Any help appreciated
A bit of a mishap, a friend has put diesel in their mk3.
Diesel has been removed and car has gone through a tank of fuel, OBD readings have thrown up fault codes to no3 misfire and faults to all plugs, these faults have now been cleared and have not returned, but the EML light is on, we’ve cleared and reset the EML but it comes back on.
So is PO421 specifically the cat or could it be the oxygen sensor?
Thanks for any help as I’m nearly 300mls away from home.
Hi,
Sorry to hear what has happened, this is the type of easy-mistake problem that we can all live in fear of. Given that P0421 is the downstream Oxygen monitor, in the circumstances, it strikes me that you would be right to believe that (for once!) it is likely to be telling you the truth… i.e. the Cat is probably ‘goosed’.
However, as with all these things, (& without trying to teach anybody to suck eggs, as you clearly know what you are doing) it now becomes an issue of damage (cost) limitation and obviouslyy the Cat would be the most expensive item to buy as a first shot at elimination. But… just before you dash off to the cash-card machine … if you do have a multi-meter to hand & gas torch / bench test facilities (300 miles from home?), you could try the Oxygen sensor tests on the downstream sensor as described in the FAQ @
http://www.mx5oc.co.uk/forum/default.aspx?g=posts&t=230
If you don’t have the facilities which the FAQ describes, propane torch etc… you might consider just buying a new downstream O2 sensor to prove that it IS the Cat at fault… the sensor is a fraction of the price of the Cat and in the long run, the sensors don’t last forever anyway (they just get lazy)… so the price of an 02 sensor not a complete loss over an extended period of ownership.
In the absence of any other codes, it sounds as if you might well have been very lucky with the upstream 02 sensor!.. and you could logically deduce that if the upstream 02 sensor has survived, the downstream is also more than likely to have survived & hence ‘should be telling the truth’… (as much as we would all perhaps not want to believe it)
For further ‘interesting reading’… see also:- the thread covering Sarah’s (Mazda Chick) similar issues with an 02 sensor… by entering P0421 into the Search feature on the MX5 Owners Club Forum.
Best of luck,
Steve
Afterthought…, five minutes on your 'local friendly neighbourhood garage / MOT testers Exhaust Gas Analyser will tell you all you need to know about what is coming out of the exhaust pipe. Depending on your working relationship with the man… might only cost you the price of a drink!
Very much appreciate your reply.
No1 there is a cat on eBay at the mo.
No2 the rear sensor is £95.
No3 the front sensor is £137
This is only a thought bearing in mind the initial problem, would you bite the bullet and plump for the cat?
Only asking because somebody may pick up on this and grab the cat before we do.
You don’t need the Cat until MOT time if it is faulty, the labour involved to swap the primary is quite expensive as basically it’s a manifold swap. The car should pass the emission test on a functionary secondary alone, the secondary sensor lies between the primary and secondary cats, it does not monitor the function of the secondary. There are a few of us who don’t run a primary cat or secondary sensor at all on a mk3 ie those with a 4-1 or turbo manifold, the sensor is deleted by ECU remap.
How’s the car running?
I believe the car is running perfectly, as I stated initially this is for a friend who happens to live 150mls from where I am now and possibly 250mls from my home.
I’ll find out when the mot is due, in your opinion would the cat possibly burn back to a clean state if used spiritedly ?
Again thanks.
I’m not a mechanic but the ‘title’ code may clear as the diesel clears.
This is exactly what we are hoping for if time will allow.