HEAT waring light + misfire (Should Not Have Steam Cleaned the Engine)

Looking for some advice. So my engine bay was a little tatty and a friend suggested I get is steam cleaned. Knowing what I know now I probably should not have done it, but thats why I am here. Car is question 1995 MX1 na (import), 1.8 manual, 70k miles - no oil leaks and no prior mechanical problems.

So took it to car wash and asked to steam clean. Guy asked to let the engine run (weirdly after asking if it was a diesel…). I left it running and went for a coffee. After 1h or so I come back and the car is nice and clean and I set off. Obviously applying cold water on a engine at operating temperature was not a good idea, but its done now.

There was some rough ideling straight away, but I thought it might go away. After driving 15mins or so I stop to go to a shop and after 20mins come back and start the car. It is not running rough and I get the “HEAT” waring light as well as misfire. I suspect it felt a little underpowered.

So after 20 mins drive I get home and leave the car for couple of hours to cool off. After starting it again the heat warning light still there and misfire. Same thing after 10hours.

It seems I got a misfire issue after the wash that led to me damaging my cat. So, any advice as to what might have happened and what I should do would be much appreciated.

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Welcome to the forum, I would guess that one or more plugs have got wet during the clean and were not firing, hence rough idling etc. This would dry out eventually but the unburnt fuel causes the Cat to overheat, and the sensor on the Cat turns on the light on the dash. Once on it stays on but you can unplug the wire to the sensor underneath the car. Again I would guess that an emissions test will tell you if the Cat is damaged. I am sure someone will be along with a better explanation if I am wrong.
Don’t ask me how I know but I still have the T shirt!

About right. If the cat is wrecked, I guess you will find out at the next MOT. Replacements are cheap, only £60-80, so not the end of the world.

Yes, agree with above.
You need to check the ports in the coilpack where the spark plug leads go. Remove each in turn to make sure all clean and corrosion free; there will likely be no problem here. More likely is water ingress into one or more of the individual spark plug wells where the other ends of the spark plug leads go. Remove each lead and I would run tissue or similar into the rather deep holes to ensure that any water pooled in there is removed before checking each spark plug and lead connector carefully. Do not remove the spark plugs until you are sure there is no water in the plug wells.
The ‘heat’ light indicates that the one time temperature fuse attached to the original cat has blown. This would have been caused by the engine misfiring. Injected fuel is not burnt/ignited by the spark hence the misfire and is then ejected via the exhaust valves, through the engine exhaust ports into the exhaust system. It ends up hitting the hot catalytic converter and ignites. In a continuous cycle, the cat will superheat and the ceramic core will melt at the surface and eventually break up. The melting causes partial blockage leading to increased heat and somewhere in this process the heat builds up to a level where the one time fuse blows. Once that light is on, where the fuse is fitted, it will not go out.
The good news is that this is an original Mazda cat so 20 - 30 years old and probably at the end of its life anyway. I would buy an aftermarket UK spec 345/375mm cat without the fuse to replace and disconnect the broken fuse, taping up the loom wiring and remaining connector.

If you do need a new Cat I would recommend


Most helpful, as you have to make sure that you are ordering the correct size for your car, had mine over three years now and still going strong. (touch wood)
Good luck

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The reason the heat light is on is simply because of the misfire,so the the cat is fine and the sensor is doing its job, check for water in the spark plug wells which sounds like the case,pull the HT out off the spark plugs, soak up as much as you can with rags wedged down , turn out the plugs and let what ever is left into the engine( yes it will be fine) let everything dry off and put it back together and start her up she will not like the little water but she will push it out and mix with the fuel, and go and give her a good blast , also check for any furring/oxidising in the coil pots that might be giving a weak spark.

Water and spark plugs is never a good combination.
If after all this time you still have the misfire then you probably still have water somewhere.
Start pulling leads off and spray the contacts with WD40 etc.

As for the heat light, whilst everyone has suggested that you fried the CAT, Rhino is suggesting that a thermal fuse may have blown.

Is it possible to replace this fuse, or wire something in to fool the system

Maybe the CAT has survived.

Cats fine. it is not fried, thermal fuse is fine buddy…read this, it is just that they have done it with a jet wash, rather a bit safer by hand…http://www.mazdamenders.net/forum/viewtopic.php?f=64&t=5749

Thanks for all replies. Will check the spark plugs later on today. Hopefully will be a simple fix.

Could you also advice how to bypass/diasble thermal fuse, and where it is located? I guess I could fool the sensor with a resistor?

Don’t worry there is nothing wrong with the thermal sensor or fuse…start with the basics, the spark plug wells have water in them thats causing a spark to go where it wants rather then where it needs to be which is the spark plug and not earthing out to the block giving you a misfire,and please do NOT use old school WD40 everything needs to be dry and clean .

The one time fuse is spent because the cat overheated. If you turn your igntion on with the broken fuse in place and no heat, the ‘heater’ light will still come on and stay on - it cannot be reset without disconnecting the thermal fuse plug from the loom.
It came on in the first place because the cat overheated. Given the misfire and associated neat fuel in the exhaust, we know why this is. Typically a cat superheating not only causes the core to melt where hottest on the side nearest the engine but as a side effect blocks off some of the 2mm hex holes causing partial blockage and more heat buildup. The heat/partial blockage combination will start to fracture the fragile ceramic core leading to a break up of said core and further problems down the line. You may have been lucky and the ignited fuel simply overheated the cat and melted the thermal fuse?

Thermal sensor is located on the side of the Cat, prop shaft side

You can see it sticking out on this photo.
It looks like this when removed

It can be unplugged and the wires secured out of the way for safety which will turn off the light. As far as I am aware it is a one time fuse/switch and once activated it can’t be reset…

Your car is a 95 and depending upon when it was built/registered it may well not need a CAT test, although if it has failed it is not good for the environment.


Thanks for all advice. All sorted now. Couple of the plugs were moist so was probably the cause of the problem. Nothing was flooded or anything like that, just little dampness.

I wiped the spark plug wells and the the leads clean then removed the spark plugs. Then cleaned the spark plugs and left everything out to dry with the car bonnet open for about 1-2 hours. After that connected everything back and all was good!

Still had the heat warning light come up, so disconnected the sensor and cut off the end from the sensor to avoid any rattles (since its single use its pretty much redundant)

Just a side note is there any way to know if my cat has been damaged significantly?

Would the Eunos benefit from partial decat perhaps? Good opportunity to make it sound a little nicer?

That looks like an exceptionally clean cat. Hard to believe that is the original 24/25 year old part. I would expect to see a rusty mess with corrosion holes in the heat shield. We still have an original one that passes MOTs with no problem on our 1991 1.6 Eunos that could in theory be decatted. I think the rule regarding mandatory cat fitment changed sometime in 1993 so your 1995 car well outside. In addition there will be more stringent emissions rules applied than to earlier cars that do not need a cat.
Perhaps leave it for a while and see how you get on at MOT. If in the meantime you hear any rattling or suspect any other issue with the cat replace it. Provided the flow is good and the core does not break up you will hopefully be OK. Ideally it should be inspected to see if any issue but hopefully you have got away with it. Worse case scenario the core moves and blocks flow or breaks up and blocks the exhaust.
I have not taken one of these thermal fuses apart but it seems that the melting of the fuse material is creating a connection rather than breaking it. Would be interested to know how that works on this part.

Had a cat break up. Lead to severely dented performance and the exhaust spitting stuff out.

Compared to a blocked cat, a cat pipe would be an improvement, Compared to a good car, a cat pipe will be worse (noisy, no performance gain). 1995 is the cut off on cats for imports, but morally you should replace a catalytic converter with a catalytic converter. And its no good for anyone’s health arguing with a mot tester that you don’t “need” a catalytic converter (truthfully, we all need a cat).