So have had the engine management light come on probably 3 times, all times no loss of power or performance.

First time, garage scanned it and found a code but cleared it and couldn't find anything seriously wrong, so told me to drive it and to bring it back if the light came back on (they didn't charge me).

Car was ok on short trips but a few weeks later light came on again and only did so after a motorway trip of over 50 miles. Took it back and they said it might be the cat, cleared the fault, ran cat cleaner through it and test drove it, but obviously couldn't do longer mileage and the light didn't come back on. They did advise me if this didn't work, then it would mean a new cat and that an OEM part would be preferable.

It's been a few weeks and only done local mileage but yesterday did some motorway mileage and light was back on after 50 miles or so, still with no loss of power.

I scanned it today and got the P0420 error code. Having read other posts, people have suggested replacing a lambda sensor first (the cheaper option).

So, having looked at OEM parts (lambda was silly money and couldn't find a Ford cat), what aftermarket lambda would people recommend and what cat has anybody used if they've had to replace one?

Sorry.....2004 Fiesta 1.4 Zetec!

19 years is a fair old age for a Cat 🙁

Guess so. Only done 39k though. 

It'll more likely be the sensor.

Don't buy cheap

Have you changed fuel recently?

Are the HT leads, spark plugs and coil pack in good condition?

Consider changing those, first, too

The ignition side of things was taken care of when I broke down before these current issues. The engine management light came on and the car was a dog to drive. Plugs and coil pack were replaced. 

I didn't have any issues with the cat before this and am thinking that might've caused these current problems?

I appreciate don't but cheap, but there's such a wide gap in pricing, from over £200 for OEM, down to about £30 for a Lucas. I try and but original of possible, but not willing to shell out over £200 of it turns out to be the cat!

34 minutes ago, NobbyC1380 said:

I try and but original of possible, but not willing to shell out over £200 of it turns out to be the cat!

I doubt very much it's the cat

Maybe source a known working 2nd hand original sensor

P0420 is catalyst not working correctly, but could mean it's the post cat sensor, not the front one.

It could be carboned up

Has anyone visually checked the cat?  At 19 years old, I wouldn't be at all surprised if it was blowing through a rust hole, that'll throw the lambda readings out.

1 minute ago, TomsFocus said:

Has anyone visually checked the cat?  At 19 years old, I wouldn't be at all surprised if it was blowing through a rust hole, that'll throw the lambda readings out.

That is a possibility.

It's definitely not the front sensor though, yes get the cat checked

33 minutes ago, DaveT70 said:

I doubt very much it's the cat

Maybe source a known working 2nd hand original sensor

P0420 is catalyst not working correctly, but could mean it's the post cat sensor, not the front one.

It could be carboned up

 

They ran a cat cleaner through last time it was in, so hopefully would've removed some of the carbon if there was any present. 

4 hours ago, NobbyC1380 said:

They ran a cat cleaner through last time it was in, so hopefully would've removed some of the carbon if there was any present. 

I'm thinking more that the post cat sensor may be carboned