Mot fail co reading .700

321p123

Oh you mean fit the cheapest aftermarket I can find? 

Mildly_Miffed

321p123 said:

Thanks all, I found this on ebay, https://www.ebay.co.uk/itm/325587874994

Would you recommend or spend a bit more for a better part.

Shipped from Lithuania, with the possibility for VAT and duty on import.

born_again

321p123 said:

I want to fix/find the source of the problem, not bodge it, that's why I'm asking, I just don't want to pay thousands on something if its not that issue, I used redex as a one off just to help clean an old engine, not as a bodge. 

Well you need to find a good local garage for that. All people here can do is throw idea's at you. Not give a 100% this is what it is..

Given it has been happening for a couple of years. As someone else said, the issue maybe far worse now & cost far more than it would have, had it of been fixed when it 1st happened.

Would be better finding a Forum for you car & asking there, as you might just get lucky & someone had the same issue.🤷‍♀️

321p123

Just taken heat shield off and nuts and threads are corroded heavily..... would a mechanic struggle with this, got some wd40 on them but they look rough 

born_again

321p123 said:

Just taken heat shield off and nuts and threads are corroded heavily..... would a mechanic struggle with this, got some wd40 on them but they look rough 

You need penetrating oil, not WD40.

Goudy

There is something going on with the fuelling to give you these emission reading and the P0420 doesn't mean for certain the CAT is duff.

P0420 is basically the post CAT O2 sensor reading the CAT is below it's threshold which is backed up by the MOT emissions test.
What is coming out of the CAT is above what it should be.

There are a couple of reasons for this and plenty of explanations why it happens.
Yes one is an under performing CAT.
Another is what is going into the CAT is wrong and the CAT just can't cope.

The CAT is a funny part, it doesn't just work off the bat and it can't deal with more than it's designed to deal with.
It needs plenty of heat to work, so what comes out of it cold is basically ignored until the engine and CAT has had time to heat up.


An imbalance of fuel and air entering the engine means there could be an excess of emissions leaving the engine and hitting the CAT.
The CAT can't cope and the extra emissions are being emitted at the exhaust. 

This imbalance could be due to lots of things.
A lazy and slow switching pre CAT O2 sensor would force the fuel trim over to rich.
The cars fuelling is determined by a few things but the fine trim when warm is what the pre CAT O2 sensor reads.
It should switch back and forth between lean and rich (0.1v to 0.9V) about once a second.
If it's not the system will almost certain default to rich as it's safer to the engine than lean.

An small air leak on the intake side of combustion will allow air to flow through combustion causing a lean fuel trim at the pre CAT O2.
This air in unmetered so it just hasn't fuelled for it, so detects lean and pumps more fuel into the mix.

An exhaust or manifold leak allowing air to interfere with the O2 so the ECU may try to enrichen the trim as above.

The problem is most fuelling sensors is they reset with the ignition and to flag a problem the same fault needs to be detected a few times in the same drive cycle.
So starting and driving about is altering the fuelling constantly so the ECU finds it hard to detect a fault with anything upstream of the CAT.
What comes out of the CAT is staying pretty constant (when the CAT is hot) which is why you just have the P0420.

Before condemning an expensive part like the CAT you first need to run some diagnostics, or better still find someone that can do this and understand the readings.

They aren't easy to understand but the fuel trims and O2 sensors could do with some logging and some checks on things on the intake side, like vacuum lines, inlet manifolds and exhaust/exhaust manifolds need checking. 

I suspect there's a fuelling issue rather than a CAT issue as it's far for more common.

321p123

OK thanks guys, another question, with my readings of co being.700, would a bottle of cataclean lower this enough just to get emissions through mot ? 

Goudy

Your car is running rich.
The post CAT O2 says it is, the MOT stations analyser says it is.

MOT limit at fast idle is 0.3, yours is over twice the limit.

If it was mine, first step would be to test the upstream O2 sensor with a warm engine at idle.
With a OBD dongle, log the Bank 1, Senor 1 O2 sensor.

If it's averaging .45v (mid point between 0.1 and 0.9v) and switch up and down every second or so, the fuelling is good and perhaps look at the CAT.

If the fuelling is averaging anything significantly different, it's either lean and adding fuel or rich and trying to take it away, so that needs investigating.

321p123

Thx, so on my code reader 02 bank 1 sensor one is varying from 0.1 going up to about 0.85 then down and so on sensor 2 is still at 0.425, this is on idle. After a few minutes sensor 2 bank 2 started to fluctuate between .3 and .8 as the other one 

Goudy

321p123 said:

Thx, so on my code reader 02 bank 1 sensor one is varying from 0.1 going up to about 0.85 then down and so on 

This looks pretty normal.
The switch will look like it's once a second or just under as that's what your diagnostic tool can handle, with a specialist scope it would switch far far fast, but this is a good indication of a reasonable "closed loop" cycle.

Prefect air to fuel ratio is around 14 to 1 and on an average signal taken from the pre CAT O2, that's at the mid point which is around 0.45v.

There's a image here of oxygen sensor waveforms to compare, about half way down page.
Normal, Rich, Lean and FUBAR.
OXYGEN SENSORS: HOW TO DIAGNOSE & REPLACE

I forgot to add to my initial post, but is the fuel status using "closed loop" when you take this reading?
(closed loop is it actually using the pre CAT O2 sensor to trim the fuelling)

It should start in "open loop" (a rich start up mode that ignores the pre CAT O2) and after a short time it should trip in to "closed loop" when all the O2 sensors heaters have warmed them up.

Then at wide open throttle revert to "open loop" and back again in to "closed loop" when throttle is closed.

A fault with certain parts of the fuelling system can see the fuel status trip into "open loop - system fault".
That's the rich, emergency, get me home, I'm ignoring the pre CAT O2 signals that tries to save the engine from heat damage by possibly running too lean.

Most diagnostic tools have a "fuel system status" setting to see what the fuel system status is.

321p123 said:

sensor 2 is still at 0.425, this is on idle. After a few minutes sensor 2 bank 2 started to fluctuate between .3 and .8 as the other one 

This post CAT sensor reading is always a little tricky to understand as the CAT requires lots of heat to work efficiently.
This means it's not really active until the CAT and post CAT O2 heater has had time to warm up, hence the signal doing nothing for a while.

But once hot and at idle it should not mimic the pre CAT sensor.
At idle is should lean out the reading to a steady low voltage and not have this prominent switching effect.
Open throttle will see it return a steady high voltage.
Snap the throttle open and closed and it will follow the above (up than then back down) after a small delay.

This throttle snap result is a good indication the post CAT O2 is working.

If the post CAT O2 sensor is mimicking a similar frequency and voltage switch as pre CAT O2, this means what's coming out of the CAT is the same as what is going in and you're starting to confirm what the issue is, the CAT.

Vauxhalls from around this time are noted for the CAT's start breaking up inside, but it's an expensive leap of faith to replace one in hope without at least trying to determine if anything else is at fault first.

If you can get the CAT off and bits of the internals fall out of it, then there's the definitive answer!
I'd say if you can do this yourself and you're happy with how the O2's are working, this might be the next step.

If you replace the CAT and are going to keep the car, I'd replace the O2's as well.