This document is based on my experiences with a Bosch 800 series dishwasher, built in 2015. It probably has wide applicability to other models of Bosch dishwashers over many years.

The E18 error code means "low water level". The user manual suggests investigating the following three possible causes:

1. Kinked water inlet supply hose.
2. Blocked filters in the supply hose.
3. Water tap not turned on, or water pressure is too low.

None of these issues were relevant in my case.

So, if the water level in the dishwasher is too low, maybe there is a problem with the water level sensor?

However, the dishwasher doesn't have a water level sensor. At least not a direct sensor.

The dishwasher senses the presence of water indirectly by spinning up the water circulation pump (sometimes called the water heater pump) and measuring the current it draws. The pump is driven by a three phase BLDC motor, and the current will be low if there is no water, and higher if water is present. This is actually a smart design since (1) it eliminates a separate sensor, and (2) it gives what should be a reliable measurement of water actually being in the pump, which avoids destroying the integral heater element that would probably burn up in seconds if it was not immersed in water. You could try to sense the water by PWMing the heater and examining the rate of temperature rise, but this may be problematic, e.g., the top of the element may not be under water and insufficiently monitored by its thermistor.

Removing the water circulation pump is easy. You can then check the winding resistance, heater resistance and thermistor resistance. You can also inspect for the presence of foreign objects, and for free rotation of the impeller. In my case, the impeller shaft had a lot of side-to-side play, so I replaced the pump. E18 still occurred.

Other possible causes of an E18 that Bosch won't tell you about are:

4. A faulty aquastop mechanism. This is the device that connects to the tap from your mains water. It contains a removable filter (which you should clean), a secondary filter which appears to be non-removable, and a solenoid. The aquastop is a cleverly designed device that passes water at precisely 2.5 litres/minute, provided the inlet pressure allows at least 10 litres/minute to flow. By doing this, the dishwasher can regulate the amount of water added by simply timing the opening of the solenoid. You can verify the inlet pressure by filling a bucket and timing it. You can do a definitive check by removing the inlet hose from the heat exchanger (the plastic tank on the left side of the machine) and filling a bucket. I found 2.5 litres/minute. The inlet water flow is also measured by a spinning magnet and a reed switch. Apparently this sometimes fails, but shouldn't give an E18 code. You can easily check this mechanism with a small magnet and an ohmmeter.
5. The heat exchanger has one or two solenoid valves that control the water flow. You can check the solenoid coils with an ohmmeter (typically a few thousand ohms), or feel the temperature rise on them when operated (obviously take extreme care with mains voltage - only do this if you are qualified). It is possible that the coil(s) are OK, but the plunger is sticking or otherwise not working.

After all these checks, I was still stuck with an E18.

The problem turned out to be the "water points" mechanism. This consists of a mains operated geared AC synchronous motor that rotates a thin plastic disk with a number of holes in it. The mechanism accepts water from the circulation pump and directs it to various spray arms and other places in the dishwasher. If faulty, the pump may be driving water into an unexpected flow restriction, which would change the pump current and possibly signal a low water level condition.

The "water points" motor rotates at about 4.5 RPM, and the position of the plastic disk is sensed by a "pulse transmitter". According to Bosch, it is not possible to check the operation of the pulse transmitter. Not true. The pulse transmitter is a simple spring loaded contact that is activated by a cam mechanism on the final output shaft from the geared motor; there is a 1:1 gear connecting the output shaft to the plastic disk. One of the two wires from the motor passes through the contacts. This means that by monitoring the voltage on the other side of the contact, while continuously driving the motor, the electronic control module can determine the output shaft position by noting the characteristic timing of the on/off pulses as the shaft completes a revolution. Furthermore, by swapping the motor power connection to the contact when it is on, the motor will then stop when the contact is broken. This provides a nice simple indexing system. Note that my analysis here is based on guesswork, and it may be that the index position is not defined by the break of a contact.

In my case, looking at the contacts showed clear signs of blackening where sparks had occurred during 7 years of operation. Checking with a multimeter showed that the contact impedance was poor and variable. I cleaned the contacts by using a piece of paper impregnated with a small amount of Brasso, and passing the paper between the contacts when closed. I then cleaned off the residual Brasso and sprayed the contacts with CRC contact cleaner and lubricant. This fixed the E18 error.

A further symptom of a "water points" problem is that the dishes weren't always cleaned very well, which may be explained by one or more spray arms not being activated during the cycle. Which reminds me that you should also check the spray arms for any blockages.

What Bosch should do: the electronics module should easily be able to detect a variable timing problem with the "water points" that would have identified this error. The module also keeps track of lots of other diagnostic information - this should be available via the dishwasher info screen and shouldn't require a special purpose scan tool. Bosch should make service manuals available, or at least give a simple description of the operation of the dishwasher as I have done above. Bosch should explain what an E18 error code really means. My water level was not low.

That said, the design of the Bosch dishwasher is very nice, and it is easy to remove the various panels and components (apart from the heat exchanger).

Incidentally, if your drain pump has an open phase, it is most likely due to a break in the wire from the coil where it is connected to the PCB pin. It is possible to clean off the insulation on the remaining wire from the coil and use another piece of wire to remake the connection. Then cover the connection with the gloop that is used over the pin. I had to do this twice, but bought a replacement pump in case it failed again.

I hope this has helped someone.

Two weeks after fixing the dishwasher, it gave an E27 error code, see here for how I fixed this: diagnosing a Bosch dishwasher E27 error code

Michael Ashley 19 August 2024