I have a 2009 Fiesta 1.4 zetec. Does anybody know of any problems with high fast idle CO numbers? 
It has been fully serviced and had both oxygen sensors replaced.I have checked the most obvious stuff like PC valve etc. It has no engine management warnings and no fault codes displayed. The CO will not come down below 0.29%. 
any help would be greatly appreciated.

Plugs & Leads OK?

Air filter clean?

MAF clean?

Throttle body could be dirty and sticking, try cleaning that

Fast idle CO is probably the cat losing efficiency.

Was the lambda & HC decent or did either of those only just scrape through?

Thanks for replying. Lambda was ok. HV was 45. Temp sensor is new. MAP readings are ok from my scanner. I’ll try cleaning the throttle body I think.

2 hours ago, DaveT70 said:

Plugs & Leads OK?

Air filter clean?

MAF clean?

Throttle body could be dirty and sticking, try cleaning that

Thanks. I’ll try that.

The problem with all of the recirc rebreathe anti-emissions systems is the overcomplicated pipework in the designs, across all makes of car.

You could easily have one tiny split in a pipe that you can't see or feel, which will only 'leak' under fast idle conditions.

I'd be tempted to take it somewhere that can do a proper smoke test; find a local specialist that has real recommendations on some social media somewhere.

Otherwise you are just going to continue replacing hardware that has nothing wrong with it.

I don’t know the answer. But I wonder if you used a elm327 obd fault code reader (eg from tunnel rat electronics) for about 20 quid with forscan software (free for windows, about 6 quid for Apple/android) you might be able to look at live readings that might give a clue eg voltage being created by the gas sensors or maybe some other readings. I’m certainly no expert but someone might be able to give you more guidance on this.  Also I wonder if you used decent new sensors or the cheapest ebay ones.