I have a Focus 1.4 CL, 2001, approximatley 110,000 miles on the clock. When the car is started from cold there is no problem for 4 to 5 minutes. It then begins to hesitate on acceleration and feels like it has lost some of it's power. I have had the coil pack, ht leads, plugs, battery, air filter and fuel filter renewed. None have solved the problem. I have had it hooked up to a code reader which reported no abnormalities. The engine management light is not on. Any help would be greatly appreciated. It's costing me a fortune. Thanks Chris.

If you hadn't said there were no fault codes registered I would have suspected a HOS (Lambda sensor) fault. The reason is that because this sensor doesn't give a useable signal until it gets up to temperature the engine management system fuels the engine open-loop for the first few minutes. After this the fuel/air ratio is adjusted according the the Lambda signal.

Thank you for your help. I will have the O2 sensor replaced. See if that does the trick. Regards Chris.

I think theres 2 of them, without either giving fault codes its hard to say which might be faulty and at about £100 each they arent cheap.

There's probably one either side of the cat but I would guess that the one that controls the fuel/air ratio would be the upstream one. I think the downstream one is used to monitor the performance of the cat.

The only doubt is that I would have expected a defective one to register a fault code. Maybe it's failing but not bad enough to register yet.

Hello again guys. I have changed the O2 sensor but the problem is still there. Any more suggestions would be appreciated. Thanks Chris.

Presumably you've reset the ECU to clear any settings that the faulty sensor (if it was indeed faulty) might have produced?

Yes, I unhooked the battery for 15 mins.

Think you're looking at engine sensors now then mate.

Thank you once again for your reply. Engine sensors? I only know of three, camshaft, crankshaft and knock sensor. In which order do you recommend swapping them?

In addition to the three you mention there are, according to the Haynes, a Mass Air Flow sensor, a cylinder head temperature sensor, an air intake temperature sensor and, of course, the throttle position sensor.

I would guess, in terms of cost, the cylinder head temp sensor would be the cheapest to start with and is probably the next most likely candidate since your problem seems to be related to engine temperature. Mind you, this is all guesswork. Have you considered getting a diagnostic done by a Ford specialist?

Yes, but I have heard that these are really expensive?

No more expensive than replacing sensors one after the other to find out its not the one. Most Ford dealerships charge for the diagnostic, then if you let them do the work it finds they knock the cost of the diagnostic off the end price, in effects making it free.

Thanks for the advice. I think I'm going to take it. Bite the bullet and pay the big bill. Thanks again mate.

We'll be interested to know how you get on.

Hi guys. I have posted this problem in more than one forum. Replies from the other forum sugest that I got a scan tool and post the results. I do wish to learn more about modern day engine management so I have bought a ELM327 scan tool. It's problebly a waste of money but maybe I'll learn something. Will keep all of you upto date as things progress.