Zanussi Dishwasher Not Heating

[kcr123]

My five year old Zanussi ZDF26011WA dishwasher is not heating correctly. On the Auto cycle, the water heats to around 38 degrees near the start of the cycle, but never reaches full temperature, and finishes cold (normally finishes hot and steaming when opened).
I have checked the following:

• Temp sensor appeared to return the correct resistance readings. I tried replacing it anyway, which made no difference
• Heater measures 24 ohms, which seems to be about right for the 2kW rating
• I can measure 12 V on the control board heater relay in the early part of the cycle, and 240V going to the heater, so the heating circuit appears to be OK, but it only seems to heat for a short period at the start of the cycle
• Pump appears to be working fine and water is spraying
• I have cleaned everything out and checked the channel to the pressure switch is clear

So the components (as far as I can check) seem to be OK, it just isn't heating for very long. Can anyone suggest what might be wrong, or anything else I can investigate?
Thanks

 

[telectrix]

looks suspiciously like a control panel PCB fault.

 

[brianmoooore]

May or may not be relevant in this case, but weird faults in washing machines and dishwashers are often caused by a blockage or partial blockage in a pipe to a pressure sensor used to determine water levels.
Pressure sensor is usually a round device, about 60mm in diameter, mounted near the top of the machine, with a hose of about 8mm diameter connected to it. Pull the hose off of it and blow hard into it, reconnect and see if it makes any difference at all. If it does, remove the hose, find and clean the blockage properly.

 

[kcr123]

Thanks for the responses.

I wondered about the control panel, but it seems a very specific fault in the programme sequence rather than an obvious component failure; I can definitely see the relay being energised and supplying power to the heater, it just doesn't turn it on for long enough. Everything else in the control sequence seems to be working correctly.

The pressure switch on my dishwasher appears to be mounted in the base and connects to the sump via a short pipe moulded into the sump. I took the switch out, which appears to be clean, and I pushed a zip tie back through the connecting pipe to make sure it was not blocked, but this didn't seem to make any difference. I was assuming the pressure switch was probably working because the heater does kick in for a short while (i.e. the dishwasher is measuring that there is enough water to start heating)?

 

[brianmoooore]

Maybe it's the opposite of what I suggested, and the pipe is leaking pressure. This would make the ECU think the water has drained, and it would turn off the heater.
I'd make up a circuit with LEDs to monitor the switching of the pressure switch, to see if it's doing what it should.