l checked the radiator levels there fine... my heaters keep blowing cold air even when l have been driving for a while. Could there be something else that wrong with it?
How do l check the heating core?

your heater core may be bad, have you checked it lately?

thermostat may be bad, or the control valve.

Yes, there could be. Sounds to me like the thermostat may be at fault. A easy to check this is to put a piece of cardboard directly in front of the radiator (the cardboard needs to cover the entire front of the radiator). Then start the car & put on ur heater, & wait & see if it blows hot air after awhile. If so, then ur thermostat is bad. Also u may have issues with ur water pump, as well. As for the heating core, well u almost have to take it to a radiator shop, so they can pressure test it for you. This will detect any problems with the heating core itself.

Depending on what year car it is, it could well be the heater control valve as it is a common fault.
It is quite easy to change yourself, just 4 pipes & a connector then top up the coolant.
See link below or go to Ford dealer
http://cgi.ebay.co.uk/HEATER-VALVE-FIESTA-KA-PUMA-TRANSIT-1-YEARS-WARRANTY_W0QQitemZ270307694857QQcmdZViewItemQQptZUK_CarsParts_Vehicles_CarParts_SM?hash=item270307694857&_trksid=p3286.c0.m14&_trkparms=72%3A1299%7C66%3A2%7C65%3A12%7C39%3A1%7C240%3A1308

hmmm sounds like u had the same problem as me.

when my fiesta heater was blowing just cold air l just fiddled around & noticed that where the wipers r under the bonnet is the heater valve where the heat should be coming from.

i took the heater valve off (should be two rubber pipes coming from the engine to the valve then from the valve to the heater control inside the car.) & tried my luck in hoping it was at fault & just replaced it by buying one from ebay (brand new).

i replaced it & it worked after 2 Min is of leaving the engine on.

also l should point out im no expert in fixing cars so if l can do it so could other people.

if u think the Valve is at fault & l did not give enough detail on what to do then just email me & ill try & help as best l can.

hope this helps