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2018 CX-5 Check Engine Light - P0420 Code - Catalyst System Efficiency Below Threshold

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Mike T. · rep 0 · May 25, 3:31 AM · 1 replies · solved · Question

Vehicle: 2018 Mazda CX-5 Grand Touring

Hey everyone,

My 2018 Mazda CX-5 Grand Touring, with about 75,000 miles, just threw a check engine light. I used 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 also seems normal for now. I've tried clearing the code once, but it came back after about 50 miles of driving.

Could this really be a failing catalytic converter already, or are there other common causes for P0420 on these CX-5s that I should check first? Any advice on diagnosing this further before I take it to a shop would be greatly appreciated. Thanks!

✓ Accepted solution

ClearTheCode Admin

ClearTheCode Team · rep 0 · May 25, 3:31 AM

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ClearTheCode admin answer:

The P0420 code indicates that the catalytic converter's efficiency is below the required threshold, meaning it's not effectively converting exhaust gases. While it often points to a failing converter, several other issues can trigger this code.

  1. Check for Exhaust Leaks: Inspect the exhaust system for any leaks, especially before and after the catalytic converter. Leaks can introduce oxygen, confusing the O2 sensors and leading to a false P0420 code. Listen for hissing sounds or look for soot marks.
  2. Inspect O2 Sensors: The upstream (pre-cat) and downstream (post-cat) oxygen sensors work together to monitor converter efficiency. A faulty or sluggish O2 sensor, particularly the downstream one, can inaccurately report efficiency. You can monitor their live data with a good scan tool to see if they're switching correctly.
  3. Engine Performance Issues: Problems like misfires (P030x codes), excessive oil consumption, or a rich fuel mixture can damage the catalytic converter over time by sending unburnt fuel or contaminants into it. Ensure your engine is running optimally.
  4. Fuel Quality: Poor quality fuel or consistent use of fuel not recommended for your vehicle can sometimes contribute to converter issues.

When to see a shop: If you've checked for exhaust leaks and O2 sensor data looks normal, but the code persists, it's advisable to visit a qualified technician. They have specialized diagnostic tools and experience to perform a back-pressure test on the catalytic converter and more accurately determine if it's truly failing. Replacing a catalytic converter is a significant expense, so proper diagnosis is crucial to avoid unnecessary repairs. Ignoring a P0420 can lead to increased emissions and potentially fail an inspection.

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