My dishwasher stopped working yesterday. The symptoms were the wash started as normal but then stopped half way through flashing the 4th light. The water wasn’t hot and the soap dispenser hadn’t opened.
Some Googling showed that this is caused by the water not reaching the desired temperature. The dishwasher starts with a 15-20 minute cold wash, then it heats the water and starts the main wash. As the water wasn’t reaching the desired temperature after a certain time it gives up, abandons the wash and flashes the error 4.
There is 3 likely culprits for the water not heating. The heating element, the cut out thermostat and the PCB Timer Module board.
The heating element and thermostat can be accessed by pulling off the black plastic covers on the bottom of the dishwasher. On the right is the heating element connector which can be tested by pulling off the two connectors on the bottom and testing for continuity with a multimeter. Mine was fine showing a few Ohms.
To test the thermostat is a bit trickier, it is on the back right side of the dishwasher. You need small arms to get in, disconnect the two connectors then it unscrews. It is supposed to be hand tight but I had to use a spanner to loosen it. Once disconnected test for continuity with a multimeter on the lowest Ohm setting. Mine was fine, to further test it I put it in some boiling water and I heard a audible click after a few seconds, the multimeter then went to 1 showing no continuity. Once it cooled down it went back to reading a few Ohms.
This left only the PCB, by reading the white goods forums I had discovered that first of all this dishwasher is a piece of junk and secondly that in this scenario it’s usually the PCB that is broken. Apparently thermostats and old style heating elements rarely break. The bad news is a new PCB Timer Module costs £60-£70!
Someone had mentioned on the forums that they fixed their board by changing a zener diode on the circuit board, unfortunately there was no further details. As it turns out, although I didn’t know it yet, I had exactly the same problem with my PCB. Below is detailed instruction how to test and replace the diode.
On my board there were three visible zener diodes. I decided to test them 1 by 1 with them in the board. To test them properly they should be removed from the circuit, but I was hoping to narrow down the culprit to save me having to remove them all to test.
In theory there should only be continuity one way on a diode. I tested all three with the black test probe to the line side of the diode, all three had continuity which is correct. I then reversed the test with red to the line side of the diode, two of them reported no continuity, but one of them read about 45 Ohm. I removed the suspect diode and tested it again outside the circuit and confirmed the same readings. This diode was broke.
On the side of the diode it says C 12 PH. This means it is 12V and Philips. Maplins sell two 12v zener diodes, one is 0.5 watts and the other is 1.3 watts. I went for the higher wattage one BZX85C 12V 1.3W which turned out to be an exact physical size match to the original. After soldering it back in I am happy to say the dishwasher started working perfectly.