I have recently done the local government car test and my car keeps failing on emissions which shows high on low idle @ CO 9.35% and still higher on high idle @ 10.25% so both should be below 0.30% help. It is a 2003yr model and it has failed 3 times

 

High CO emissions indicates your engine is running rich. Are you a DIY guy or would you prefer to bring it in for service? Does the vehicle have a check engine light on? ... And why would you bring your car back to be emissions tested if you failed the first time? Did you have any work done to it before bringing it in 2 more times?
First thing any DIYer should do is check the engine air filter, and spark plugs. If those are clean and/or are just fine then its time to start looking into the fuel and electrical side of it. Are all the sensors working properly? Is there bad fuel in the system, or are the injectors working properly?

If you're not a DIYer, the answer is simple. Bring it to the dealer and/or a European Specialty shop and let them know your concern. "I'm failing the emissions test because my CO content is very high. I think it's running rich. Can you diagnose and potentially fix this issue?"

 

After the first time I did a service and put an additive in the fuel VB26 then drove it at high rev b4 taking it for a test but it failed. The last one I changed one leaking solenoid valve and cleaned the loom connector to it but due to time constraints to do the retest I could not access the Knox sensor. But now since the next time I book for a test it will be a fresh one so this time am just looking for possible advices on what to look for because I have been in contact with different mechanics and all have different opinions from lambda sensors, catalytic converters, pistons and generally fact finding which would turn expensive with no guarantee.

 

Fuel additive, and "italian tune-up". Not necessarily a great first action on diagnosis and repair, but okay. Gotta start somewhere.

Vague; Solenoid Valve to...? Do you mean the Fuel Solenoid Valve, Fuel rail or injectors, or something else?
"Knox Sensor", I'm going to assume you meant Nox Sensor, or Knock Sensor. Two different sensors, both could attribute to rich condition. Was there ever a check engine light on, are you are just using a "Parts Shotgun" and throwing parts at it you think might fix the issue? That's an expensive way to go about it.

My advice is to pin down exactly the kind of diagnosis procedures you should utilize. There are dozens of possibilities that could cause rich running condition. It could ultimately be more than one part causing an issue. Honestly if there's no check engine light then you're in for a ride, metaphorically.

I'd check all the sensors I could check first. MAF/MAP, Intake Temp, Throttle Body/Position Sensor, O2 Sensor, Fuel Pressure Sensor/Regulator, Coolant Temp Sensor... Check for vaccum leaks. Check the fuses in the multiple fuse boxes. This is a difficult job - real engine diagnosis like this is not easy. A mechanic with a diagnostic scanner would be able to read the live data and determine where the fault lies and make a determination on what needs to be replaced. I wouldn't continue to throw parts at it until you know for sure what needs replacing. God forbid you invest a lot of money throwing parts at it only to find out something like the head is shot or your piston rings are blown, then you'd have wasted money on an engine that may need a rebuild.

 

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