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2018 Ram 1500 Laramie - P0420 Code and Check Engine Light

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Melissa G. · rep 0 · May 22, 3:30 AM · 1 replies · solved · Question

Vehicle: 2018 Ram 1500 Laramie

Hey everyone,

My 2018 Ram 1500 Laramie, with about 85,000 miles, just threw a Check Engine Light. I hooked up my OBD-II scanner, and it's showing a P0420 code: "Catalyst System Efficiency Below Threshold (Bank 1)."

The truck seems to be running fine, no noticeable loss of power or strange noises. Fuel economy might be slightly worse, but it's hard to tell for sure. I've heard this could be anything from an O2 sensor to a failing catalytic converter.

Has anyone else experienced this with their Ram? What was the fix? I'm trying to figure out if this is something I can tackle myself or if it's definitely a shop visit.

✓ Accepted solution

ClearTheCode Admin

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

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ClearTheCode admin answer:

The P0420 code on your 2018 Ram 1500 Laramie indicates that the catalytic converter on Bank 1 is not operating at optimal efficiency. While it can sometimes be a false alarm, it often points to a real issue. Here's a breakdown of common causes and steps:

  1. Oxygen Sensors: The most common culprits are the upstream (pre-cat) and downstream (post-cat) oxygen sensors. A faulty downstream O2 sensor might incorrectly report the catalyst's efficiency. A failing upstream sensor can cause the engine to run rich or lean, damaging the converter over time.
  2. Exhaust Leaks: Leaks in the exhaust system before or near the catalytic converter can allow unmetered air in, skewing O2 sensor readings and triggering the code.
  3. Catalytic Converter Failure: Over time, catalytic converters can become clogged or internally damaged, losing their ability to convert harmful emissions. This is often the most expensive fix.
  4. Engine Misfires/Running Rich: Persistent misfires or an engine running excessively rich can introduce unburnt fuel into the catalytic converter, causing it to overheat and fail prematurely.

What to do:

  • Inspect for Exhaust Leaks: Visually check the exhaust system for any cracks, holes, or loose connections, especially around the manifold and catalytic converter.
  • Check O2 Sensor Data: If your scanner allows, monitor the live data for both upstream and downstream O2 sensors on Bank 1. The upstream sensor should fluctuate rapidly, while the downstream sensor should show a relatively steady, higher voltage if the converter is working correctly. If the downstream mirrors the upstream, the converter is likely inefficient.
  • Consider a Fuel System Cleaner: In some mild cases, a high-quality fuel system cleaner designed to clean catalytic converters might offer a temporary solution, but it's not a guaranteed fix for a failing converter.

When to see a shop:

If after checking for exhaust leaks and monitoring O2 sensor data, the P0420 persists, it's highly recommended to take your Ram to a qualified technician. They have advanced diagnostic tools to perform back-pressure tests on the catalytic converter and more accurately diagnose O2 sensor health. Replacing a catalytic converter is a significant repair, and proper diagnosis is crucial to avoid unnecessary expenses.

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