Co2 reading high? why

[frenchdave]

MGTF 135 year 2003 64k miles
I took the car for its french MOT today, passed except Co2 reading was 0.7 when its a 0.3 max.
Inspector thinks it might have an ex leak before the lamba sensor?

history is that last week we put a new head gasket with timimg belt and water pump.
Used a good head from a 1997 120 bhp car had it skimmed , and used the cams etc from the 135.

Apart from that no issues.

Where to start with looking for and solving this?

[talkingcars]

I would start by looking for air leaks.

[frenchdave]

I would start by looking for air leaks.

Inlet or exhaust side?

[Plezier]

I was told that HC being high would be the result of leaks and not the CO2 readings.

[talkingcars]

An exhaust leak before the first lambda probe can cause a misrepresentation of the exhaust gases which causes the ECU to adjust fuelling.

[frenchdave]

Just found that when we finished the head gasket repair, forgot to connect the air box to the throttle body
Lots of air going in and maybe a high Co2 reading?

[Plezier]

Cannot see that making much difference. On Blue it was a faulty CAT................ a faulty new CAT at that.

[talkingcars]

My VVC failed its emissions test once due to a leaking gasket betwixt the exhaust manifold and downpipe, and a very small leak at that.

[John SS]

Cannot see that making much difference. On Blue it was a faulty CAT................ a faulty new CAT at that.

Ditto. Replaced mine with a Bose cat and all good.

[frenchdave]

First thing will be put it on the ramp we have here, and look for exhaust leaks.
then see where we go.

[Plezier]

Good luck as I know full well just how frustrating tryign to solve this can be.