High co2 from exhaust Failing MOT. Subaru impreza 96 have put new exhaust downpipe on, it has noo cat on it, any suggestions!

did you install a O2 sensor or a heated O2 sensor
they are different items and do different jobs
The heated oxygen sensor (HO2S) reads the exhaust gas composition and reports to the ECM to adjust the air /fuel mixture to obtain maximum economy and performance
It is found in the exhaust manifold or before the cat converter
the oxygen sensor (O2 ) is found after the cat converter and reports on the cat converter operation
after installing the correct sensor use a scan tool to reset the ECM

code p 0137 refers to heated oxygen sensor (HO2S) 2 bank 1---low voltage ===causes --exhaust leak---wiring short to ground---HO2S --ECM
it also refers to oxygen sensor(O2S) 2 bank 1--low voltage ===causes exhaust leak---wiring short to ground--O2S-- ECM
HO2S sensors are in the exhaust manifold before the cat converter and O2S are in the pipe after the cat converter there is a flex section of exhaust pipe covered by a stainless steel mesh and they crack under the mesh
I suspect that it is the HO2S that is faulty as it reports the gas composition of the exhaust gas so that the ECM can adjust the mixture

Have you had the ECM checked?

The ECM controls your fuel to air mixture as well as monitoring the CO2 levels.

Also, have you checked the oxygen sensor, when there's too much oxygen intake,,, the ECM increases fuel to compensate for the excess air, and thus results in more CO2.

Another check would be to find the diagnostic port, (usually under the dashboard), and, to either hire, or borrow an OBD diagnostic tool and see if there is an error code displayed, beginning with 'P', the code will show you what component is likely to be in error for producing the high CO2.

check that the cat converter is not failing and blocking up the exhaust system. or not doing it,s job. replace both HO2S and O2 sensors

retest could be a contaminated gas anilizer high co2 isnt a bad thing it means teh cat is working high CO is a failing cat whats the hydrocarbons at

co2 sensor/s and egr might need cleaning.

You have one right by the firewall. Its actually mounted on your downpipe by the turbocharger. and the other one is underneath the car on your exhaust. If you get under the car and look down the exhaust system you will see it plugged in.

if not skipping and losing power take it to a reputible exhaust shop to check for exhaust flow.. possible stopped up cat. could also be a timing problem

I think you will find it is 2.5 inches. The best exhaust to go for is a Haywood & Scott stainless system. Make sure you replace the downpipe . The design of the factory one is very restictive. You can then go for a 2.5 or 3 inch sustem. They sound wonderful . Also if you are there you might as well go for a set of GT Spec Headers. Unequal length to retail the Impreza burble.

for a subaru this can be a common problem and can cause the engine to stall or cut out my opinion is get a second opinion from a mechanic i will let you no cats can be very expensive depending on the type and cc of your vehicle