Could a faulty diverter valve cause lockout when the taps are turned off?

I have a potterton puma 80e. The lockout light is comming on after a shower has been taken. It lights once the taps are turned off. The pipes to the central heating get very hot after a about 30 seconds once the taps are turned off, I seem to remember the pipes getting very hot much quicker than that previously. The diverter valve appears to be sticking or working very sluggishly the lockout happens because of over temp. The fan works, the pump is on.
thanks for any advise.