My 99 Fiesta heater is stuck on hot. Replacing the control valve did not help.

Checking the power to the control valve I get a steady 12v with the heater temp knob in either hot or cold positions.

I don't think this is right. I believe the voltage should pulse in one of these knob positions. Mine doesn't.

Can anyone confirm this should be so.

Does this then mean the temp switch rheostat is broken.

I believe it's possible to replace the circuit board for this switch rather than buy the whole console from Ford.

Can anyone confirm this is possible and perhaps suggest where I may find one.

Thanks for any help you may be able to offer.

Tried a new control valve from a Ford dealer, no change.

Fitted another control panel circuit board from a breakers, no change. Checked the voltage from it which ranged from 0 to 1 v. Obviously broken.

Took it back and got another, checked the voltage, pulsing towards the cold setting, great.

Fitted, no change.

Checked the actuation of the new control valve with this new panel. no movement.

Tried this new panel with the old removed control valve, movement, great, fitted, got hot and cold.

Except back to front !

Remounted circuit board to temp switch correctly, success.

Now returning duff control valve to Ford dealer.

Surely it shouldn't be this difficult.

Tried a new control valve from a Ford dealer, no change.

Fitted another control panel circuit board from a breakers, no change. Checked the voltage from it which ranged from 0 to 1 v. Obviously broken.

Took it back and got another, checked the voltage, pulsing towards the cold setting, great.

Fitted, no change.

Checked the actuation of the new control valve with this new panel. no movement.

Tried this new panel with the old removed control valve, movement, great, fitted, got hot and cold.

Except back to front !

Remounted circuit board to temp switch correctly, success.

Now returning duff control valve to Ford dealer.

Surely it shouldn't be this difficult.

The heater on my Fiesta was stuck permanently on hot. I initially changed the solenoid valve (located between the windscreen and the bulkhead under the bonnet, it has two pipes in and two out) but with no benefit. Last time I had a similar problem this did fix it and it is a well known fault for these valves to stick. So, it seemed to be a problem with the voltage driving the valve. This can be measured at the plug connecting to the valve. With the engine running, the voltage across the two terminals should be 14v with the control at cold, 0v with it set at hot and 14v pulsing down to a lower voltage with the control set mid way.

By the way, it is possible to check if the valve is working. If 12v is applied across the valve terminals (+ to the terminal furthest from the engine), the valve should click audibly as it activates. This can be done by connecting directly from the battery - the engine does not need to be running. If it is stuck, you can remove the 4 screws and pull off the solenoid windings. The armature is then exposed and can be manipulated to unstick it. Coolant will probably leak out so it might be worth removing the valve from the car first. I think that they stick because of failure of the O-ring. The latest Ford supplied valves have a slightly modified design.

On my car, the voltage was reversed with 14v at hot and 0v at cold, and there didn't seem to be enough current to move the valve at all. I initially checked the earth (this can be found under the plastic trim over the inner sill and A post on the driver's side, just inside the door). There are two earthing points there, the rear one is for the heater control panel. Improving the earth made no difference. I tried a replacement, secondhand, control panel but again it didn't work. Then I took the circuit boards off the back of the control panels and found that both had a small blister on the component labelled PH06 52-10A. I went back to the scrap yard and got the faulty panel changed. Before fitting the replacement, I checked the circuit board and found that this component was OK. After fitting, the voltages were correct and the valve worked. Fantastic!

In actual fact, the working panel came from a Ford Puma. It seems to be compatible with the Fiesta. Because it was silver rather than the wood effect from my Fiesta and didn't have a cigarette lighter socket, I fitted the Puma circuit board to the old Fiesta panel. This worked fine, but the arrangement of LED lights is different so that the A/C and circulate buttons now lit up with the side lights on (panel lights) but did not indicate when activated. I drilled holes in the top of the buttons so that the second set of LEDs on the Puma board (the ones which indicate button activation) showed through. So, finally, we are back in action with a controllable heater.

How do you get the control panel off?

Apologies..great post and very informative.