Replace cat or fit extra one?

[DuncsGTi]

My 06 plate megane 225 has just failed its MOT on emissions.

About 4 years ago, the OE cat crumbled away and blocked the exhaust (weak coil was causing misfire under load and allowing unburnt fuel into exhaust) I had the cat replaced with a pattern part which seems to have failed already.

Now, to replace the cat on one of these is an absolute nightmare, I've not got the time, tools or patience to do it myself and the book says about 7 hrs labour to get a garage to do it.

If I went to an exhaust place, got the centre box cut out and replaced with a second (universal) cat and a section of straight pipe to fit, would this approach work to drop my emissions? Would the second cat be too far from the exhaust manifold to get up to operating temperature?

[DuncsGTi]

Julian Thompson said:

As you say, the temperature will be lower and also there is a possibility that there are o2 sensors downstream of the oem cat that impact the ecu closed loop.

Why is it a nightmare?

Access more than anything else, it's just quite tightly packed in that part of the bay. Driveshaft needs removing to get the old cat out/new one in, etc.

The lambda reading was fine on the test so I'm just trying to find a way of cleaning the gasses further downstream.