I have a regular EML P0421 coming on, replaced plugs, coil packs and Lambda sensor.
The car also has a “lumpy” tick-over, it misses a beat quite frequently. When being driven, it stutters briefly under acceleration in any gear and at any rev range. The frequency varies but can be quite an issue.
Any suggestions much appreciated, I can’t even be sure they’re linked.
has the car been used, or has it sat around for sometime?
as always, nice clean fuel, and a quick run is the first step
Gremlin - used almost daily, as are most of the revs available
That code means your cat is not operating efficiently.
This could be down to one of the O2 sensors or the cat it’s self. I see you have already changed one O2 sensor, but not sure which one, but it might be worth looking at the other one.
Also has this car had a number of cold starts or short drives where it has not fully warmed up? It may just need a good drive.
This shouldn’t really affect the running of the engine to much, but there are rumours that the cat efficiency can affect fuelling, but I’m unable to confirm this and if it did, I would imagine it wouldn’t be much.
A Peugeot my wife had came up with a lambda sensor issue and it was duly replaced.
The problem soon returned though.
A bit of googling revealed that it’s well worth giving the connector a good wiggling.
Problem solved…
I’m not saying this will be the issue but its worth a try.
Hi Nick… What milage as the car done as it may be your injectors require testingand cleaning,have youhhad a obd2 check tosee if a fault shows.
Hi Tony - only 55k, yes it’s been “OBD’d” often and all we get is P0421. The engine lights off at the moment, I had it “de-carboned” yesterday and it was fine for a few miles. I had a totally knackered spark plug, the coil pack was sooted up too so I’m wondering if there’s damage to the pre-cat or might it be an accelerator issue, or both!!
Soot on the coil pack suggests sparking there from a poor connection so that might have caused a misfire and that in turn could cause cat overheat from unburnt fuel getting to it. (No experience of NCs so I don’t know how likely that is to damage the cat. )
Thanks Martin, that seems to be the current theory.
Does anyone know if a knackered pre-cat is likely to cause the hesitation on tick-over and under acceleration (only when the engine is hot).
I’d expect nothing less from a bloody Peugeot! 
Update:
So we’ve replaced the manifold with a cheapo from eBay losing the pre-cat which had a hole, presumably from the damaged plug.
That seems to have got rid of the Engine Warning Light. But the engine still seems to “miss a beat” both on tick-over and briefly when accelerating.
The guys at S.P.A. Ilkeston said there’s a small hole in the cat as well. Might this be the source of the misfire?
Help, the sun’s out and I want the car running smoothly.
PS - the cheap Manifold off eBay (Toyo sports) went on easily, great fit and seems ok.
UPDATE
So the "misfire has been sorted , car now runs well. I hoped that was the end of it but no. The EML is back on showing good old P0421.
Both sensors replaced so it looks like the Cat unless anyone has any other ideas that might save me a lot of money?
Just read the thread Nick
Given the likely misfire from your original problem, I would have expected the resultant unburnt fuel to damage the front cat(maniverter). Neat fuel ignites and superheats the ceramic cat core, melting it and/or breaking it up. Has this part been replaced/removed? If so there is the possibility that debris from the maniverter has partially blocked the second, underfloor cat.
What is the flow from your or rather your car’s exhaust like.
Hi Rhino666
It wasn’t actually a misfire, one of the many sensors had gone (don’t ask which) but the damaged spark plug may well have inflicted the damage you’re referring to. Part of the plug disintegrated but doesn’t seem to be in the engine.
The flow is fine, as is the car, it drives really well (I think).
Well you could try again on giving the sensor connectors a clean and a wiggle, as per my earlier post.
Allegedly, it doesn’t take much to impair the electrical contact as the currents involved are so small.
999to5
Will do. Having emissions tested today to see if it is the Cat.
Turning into a nightmare!!!
I’m getting conflicting advice on the exhaust/Cat system. I’ve already lost the front (pre-cat) one with a new Manifold. So if it is the Cat, it’s the one mid system. One view is that this is the main Cat, another is it’s front one.
Can I get away without the front (manifold) Cat and have a new one in the middle of the system? If so, any recommendations which one to go for?
Prices vary from £150 - £800, what’s a non technical mx5 driver to do? All I want to do is fix it and drive it.
Workshops have live diagnosis systems which can indicate what is happening as the engine is running.
Maybe this might be helpful.
I am curious about losing the front sensor with a new manifold. How do you prevent the ecu from throwing up an error due to no signal from that sensor??
I think (hope) they moved the sensor.
Just got back from the test and the Cat is working as it should, passed the emissions easily.
Anyone want an MX5?
Did you replace your plugs like for like or according to spec?
Someone else on here has discovered they had the wrong plugs in, as the 1.8 and 2.0 engines have different plugs.
The 2.0 plugs are a fair bit longer, so 1.8 plugs will spark inside the threaded ‘plug hole’ and I reckon the compression would be low too.
It was apparently a dealer who fitted the wrong plugs too…