After diagnostics they suggested it may be the O2 sensor as the fault was lambda related.

Hi, last week I took my BMW 320ci 2004 to my local halfords garage as the engine light was on. After diagnostics they suggested it may be the O2 sensor as the fault was lambda related. They ended up changing two as they said there was two fitted!? I took their word for it. After driving it for 20 minutes or so the engine light has now come on. The car runs fine, and was never sluggish or burning up fuel. Any help would be greatly appreciated! I'm fearing a new cat!

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[Nick at Pelican Parts]

If the code was P0420 or P0430 then it is probably a bad cat. If they were O2 sensor codes then you may have a vacuum leak or a dirty MAF but I'm only guessing

- Nick