Catalytic converter clarification

[Dmilne46]

Right so I just need some clarification before I spend the money to fix my car. It failed the mot on emissions and the garage said probably head gasket. I ran a compression test and the compression was really high for all cylinders. No coolant going down and no emulsification in the oil. I got an obd reader and the pre catalytic converter voltage was varying up and down between 0.1 and 0.9 volts, the post catalytic converter oxygen sensor voltage was doing the same. I read that the post catalytic converter oxygen sensor should stay at a constant voltage if the catalytic converter is doing its job. I tried some Cataclean and the voltages before and after am the catalytic converter are still varying (the same) so that points to me that my catalytic converter is inefficient still. Just want to get others opinions before I replace the catalytic converter, do you guys think it is the catalytic converter that needs replacing? It is a 53 reg Ford Fiesta 1.6L 81000 miles. Thanks in advance guys

[HustleRussell]

I agree with your deduction that it's probably not the head gasket. It'd be unusual for the car to fail on emissions due to this while exhibiting no other symptoms.

However I'm not confident in your catalytic converter diagnosis, to me (and I'm not an expert) your problem sounds more like an engine control systems issue i.e. one of your sensors.

[Dmilne46]

Ok I am open to suggestions etc. Originally after the mot the after cat oxygen sensor was faulty so I replaced it, re-mot'ed and it was exactly the same on emissions (failed). So now both the pre and after cat oxygen sensors are less than 5000 miles old. I just want to make sure I do the right thing to fix it because my money is tight. Anymore information you would like to help you diagnose?

[SonicShadow]

Can you post a photo of the emissions test results?

That car will have OBDII - have you checked for any error codes? The combination of these two things should help narrow the list of suspects down.

[Dmilne46]

Fast idle
Co= 0.6
HC = 95
Lambda = 1.012
Second fast idle test
Co = 1.01
HC = 204
Lambda = 0.999

Max co limit =0.2
Max HC limit = 200

I have an OBDII scanner and when I initially plugged it in it told me oxygen sensor 2 was knackered, so I replaced that and it cleared all fault codes. I haven't had any fault codes since