DTC P0421 Catalytic Converter/O2 Sensors

[andybro13]

A check engine light with code DTC P0421 was diagnosed on my car recently. It's a mitsubishi lancer, '06, 1.6 Petrol. the garage told me i needed to change the exhaust manifold.
Has anyone encountered this problem? I know its either the O2 sensors or the catalytic converter that needs to be replaced. I'm not seeing any problems with the performance of the car. I have an NCT coming up soon which will show me the emissions from the car. I'm not going to replace anything until I know something is failing on the car. Any help/suggestions welcome!

 

[Mursal]

The garage said to change the manifold because they cant get the O2 sensor out? Or is there another reason?

Might be worth trying a manifold from a breaker and see if the fault code come back on?

 

[andybro13]

No the garages initial reaction was to change the exhaust manifold which I'm assuming houses the catalytic converter? It was my assumption that it was either the sensors or the cat. I've switched the cel off by disconnecting the battery but the fault is picked up within 40 miles or so.

 

[Mursal]

Is this the one?
http://www.ebay.co.uk/itm/MITSUBISH...arts_Vehicles_CarParts_SM&hash=item1e69cf3f54

In my opinion always best to make sure it's not the O2 sensors first, especially checking the connections for corrosion. But the garage may already have checked this?

Then go for the cat

 

[andybro13]

Theres only 60000 miles on the car so I'm hoping it's not the catalytic converter, surely it's to soon for it needing to be changed? I'm going to check the O2 sensors myself for corrosion, even they are quite expensive to replace. I might just send it through the Nct and see if the emission test fails. If that fails its more than likely the cat?

 

[Burnerman]

I simply won't admit how many lambda sensors I've changed, to make no difference at all try explaining that one to the punter!
Get the cat steaming hot prior to the test...it may just go through.
John

 

[andybro13]

Ye i heard an "italian tune up" being mentioned! Take the car out at high speeds or just rev the engine? To be honest John, I'd rather let it fail the emissions test to be sure that it's the cat thats causing the CEL. The fact that there's less than 60000 miles on the car has me hoping that its not the Cat at all! Is there such a thing as a catalytic converter cleaner? Or is it possible to remove the cat and try and clean it yourself?

 

[Burnerman]

Ideally the car needs to have had a decent run, and be thoroughly hot when it is tested. Often this means holding the car at 3000 rpm, cooling fan on, to give the cat maximum heating and usually this is enough to allow it to pass.
Its the emissions that count, not the fact that the engine management lamp is (or isn't) on.
There are potions available that are meant to clean the cat - I've no experience of them but I have grave doubts about their performance.....!
There's nothing you can do yourself.
We've all been caught out by your phenomenon - an example for me was an 02 plate Saxo....management light on, lambda fault code stored. I put a new lambda probe in, and it was just the same and the cat was recent anyway - but the lamp stayed on! It still passed the test though.
Over here, if a car fails you are given a certain time to sort it without paying again....maybe its the same with you?
John

 

[andybro13]

I've just rang the test centre and was told that a CEL light was enough to fail the test. Which is rubbish in my opinion as we don't know the source of the problem. Im going to put the car through the test without heating the cat as I'm hoping to get to the source of the problem. So if it passes the emission test i can safely assume it's not the cat that's the problem? I'm highly doubtful that is the source of my problem, at least i hope so! Don't fancy paying big money for a new cat.

 

[iwaters]

I had this error code on my old car. It is a bugger to track down. It basically means that the ECU thinks the cat is not performing as well as it should be. This can either be because the O2 sensors are faulty, the cat is knacked or usually just that the cat is on the way out.

I spent ages with my car hooked up to OBD data loggers, oscilliscopes etc, which shows the O2 sensors were fine. It passed two MOTs with that fault as the CAT was still working.

This problem can occur when people fit non OE cats as whilst they perform suffciently for MOT tests they not performing as well as the ECU thinks they should. This is especiilay a probelm with VW/Audis whose ECUs have very high standards.

I imagine most garages would start changing parts rather than trying to track down the fault because the labour is less. They will start with the O2 sensors because they are cheaper.