97 528i Fault Code 420

[cw_cannon]

On my 97 528i the check engine light comes on and it is showing a fault code of 420 which I'm told means more uncombusted gas than the allowable tolerance is passing through the catalytic converter. The car runs great and the mpg are narmal. My mechanic told me that there are 4 sensors that send this code- he said he checked the sensors and they checked out ok, but if it kept coming on we could replace them all @ $145 each sensor or we could try replacing one at a time until the check engine light stays off for good. I told him to just reset the light and we'd see if it came back on... and low and behold 200 miles later it's back. Any ideas... it seems that there would be a better way to target a bad sensor than just trial and error. Anyone else had a similar problem?

[billB]

CW to the forum

My car has identical set up; I bought 4 Bosch O2 sensors from this guy below for $90 each; had no problems since last year.
http://www.alleurasianautoparts.com/

And yes there are different codes to pin point the bad sensors like PRECAT and POST CAT codes Just make sure that is what you need.

Good luck
billb

[grtmoby]

fault code 420 means: Catalyst System Efficiency Below Threshold. So it is either the O2 sensors or the catalyzer itself....

[mrogers]

I'm thinking I may have a similar problem because my light came on last week and I've been smelling a bit of gas from the exhaust. This may be a dumb question, but is there any easy way to get the fault codes from the E39 without a tool? Like on the E34 for example, you put the key in aux position and then stomp on the pedal five times and it will blink the code at you. Is there a trick like that for the E39?

[cw_cannon]

Thanks for the info. What's weird is that my wife drove my car the other day and filled it up with 87 octane and the light has since went off. Hopefully it will stay off...

[grtmoby]

Originally posted by cw_cannon@Dec 26 2004, 06:01 PM
Thanks for the info. What's weird is that my wife drove my car the other day and filled it up with 87 octane and the light has since went off. Hopefully it will stay off...

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My guess is that if u stopped using the 87 octane it wil come back again. This makes me think besides the catalyser or O2 sensor that there might be an error in the ignition system including ignition timming...

[mum528]

I am having exactly the same problem with my 97 528. I hate to just start changing sensors until the light stays out especially since the performance is great.