Ok so OBD2 code reader arrived. Fault appears to be an engine misfire, am I right in thinking this is likely to be spark plugs?
I have added below the freeze frame data if this means anything to anyone?
Thanks for looking!
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What causes the P0300 code?
- Spark plugs that are damaged or worn
- Spark plug wires and/or coils that are damaged or worn
- Distributor cap that is damaged or worn (only if applicable to the vehicle)
- Rotor button that is damaged or worn (only if applicable to the vehicle)
- Fuel injectors that are defective
- EGR valves or tubes that are clogged
- Ignition timing that is off
- Vaccum leaks
- Fuel pressure that is low
- Head gasket(s) that are leaking
- Distributor cap that is cracked
- Camshaft sensor that is faulty
- Crankshaft sensor that is faulty
- Mass air flow sensor that is faulty
- Oxygen sensor that is faulty
- Throttle position sensor that is faulty
- Catalytic converter that is faulty
- PCM is faulty
What are the symptoms of the P0300 code?
- Check Engine Light that is on or flashing
- Car may take longer to start up, or does not start up at all
- Car dies while stopped
- Car idles rough
- Car may hesitate while accelerating
- Lack of power while driving
- Increase of fuel consumption
How does a mechanic diagnose the P0300 code?
- Uses OBD-II scanner to retrieve freeze frame data and all of the trouble codes that have been stored by the Primary Control Module (PCM)
- Completes a test drive of the vehicle to see if the P0300 trouble code comes back
- Reviews live data to identify which cylinders are showing misfire data
- Inspects spark plug wires or coil packs for wiring that is damaged or worn
- Inspects spark plugs for excessive wear, cracks, or breaks
- Inspects coil pack wiring at the harness and connector, for breaks, or corrosion
- Replaces spark plugs, spark plug wires, coil packs, coil pack wiring at harness and connector, as necessary
- If the P0300 trouble code is still present after the spark plugs, spark plug wires and/or coil packs have been replaced, they will inspect the fuel injectors and fuel injector system for faults
- Older vehicles have a distributor cap and rotor button system. For these vehicles, the mechanic will inspect the distributor cap and rotor button for damage, cracks, or excessive wear
- If other related trouble codes have been stored by the PCM, the mechanic will need to diagnose and repair these issues as necessary. Test drive the vehicle again to see if the P0300 trouble code comes back.
- If the P0300 trouble code returns, the compression system should be checked. A fault in the compressor is rare but possible for this code.
- If the P0300 trouble code still persists, there could be a problem with the PCM. This may require replacing or reprogramming. This is also rare.
Common mistakes when diagnosing the P0300 code
One common mistake is ruling out the possibility of a faulty cylinder, fuel injector, or PCM. It is also common that the diagnosis and repair of other related trouble codes, is not completed.
Other related trouble codes, a faulty cylinder, a faulty fuel injector(s), and/or a faulty PCM can all cause the misfiring problem.
How serious is the P0300 code?
The P0300 trouble code is considered serious. The reason for this is the resulting driveability problems that are present when this trouble code is stored in the PCM. The car may not start and/or cause hazardous conditions during the operation of the vehicle. For these reasons, the P0300 trouble code requires immediate attention.
What repairs can fix the P0300 code?
-
Repairing vacuum leaks
-
Repairing or replacing leaking head gaskets
-
Diagnosing and repairing other related trouble codes
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Replacing distributor cap and rotor button, as well as wires, coils, and plugs, (only if applicable to the vehicle)
-
If necessary, repair fault internal engine components.
-
Replacing engine (if cylinder damage exists)
Additional comments for consideration regarding the P0300 code
Do not assume that the spark plugs and wires are responsible for the P0300 trouble code. There are other components of the system that could cause this error code that should be diagnosed and inspected prior to repair.
Also, there are other related trouble codes that could cause the P0300. After repairs, if the P0300 trouble code returns, inspect and diagnose these other trouble codes if applicable. Always test drive the vehicle to see if the P0300 trouble code returns after making any repairs.
Make sure the spark plugs are gapped properly and with precision. To gap the spark plugs properly, it will be necessary to use a feeler gauge.
If the spark plug gap is not precise, this will cause a misfire.The vehicle manufacturer provides the spark plug gap specifications. These specifications can be found on a sticker under the hood of the vehicle or obtained from any local part store.
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Thank you so much, I didn’t realise one could look up the codes, behold my ignorance!
This all seems rather more concerning than I expected, I imagined I could just change the plugs…
Seems like the most expensive part will be a mechanic’s time eliminating all possible issues?
Seems like the most expensive part will be a mechanic’s time eliminating all possible issues?
Oh yes…he’ll love you to bits if he is a charlatan
Only 1 of the massive list will be your problem.
Start with stuff you can easily check without new parts expense.
I’d pop back to this Colin guy with what you now know and get him to have a look at the plugs, CAS, COPs etc. He’ll know…
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Thanks, I’m sure Colin will be able to sort me out but have to say I’m not enthused at the prospect of paying for another few hours of his time…
Is there anything on this list I might be able to cover off myself I. The next day or so before the cat goes back in on Thursday, maybe save Colin some time?
Please bear in mind I’m basically clueless and can follow only the simplest of instructions.
That list is perfect, exactly the right way to sort it, logical and with most likely items first (assuming someone has not already had a go at it and damaged something).
A quick rule-of-thumb check on the plugs, leads and coils used to be to run the engine in the dark with the bonnet up and look for where else the sparks are going - blue sparks wandering around, very pretty, but they should not be there.
With yours you have a good chance of seeing this if its the fault. However I still suggest asking Colin to fix the fault.
If no stray sparks visible, then the only solution is to work though the diagnostic list to find a positive diagnosis BEFORE attempting a repair. Some items are easier to fix than others, and the list is definitely not in cheapest or ease-of-fix order.
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A CAS should be considered a standard consumable for our cars. They’re pretty cheap and take two minutes to swap with a 10mm and are found on top of the rocker cover, so they’re even simple to change. For the effort of ruling it out and the fact that it can become necessary at any point to do so, it’s thoroughly worth keeping one to hand. 
Thanks, I will check for sparks tonight after work!
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By CAS I assume you mean Camshaft angle sensor? I will consult my Haynes manual tonight but I think from a quick google that may be within even my limited capabilities!
That’s the one.
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Thanks for your patience with a total novice! Will let you know how I get on.
Ok so no sparks in the engine bay, took it to the mechanic and I’ve had an absolute result he’s going to guide me through fixing it remotely and send me parts as he is too busy!
Really excited to have a go at some of this stuff even if it’s very basic for some of the guys on here.
First up I have been recommended to try replacing the coil packs one by one and see if that fixes it. I have a coil pack and will try to make the first swap tonight if my son ever goes to sleep! These sunny evenings are a nightmare for a toddler without a blackout blind…
Ok so changed the nearest coil pack, cleared the codes and went out for a spin. Unbelievably the first thing I’ve changed SEEMS to have sorted the problem. I’m taking the car out for a more extended run tonight and see if there are any further issues.
Any tips on how to try and stress an engine into a misfire? I’ve taken it almost to redline in first three gears, tried starting off in second and running in fifth all the way down to ten miles an hour.
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Oddly for me to be right for a change…I had a sneaky instinct based on nothing at all it was a COPS as in my first post.
As for inducing stressors to a misfire? Pointless.
These engines if sorted are (over) engineered to take anything you or I could chuck at it an laugh it off.
Based to take a turbo application I believe…except it’s pretty much having a holiday without it.
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Some people are just born to be right! 
Thanks for the advice on stressing the engine, I’m sure my gearbox will thank you! If the light doesn’t come back with another half hour of driving, fair to say it’s probably sorted?
Believe me…I ain’t one of them.
Yes to your question.
Time to enjoy…time to quit fretting!
Have fun now.
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I will do, thanks for the advice but watch out, being helpful leads to more help being required!
If you’re not careful I’ll be PMing you next weekend when I lose patience with my alternator belt continuously squeaking. It’s fairly new as well so I think I need to have a look at the tension.
The issues never truly end I suppose. 
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The code is back! The next time I took the car out and I’m having issues. I have changed the second coil pack with the other one I removed previously, cleared the code and on a quick test rive the engine light returns quickly so I think my next port of call is a new CAS. I would have done plugs first but they were changed literally 2 months ago, just before I bought the car apparently so I’m more inclined to try at CAS first.
Should the CAS not be the problem is the below kit worth investing in do we think?
https://www.mx5parts.co.uk/ignition-leads-plugs-coil-packs-mk25-vvti-p-4529.html
Would enable me to change all the plugs and leads and the other coil pack just for peace of mind?
Ok, CAS changed and within 5 miles the engine light is back. Seemed to come on when going around a corner at low revs. Might just be a coincidence.
Either way, I will try removing and putting back all of the spark plugs later tonight and if that doesn’t work I will have to buy a whole new set and try that. Any recommendations for mk2.5 VVT spark plugs that I can buy from Halfords tomorrow?
Have you checked the air gap between the crank position sensor and the lugs on the crank pulley plate? Air gap should be 1mm. Check against all 4 lugs.
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