement. Driving in limp mode for extended periods is not advisable.

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