Volvo T5 (Focus ST) PO420

Hi everyone,
I am chasing intermittent 'hesitation' off the line, jerkiness and 'power loss' on my MK2 Ford Focus ST225 (Volvo T5 2.5T engine). 
Specifically, the sensation of 'shove' from the turbo and the sharpness of 'throttle response' just is not there from start up and the cabin has some vibrations.

Randomly for a few minutes or a few days, power occasionally comes back. It feels tight and responsive and all of the above is gone.
I often find when i return to the car after parking it up for an hour it's better, but if i leave it overnight it's rough again.

The code I have is P0420 - Catalyst below threshold

I have replaced both Lambda and O2 sensors and stuck a brand new CAT on.
Same code, same fluctuations on the post cat o2 sensor.
What do you suggest could cause this wild readout (typical sensors have been done such as MAF, MAP, FPS, pretty much everything)

I am thinking a leak? intake? manifold? Thank you.

Have you checked for exhaust leaks? "O2 1X2" is the downstream, correct?

Yes this is the downstream sensor - post cat.

I've checked the joins from the turbo downpipe backwards and there's no blowing.... the only area I cannot be sure of (as I've had 2 totally different opinions) is the exhaust manifold. One garage said 'you've got a leak at the manifold' the other garage said 'no, I cannot hear anything up there'.

I've booked into a specialist to have it confirmed either way - along with the intake path inc. gaskets.

I was curious whether this type of graph would indicate an issue with the engine breathing or exhaling. Something about extra air fooling the downstream sensor?

The best I have at the moment is my bluetooth device and torque pro. My usual more detailed approach (forscan) is not playing nice with my ELM adapter.

An exhaust leak after the upstream O2 but before the downstream can cause a below threshold code. I've heard a few people say a bad rear O2 can cause it, too (I know you said you changed yours), but I've personally never seen that.

A good way to test for exhaust leaks is to hook up a leaf blower (or Shop Vac on the blow setting) to the tailpipe, crawl underneath with a spray bottle of soapy water and start spraying around. I would concentrate on the area I mentioned.

In my experience, most of the time there's a P0420 or -30, unfortunately it's the cat. Did you replace yours with an OEM one? You may want to try to compare graphs for the upstream and downstream O2s and see how much they differ. Paul has a tutorial on how to do it and what to look for :

Hey,

Thanks for your reply.
It does have a high quality (not an ebay special) 200-cell sports cat, as the car is lightly modified and remapped, but it's been like this for over 2 years and never thrown a P0420.

I can only assume, having replaced o2 sensors and my old 200 cell cat like for like, that I have done this unnecessarily.
The old cat's honeycomb was absolutely fine and ran for 2 years with no P0420; it also passed the vehicle emissions test every year.

I messaged the manufacturer of the 200 cell cat for their opinion.
I was told, based on my graph, their technicians would be looking into a gasket, injector, or a leak somewhere. They said that is not a downstream o2 graph they would expect to see even with their high-flow sports cat fitted.

My best guess at the moment:
I don't think the P0420 is causing my jerkiness, or hesitation, but i suspect the CAUSE of my hesitation and jerkiness is also triggering the P0420 code.

I cannot really compare to the upstream sensor, because it is a wideband Air/Fuel sensor and ONLY shows the air fuel mix, or equivalence ratio as 1 or 0.
It certainly doesn't wobbly about as much as the downstream sensor, but I wouldn't expect it to.... and fuel trims are within spec.

I would check for the exhaust leak first- anything before the rear O2- then go from there. After that, if the problem still persists, you can check the lean/rich response of that rear O2 to see if it's responding appropriately.