Hi,
I’ve got a 6 year old Mazda MX5 Miyako. Bought new in 2010 - in October 2014 both catalytic converters went and needed replacing (expensive out of warranty). It’s now less than 2 years since the CC’s were replaced and yet again, engine management warning light comes on, rattling noise, fuel consumption getting worse - likely to be the Catalytic Converters yet again.
The cars done just over 51,000 miles - my usual ‘useage’ is my commute to work each day (50 mile round trip) and i do a 300 mile round trip to see family every 6 - 8 weeks.
Is it usual for cats to have to be replaced several times (especially on such a ‘young(ish)’ MX5) ? Is it acceptable?
My query is - is this a known problem with the MX5’s? I’ve googled it and cant find any info about it. I’m not sure if i’ve just got a ‘dud’ MX5
Debbie
It would appear you have an engine management fault.
Hi, yes I do, the EML came on, the OBDC fault code is P0141.
Hi, yes I do, the EML came on, the OBDC fault code is P0141.
Hi Debbie
No known issue with MK3 / MK3.5 cats as far as I am aware. I sell these so would likely have heard if there were a general problem.
Your car has two cats in series which should last a lot longer than yours have, particularly considering the type of driving you are doing. I can only suggest that there is a problem with the car, misfire or similar causing unburnt fuel to get into the exhaust system that has superheated the cat ceramic cores and caused a problem. A rattley cat indicates a loose cat core. If this is now coming from the rear cat, it could be because it was not commissioned properly when first fitted. The ceramic core is held in place with matting which needs to be expanded on first use to support the core. This mean that the first run after fitting needs to get everything really hot to expand the matting sufficiently - particularly important with cars like yours where there are two cats in series meaning it takes a serious run to get heat to that back cat. If the matting is not properly heated up when first commissioned the cat will fail prematurely.
You are going to have to replace one or both cat to cure this problem. Make sure the damaged ones are properly checked to diagnose the problem. Most cats have a two year warranty - are your replacements still covered by this. Certainly my aftermarket UK manufacturer would want to get these back for checking if they failed within the two year guarantee period.
I would add that P0141 is a heater circuit failure on the lambda sensor. If this is bank 1, sensor 2, its the post cat lambda sensor so no relation to the cars running or your current issue.
It sounds like you have a fueling issue and the excess unburnt fuel is damaging the cat a faulty sensor after the cat could be the cause if the sensor is under reading the engine management system might be thinking that the mixture is running leaner but I’d have thought sensor one would solve that so it could be sensor one is faulty too
Unburnet fuel is usually the symptom of a misfire rather than a rich mixture caused by a sensor issue. There can be excessive fuel being injected but it’ll still be burned. High CO doesn’t kill cats, high HC does. A misfire caused by a faulty coil for example means that the fuel being injected doesn’t get burned and is pumped out though the exhaust as just fuel.
CO is a by product of combustion, so if CO is high, things are getting burned, HC is Unburned stuff so if HC is high stuff isn’t getting burned.
As an example, if you get an RX-8 (engine) hot and hold the engine at a steady fast idle of 2800rpm or there abouts, the CO will climb, the engine management increases the fueling to richen the mixture and cool the burn, this in turn cools the cat and does no damage. When a leading coil fails on an RX-8, the cat is toast!
Just asking a question re Rhino’s post.
On a new car are the cats run up to temperature in factory before being shipped out, just curious.
Running rich will damage the cats and as the op says fuel consumption is now poor I’m guessing that the lambda sensors were faulty in the first place and that’s what caused the original and current cats to fail I’d try and find a decent auto electrician who can scope read the sensor outputs
You are right by the way about hc damaging the cats and to much fuel in the mix will result in an incomplete combustion hence the lower cylinder temp but also unburnt fuel travelling into the cats most modern cars now open the egr valve aswell to reduce combustion temp and it’s that that increases the co2 count damm I’d forgotten all this shit from night school thanks for refreshing my Brain at 10.15 pm lol
Good question re commissioning new car cats. I think they would have to be as it would be unrealistic to pass on that responsibility to new owners.
All replacement cats should be supplied with proper commissioning instructions to avoid premature failure of the new cat.
P0141 relates to the post cat lambda, so no effect on fueling
HC, hydrocarbons are the result of partially burnt fuel. It’s basically soot that coats the catalytic converter and eventually stops it working. HC is part of the emissions test for the MOT. One of the conditions of returning a catalytic converter under guarantee where it has failed emissions is evidence that HC is under control. 60 parts per million is the figure my cat manufacturer deems to be on the margin of unacceptable. High HC indicates a running issue with the vehicle.
Misfires, typically caused by faulty coil units, HT leads, spark plugs, etc mean that fuel has been injected into a cylinder but not burnt as no spark. The unburnt fuel is then ejected into the exhaust where it inevitably ends up igniting on the top of the catalytic converter core. If this continually happens the resulting superheating will melt the ceramic core, partially blocking the exhaust. The core will eventually become loose and break up if the initial core melting does not block the exhaust sufficiently to stop the car running.