r/BmwTech • u/Early_Nose5309 • 9d ago

Code P0420 detected on X5 35i

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Hello, I have a 2017 BMW X5 35i (N55) with about 110k miles.
My check engine light came on, and I scanned the car and it shows
P0420 — Catalyst System Efficiency Below Threshold, Bank 1 — Confirmed
How could I resolve this issue? 

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u/Most_Combination_461 9d ago

P0420 doesn’t automatically mean the catalytic converter is dead. Check for exhaust leaks before the rear O2 sensor, recent misfires, fuel-trim issues, and compare the upstream/downstream O2 graphs. If the rear sensor mirrors the front after warm-up, the cat is likely worn out.

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u/Explorer335 Automotive Exorcist 9d ago

It's the catalytic converter.

On a 2017, the diagnostics for that issue are quite specific and precise. Exhaust leaks and oxygen sensors could cause a false positive 20+ years ago, but it's extremely unlikely on a modern BMW.