E39 528i Error Codes for Catalytic Converter - Solved!
Take this information for what it may be worth in your instance: My E39 check engine light came on when the vehicle was out of warranty... naturally. The dealer ran the code and said that I needed an O2 sensor replacment. And $300+ later the light was extinguished but only for about a week.
Back to the dealer. This time the dealer said that the new code was for a defective catalytic convertor itself. This time it would be about $1,900 for the pair! Screw it! I set out to find an aftermarket set for much less money, maybe $500 installed for a welded in pair. Not my idea of a proper BMW repair though. In the meantime, I did some research on the web. One obsure post said to try some fuel injector cleaner. What the heck, I did. No results with the economy brand cleaner.
After another month, the orange check engine was just downright annoying. Then for some reason, I sprung for a bottle of top-shelf fuel injector cleaner and got some Techron brand cleaner. I put it in and much to my amazement, the light went off in about a half a tank of driving! And stayed off!
I will admit that I add some Techron every month or so just out of anxiety. I hope to never see that orange light on again. To me it looks like $1,900 saved as long as that damn light stays off. Good Luck with your catalytic error code situations! I would highly recommend trying this inexpensive solution first before shelling out $1,900. Posted by Don Ladanyi
PS. I am not affiliated with Chevron or Techron in any way.