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Thread: CEL P0300 random misfire

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    Member Scottobot's Avatar
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    Exclamation CEL P0300 random misfire

    Threw a CEL shortly after startup and pulling out of the parking lot after work today. Car is running very smoothly.. no evidence of performance issue. I googled a bit and found a suggestion to clean battery terminals. I checked and one of them did have corrosion on it so I cleaned it up. Any other recommendations? I don't want to take it to the dealer to have gen charge me a $100 diagnostic fee and dismiss it as a random one-off.

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    Sr Member Flagrum_3's Avatar
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    Default Re: CEL P0300 random misfire

    Quote Originally Posted by Scottobot View Post
    Threw a CEL shortly after startup and pulling out of the parking lot after work today. Car is running very smoothly.. no evidence of performance issue. I googled a bit and found a suggestion to clean battery terminals. I checked and one of them did have corrosion on it so I cleaned it up. Any other recommendations? I don't want to take it to the dealer to have gen charge me a $100 diagnostic fee and dismiss it as a random one-off.
    You googled it? P0300 is caused by random misfiring and has zero to do with the battery connections. Check your plugs first or change them out , then check your coil packs. After that you'll have no choice but to take it in and have it properly diagnosed.

    p.s It is considered a serious issue, as it could cause drivability issues, so don't sit on it.


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    Default Re: CEL P0300 random misfire

    Thanks for the advice. Yep, found the battery reco on Google/reddit:

    https://www.reddit.com/r/MechanicAdv...3_mazdaspeed3/

    This has every indicator of a bad battery. The ignition coils will often not fire correctly because the low voltage is affecting the control system. Get the battery checked for free at any parts place. Batteries will do very weird things when they are going out. Remove the battery terminals and clean them thoroughly. Tighten them well. This may cure it and it is free to try.

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    Default Re: CEL P0300 random misfire

    If you aren't experiencing issues with other systems misfires are typically plugs, coils, wires (if present) or 02 sensor related.

    If you're blaming a misfire on a bad battery you really need to look into the full charging system too...

    That said... plugs are cheap and easy to DIY.

    It's also easy to swap coil packs around, though right now that's a stab in the dark if it's not throwing a cylinder specific code with it. Check the cabling going to the coilpacks and their connections.

    I've had random misfires attributed to 02 Sensors on their way out as well. Good enough to work and not throw a code for itself but bad enough to start messing with the ECM and how it thinks it's controlling the engine.

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    Default Re: CEL P0300 random misfire

    So the dealer is saying it's the EGR valve and it will cost $750 to fix it. Surprising.. does that sound reasonable?

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    Default Re: CEL P0300 random misfire

    Get a second opinion and quote at a private mechanic to compare.


    Sent from my iPhone using Tapatalk

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    Default Re: CEL P0300 random misfire

    Quote Originally Posted by Jackal View Post
    Get a second opinion and quote at a private mechanic to compare.


    Sent from my iPhone using Tapatalk
    ^I second this advice..

    An EGR cost like $80 list, so they would like you to pay $700 labour to replace it...Total scam!

    Are you getting any CELs other then the P0400? A bad egr would give you a P0401 code.

    You could try cleaning the EGR and other things plus check connections before doing anything else, there are vids on YouTube showing how to do so, but definitely get a second or third opinion.


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    Default Re: CEL P0300 random misfire

    OEM EGR costs $500 and 2.5 hours labour, verified with two other dealers. Guess I'm screwed.

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    Default Re: CEL P0300 random misfire

    Quote Originally Posted by Scottobot View Post
    OEM EGR costs $500 and 2.5 hours labour, verified with two other dealers. Guess I'm screwed.
    God man, point was; Don't go to the dealer!

    You can get an OEM replacement EGR anywhere for $80 and up and labour costs at half or less. But get a second opinion first somewhere you can trust.


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    Default Re: CEL P0300 random misfire

    YOU MEAN THE EGR valve at the back of the engine near the turbo inlet hose? Then yes, that one is hella expensive. Still not 2.5 hours worth of labor. I know the book time is one thing but it was easy to change. It was messy because of the coolant that drained out. Just have to be quick about it and it's not so bad. If you are referring to the one by the intercooler, then no. That one is cheap as hell and SUPER easy to change
    1991 Nissan Pathfinder SE-V6 Red/red interior - trade in
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    Default Re: CEL P0300 random misfire

    EGR valve, yes, the pricey one.

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