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2018 Outback Check Engine Light - P0420 Code

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Brittany E. · rep 0 · May 22, 3:12 AM · 1 replies · solved · Question

Vehicle: 2018 Subaru Outback 2.5i Limited

Hey everyone,

My 2018 Subaru Outback 2.5i Limited just threw a Check Engine Light, and when I scanned it, I got 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, but the light is definitely on.

I've heard this can be anything from a bad O2 sensor to a failing catalytic converter. I'm hoping it's something simpler. Has anyone experienced this with their Outback of a similar year? What was the fix?

I'm trying to figure out if this is something I can tackle myself or if I need to head straight to the dealership. Any advice on troubleshooting steps would be greatly appreciated!

✓ Accepted solution

ClearTheCode Admin

ClearTheCode Team · rep 0 · May 22, 3:12 AM

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ClearTheCode admin answer:

The P0420 code on your 2018 Subaru Outback indicates that the catalytic converter's efficiency is below the required threshold. While the car might seem to drive fine, it's important to address this to ensure proper emissions and engine performance.

Here are the common causes and troubleshooting steps:

  1. Oxygen Sensors: Often, a failing upstream or downstream oxygen sensor can trigger a P0420. The downstream O2 sensor monitors the catalytic converter's efficiency. A faulty sensor might send incorrect readings.
    • Action: Check O2 sensor data with a scan tool (live data) to see if they are switching correctly. Look for erratic readings or slow responses.
  2. Exhaust Leaks: Leaks in the exhaust system before or near the catalytic converter can allow unmetered air in, affecting O2 sensor readings and tricking the ECU into thinking the converter is inefficient.
    • Action: Visually inspect the exhaust system for cracks, rust, or loose connections. Listen for hissing sounds.
  3. Catalytic Converter Failure: Over time, catalytic converters can degrade due to age, rich fuel mixtures, or contaminants. This is the most expensive potential cause.
    • Action: A professional can perform a backpressure test or use an infrared thermometer to check the converter's temperature differential.
  4. Engine Misfires/Fuel System Issues: Persistent misfires or issues like leaky fuel injectors can send unburnt fuel into the converter, causing it to overheat and fail prematurely.
    • Action: Ensure there are no other active misfire codes (P030x) or fuel trim issues.

When to see a shop: If you've checked for exhaust leaks and confirmed O2 sensor functionality (or replaced them without success), it's highly recommended to take your Outback to a qualified mechanic or Subaru dealership. Diagnosing a failing catalytic converter accurately requires specialized tools and expertise. Driving with a failing converter can lead to increased emissions and potentially impact fuel economy in the long run. They can perform comprehensive diagnostics to pinpoint the exact cause and recommend the most effective repair.

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