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2018 Forester P0420 Code - Check Engine Light On - What's Next?

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Mike T. · rep 0 · May 24, 1:24 PM · 1 replies · solved · Question

Vehicle: 2018 Subaru Forester 2.5i Premium

Hey everyone, hoping for some advice here. My 2018 Subaru Forester 2.5i Premium just threw a Check Engine Light this morning. I pulled out my OBD-II scanner and it's showing a P0420 code: 'Catalyst System Efficiency Below Threshold (Bank 1)'.

The car seems to be driving fine, no noticeable loss of power or strange noises. Fuel economy might be slightly worse, but it's hard to tell for sure after only a day.

I've heard this code can mean a few different things, from a bad O2 sensor to a failing catalytic converter. What's the best way to diagnose this without just throwing parts at it? Any common issues with this specific year/model?

✓ Accepted solution

ClearTheCode Admin

ClearTheCode Team · rep 0 · May 24, 1:24 PM

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The P0420 code on your 2018 Subaru Forester indicates that the catalytic converter's efficiency is below the required threshold. While it often points to a failing catalytic converter, it's crucial to rule out other potential causes first.

Here's a breakdown of common causes and diagnostic steps:

  1. Oxygen Sensors: A faulty upstream or downstream oxygen sensor can incorrectly report catalyst efficiency. The downstream sensor (post-catalyst) should show a relatively stable voltage compared to the rapidly switching upstream sensor. If the downstream sensor mirrors the upstream, it suggests the catalyst isn't storing oxygen effectively.
  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 P0420 code. Inspect the exhaust pipes, manifold, and gaskets for any signs of leaks.
  3. Engine Misfires or Rich/Lean Conditions: Persistent engine misfires, excessive oil consumption, or an engine running too rich or too lean can damage the catalytic converter over time by overloading it with unburnt fuel or contaminants. Check for other stored codes related to misfires (P030x) or fuel trim issues.
  4. Catalytic Converter Failure: If the above checks come back clear, the catalytic converter itself is likely worn out or damaged. This is a common issue as vehicles age, especially if the engine has had previous running issues.

When to See a Shop:

  • If you're not comfortable performing detailed diagnostic checks like monitoring O2 sensor live data with a professional scan tool.
  • If you've checked for exhaust leaks and other engine codes, and the P0420 persists.
  • A professional shop can perform a backpressure test to confirm catalytic converter restriction or use specialized equipment to analyze exhaust gases more accurately. Ignoring this code can lead to increased emissions, reduced fuel economy, and potentially more severe engine problems if the underlying cause isn't addressed.
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