Misfire and emissions light

[Alias]

Whilst doing work to get the car through the MOT we've noticed that No-name appears to have a intermittant misfire and twice now the emissions light has flashed up and gone away again. When plugged in to diagnostics its showing a P0300 error - random misfire. The car also seems to surge a little when running from cold but this seems to clear up very quickly as she warms up. the misfire doesn't happen every time and she can start fine from cold one day and not the next.

We (well, Banderson) has tried a spare coil pack but that didn't seem to help so I guess thats one part ruled out. Looking through previous threads people seem to suggest that it could be;
HT leads
Plugs
Injectors
Lambda's
Cat

I'm presuming (possibly wrongly) that it wouldn't be the injectors as I'd imagine them to bring up a cylinder specific code instead of the random misfire. Does anyone have any other suggestions or recommendations of what's most likely to be causing it and the best order to try changing things in. Just spent £300 getting it through the MOT and have also got to get the Micra done this month so don't have the cash to just start randomly buying parts to chuck at her in the hope of getting it sorted. Any recommendations are greatly appreciated.

 

[ben lee]

lambda and cat wont cause a miss fire.

would start with a new set of spark plugs.

when it missfires does it do it for awhile on idle?
are you looseing coolant?

 

[LukeW]

I had a problem like this on mine, misfiring and emissions light. I thought it was due to the new decat exhaust I had on, turns out it was a faulty injector.

Changed the faulty one and that got rid of that! But the emissions light is still on as I don't know why! I wish it would go out! Does it self cancel?

 

[niall.pearce.7]

I had a problem like this on mine, misfiring and emissions light. I thought it was due to the new decat exhaust I had on, turns out it was a faulty injector.

Changed the faulty one and that got rid of that! But the emissions light is still on as I don't know why! I wish it would go out! Does it self cancel?

The Emission Light will stay on because of the decat. Same on mine.

 

[LukeW]

The Emission Light will stay on because of the decat. Same on mine.

I have taken the decat off, the cat and standard system is back on but the light is still on

 

[niall.pearce.7]

How long have u had it off? When I took mine off for its MOT left it off for a month. Light went out after about 2 weeks.

 

[LukeW]

Nearly a month ago I'd say...

(P.S Sorry OP for slightly hijacking)

 

[ben lee]

get the codes read and reset.

Nearly a month ago I'd say...

(P.S Sorry OP for slightly hijacking)

 

[Alias]

Not loosing any coolant, it does misfire for a while on idle - but sometimes it does it and other times its fine.

Think its probably had the problem for a while now, the exhaust does smell a little rotten so I imagine the misfire has knackered the cat. Also smells like its running a bit rich and economy is a bit down.