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Discussion Starter #1 (Edited)
Disclaimer: I'm a complete novice. The story: about 3 weeks ago my SES light came on (2003 530i). I checked the error code using the BMW/Mini scanner I purchased last year. It read "insufficient heat output, oxygen sensor before catylitic converter, bank 1". The light turned off after just a day or so, but yesterday it returned. This time the same error code came back, plus an additional code reading "misfire, cylinder 3".

Regarding the O2 error- based on the research I was able to do here on these forums, I'm thinking that I will start by getting a vacuum test done for leaks before replacing the O2 sensors.

Regarding the misfire- I'm wondering if this is a related or a separate issue. Last January, I had one instance of a cold start where the engine started really rough sounding. We don't get too many cold mornings here in CA, but this was a particularly cold one. The car started fine on the second attempt, and never happened again the rest of the winter. However, it also triggered the SES light with the error code reading a misfire on cylinder 3. The SES cleared after a couple of days.

This time however, the car has been running fine - no rough starts, and fairly decent gas mileage - between 20-21 mpg on mostly city driving. I was surprised to see there had been a misfire in addition to the O2 sensor code previously mentioned.

Any thoughts or advice for me?

Thanks!!
-Ben
 

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Swap coils, see if the misfire follows the coil.

The sensors have heaters in them. Perhaps yours is bad. How many miles on them?

Does your scan tool report short term fuel trim? If so, and it is a large positive value (circa 10), you may have a vacuum leak.
 
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