Engine light - 300 & 304 code? - Mercedes-Benz Forum
 
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#1 (permalink) Old 09-21-2009, 04:29 PM
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Engine light - 300 & 304 code?

Under WOT I got a blinking engine light - after investigating I got a 300 code (misfire) and a 304 code (cylinder 4 misfire code) which is scaring me. I reset the codes with my obd2 tool and havn't seen any other issues. Could this be an issue caused by a faulty o2 sensor or the notorious maf? I have about 48k miles and the engine seems to run fine but power is down. Any ideas will be helpful - thanks

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#2 (permalink) Old 09-22-2009, 12:21 PM
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Originally Posted by leomd View Post
Under WOT I got a blinking engine light - after investigating I got a 300 code (misfire) and a 304 code (cylinder 4 misfire code) which is scaring me. I reset the codes with my obd2 tool and havn't seen any other issues. Could this be an issue caused by a faulty o2 sensor or the notorious maf? I have about 48k miles and the engine seems to run fine but power is down. Any ideas will be helpful - thanks
* Misfire damages TWC (THREE WAY CATALYTIC CONVERTOR); indicates a misfire
* Misfire cylinder 4 damages TWC (THREE WAY CATALYTIC CONVERTOR); pinpoints the misfire.

It seems to me, that your cats are taking a beating if not on there way out. O2 sensor could be the culprit. Depending on your MAF history (OEM, replaced or cleaned), it may also be the MAF. BTW, my MAF failed at 42K mi. Nonetheless, I would have a professional look at it before you completely damage your catalytic converter(s). Good luck.

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#3 (permalink) Old 09-22-2009, 01:14 PM
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I doubt it's the MAF (both banks would have problems) or the O2 sensor (it wouldn't be just one cylinder. A single instance of misfire won't hurt anything, the ECU likely shut down the offending cylinder (the flashing light says Hey, shut down so I can reset) to preclude damaging the cat, too.

I'd suggest getting it on a regular scanner that can give you some real-time feedback and LTFT data. If the LTFT data look good, you can further discount the MAF.

If the scanner can isolate cylinders, you may be able to truly pinpoint the issue.

I'd probably be tempted to take it to a good indy that has an appropriate setup (doesn't have to be the SDS) and can help you diagnose it. Either that or start throwing parts at it, which will get expensive, frustrating and time-consuming in a big hurry.

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#4 (permalink) Old 10-07-2009, 08:20 AM
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For the rest of us, LTFT data? I have the AutoTap OBD II program which hooks up and runs through my laptop. It reads and records real time data (OBD II requirements only - not the double secret password MB star data). Anyway, without knowing what the normal parameters are for the given readings - the data doesn't mean much. Is the 'normal range' data available anywhere? Great thread this one, thanks. BTW '01 E55 20,800 miles.
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