How to Repair Bosch Hot Water Controller YPRM66XB

YPRM66XB bosch controller repair fix
How to repair Bosch Hot Water Controller
YPRM66XB bosch controller repair fix

I love my Instantaneous Bosch High Flow Gas 26e Hot Water unit. It gives me and the family endless hot water whenever we want. Well until the other day when we had no hot water. I thought no problem, maybe the pilot light didn’t light or something? So How to Repair Bosch Hot Water Controller?

Well, the pilot light didn’t light up, nothing did! So I powered down the unit and restarted it and checked for hot water and I did have some so great another problem fixed. But it was short-lived and the water went cold again after a few days.

Further Investigations reviled that the Optional Temperature Controller was fried. It was blinking on and off, the numbers were all scrambled and the buttons didn’t work. Time to pull this baby to pieces and work out whats going on. Because damned if I’m going to pay $150 for a new Bosch Controller YPRM66XB. I’m going to repair the Bosch Main Controller myself.

Common Bosh Controller Models

The Following Bosch repair procedure should work for the following Bosch Hot Water Controller Models:

  • YPRM66XB
  • YPRS66XB
  • YPRM62XB

So How to Repair Bosch Hot Water Controller?

First, turn off the power to the Bosch Gas Hot water heater. Very Important to Isolate the electrons. This power switch is usually next to the unit itself, tag it out (Do Not Turn On)

You are going to need some tools. Every home handyman should have these electrical tools in their garage.

Bosch hot water controller YPRM66XB

Second, remove the outer cover of the Bosch Controller and remove the 2 wires (take note where they go) then take out the 2 screws holding the controller to the wall.

Third, take the controller to your kitchen table and start stripping the unit down to get to the motherboard inside. There should only be a few screws. Once you have the motherboard out have a good look over it to find where it is corroded. Yep, that’s what causes these units to fail corrosion.

Corrosion in electrical circuits is usually caused by a high-humidity environment. Our dryer is close to the Bosch hot water controller.

Forth, Take some methylated spirits on a small paintbrush and start dabbing at the corrosion it should come right off. Once it’s all clean make sure it’s dry and put it all back together and wire it up. Your controller should now be working.

How to Repair the Push Buttons on a Bosh Hot Water Controller

Repair Bosch Hot Water Controller

The second problem I had was my increase temperature button on the Bosch Controller YPRM66XB didn’t work. I could decrease the temperature fine but not increase it. So the unit came off the wall again to diagnose this new problem. To simulate the button, I used a multi-meter on the button set to OHMS and pressed the button. No change in resistance to something on the button was faulty.

Now, these buttons are tiny so you can solder new ones on. In the short turn, I used a small cable to touch these 2 points shown on the Picture top left (2x pen lines) drawn on the circuit board.

This is to simulate a button press while it was powered on. I don’t recommend this as you could touch something else and fry your board. It just got my temperature up so the family was happy before my new switch arrives.

Why did my Bosh Controllers fail?

Well due to the location of my controller in the Laundry, it is subject to moisture in the air due to my clothes dryer. Having an extraction fan or window vent should help with high humidity in a small space.

Other people may have Bosch master and slave controllers in bathrooms which may also have high humidity. As moisture gets on the motherboard it will start to corrode and shots out some Low Voltage components. The Mentholated spirits help remove this corrosion and it dries fast without leaving any residue. Try a heat gun on a low setting or a hairdryer to get rid of any leftover moisture.

Bosch Gas Hot Water Temperature Irregular Hot and Cold

Fluctuating hot water temp

Another problem I have had and many others, Is the Hot temperature fluctuates when having a shower. My problem only started when I had a water saver shower head installed.

So as my Bosch unit is the Bosch Hot Water System High Flow 26e. I put it down to the unit not being able to regulate the low water flow in the water saver shower head. I’m sure there is a minimum flow rate stated somewhere in the manual.

If you are having hot and cold showers try taking out the water saver restrictor in your shower head it should fix your problem. At least my family can have hot fun showers again with their color changing shower head.

Good luck with your Repair Bosch Hot Water Controller, leave me a comment if this repair procedure has helped you.

Bosch Hot Water Error Codes – Tankless

Bosch Hot Water Error Codes - Tank less

I have included some of the more common Bosch hot water error codes that I come across shown below.

Bosch Tankless Error code A2 – Problem with the flue gas limiter

The temperature of the flue gases is too high and is above 230 F. Check for gas leakages or constant flame on the burner. This temperature sensor is located at the top-right corner of the unit.

Bosch Error code A3 – Wire harness fault 

Measure the resistance on the wire harness; it must be between 115 and 120 ohms.

Bosch Error code A4 – Backflow temperature sensor

Check the backflow temperature sensor connection for rust or damage. It could also be short-circuited. It is located below the heat exchanger and ignition electrodes. Look for the diagnostic menu 1P and nC code.

Bosch Error code A7 – Outlet temperature sensor fault

The outlet temperature sensor reacts when the temperature is either below 36 F when the unit is protected from freezing temperatures. Also above 210 F in a case of overheating due to the lime-scale deposits. Check the sensor and wiring for scale, rust, or damage. You might have to flush the unit to remove the mineral scale build-up.

This fault results from the temperature thermistor on the right-hand side of the heat exchanger failing to give a temperature indication. This fault will not stop the heater from operating, but will first display ’00’ on the ICU display before changing to ‘A7’ on the ICU display. Ensure wiring connections to the thermistor. If this does not rectify the fault, test thermistor readings in ohms in conjunction with the thermistor chart to assess correct readings.

Bosch Error code A9 – Outlet temperature sensor fault

This A9 error code is associated with the outlet temperature sensor, but this time, the sensor is not sensing the expected output temperature. This is not an error code; it is a flashing status message that tells you to check; the wiring connection, gas pressure as it might be too low, voltage as it doesn’t meet the required 120/240 VAC, the unit that might not be properly grounded or the issue with the control board.

Bosch Error code C6 – Slow-spinning fan

The primary extraction fan spins slowly when the water heater is working. Check the wiring connection on the fan and control board. The voltage must be 120/240 VAC and the element correctly grounded. Does the venting system meet the code and manufacturer requirements? Ensure that the gas pressure is as required because the low pressure leads to fan rotation problems.

Bosch Error code C7 – Fan rotational speed sensor

No signal from the fan’s rotational speed sensor. The sensor might be broken. Check the wire connection on the control board and fan. The voltage needs to be 120/240 VAC and adequately grounded. Ensure that the fan and control unit is not damaged.

Bosch Error code CA – Excessive water flow

Make sure that the water pressure is less than 150 PSI and a flow rate below 10 GPM. Check the wire connections on the water valve and control panel.

Bosch Hot Water Error Code 11

The Bosch gas hot water fault code 11 signifies that there is a fault with the unit’s ignition. In this instance, your Bosch hot water heater will not operate and won’t produce hot water until Bosch fault code 11 has been investigated and repaired.

Bosch Error code E1 – Extremely high temperature

The temperature detected by the outlet temperature sensor is over 185 F. Ensure that the sensor is working correctly, and the wire connections are not loose. Another reason for the high temperature is the sediment build-up, so descaling and flushing will be required.

Bosch Error code E2 – Faulty inlet water temperature sensor

Check the sensor and wiring connections for corrosion as this sensor is in charge of protecting the unit from freezing. 

Bosch Error code E3 – Problem with the exhaust temperature sensor

The unit will shut down if the temperature of the exhaust gases is above 194 F. Check and clean the heat exchangers if needed. Reduce the maximum power and water temperature.

Bosch Error code E4 – Problem with the backflow temperature sensor

The backflow temperature sensor reacts when the temperature is over 309 F. Check the vent pipes if they meet the code and requirements, look for damages and obstructions.

Bosch Error code E9 – Overheat sensor ECO has an open circuit 

Check the wire connections on the sensor and control board. Ensure the venting system is per codes and instructions. Descale the unit if necessary.

Disconnect the wires from the temperature thermistor (over-temperature sensor) on the right-hand side of the heat exchanger, and test the resistance of the thermistor. If the resistance is greater than 0.1 ohm, replace the thermistor.

Bosch Error code EA – No sparks for the electrodes

The ionization electrodes are not making sparks when water is flowing. There are many reasons for this problem and here are some possible issues; low gas pressure, the fan speed is low, wire connections are loose, the air in the gas line, no gas corroded connections, dirty electrodes.

Failure to ignite will cause Safety Time Out. Three possible causes are

1. Ignition failure due to insufficient gas supply. Check the gas supply to the unit, Correct if necessary.

2. Flame rod not detecting pilot flame. Check the wiring connection to the ICU. Measure the flame rod current (should be 10 – 12 µA DC). Check the condition of the pilot flame, and if dirty, clean or replace the injector. If the pilot is clean, and the voltage from the flame rod is correct, replace the ICU.

3. If the pilot is failing to light, measure the voltage to the orange pilot solenoid (should be 0.9 – 1.2 Volt DC). If the voltage is correct, replace the solenoid. If the voltage is incorrect, replace the ICU. (note:- always check the voltage with the wiring connected)

Bosch Error code EC – Electrode failure

Ionization electrode failure during water heating. The reasons might be; obstructed venting system, low gas pressure, and CO2 readings, the fan speed could be too low.

Bosch Error code E0 hardware/software issues

The issue with the hardware or software. First, try to reset the error code and turn the hot water unit off and back on. There might be an issue with the computer. Check if the wire connections are properly linked to the computer board or the board is not damaged.

Bosch Error code F7 – Ionization rod problem

The problem is with the ionization rod at standby. Check connections to the electrodes for corrosion and tightness. It should be securely connected. Other reasons might include damaged elements or a faulty control board.

This fault can result from an earthing problem with the flame sensor rod or an ignition pack failure. Clean the flame rod and ensure the wires are adequately connected. If the fault remains, replace both the flame sensor rod and the ICU.

Bosch Error code FA – Gas valve circuit

The problem is with the gas valve circuit. Check the wire connections on the gas valve and control board. Measure the voltage on the wires when a water heater is ON… it should be 24 VDC. If the measurement is not as specified, the gas valve is malfunctioning. Check for correct gas pressure and that the gas valve is operating by putting 24v onto the valve.


Check out this post on DIY Bosch tankless water heater troubleshooting if your remote controller is working fine. There is also a more in-depth list of error codes included in that post.

Bosch hot water units are very reliable and only tend to fail when there is moisture in the PCB, so keep them dry. Avoid installing remote control units in wet areas like bathrooms and laundry.

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  1. If the controller has only two wires, is this just a variable resistor system? I need to lower the temp on mine in a standard unit. Bridging options only increase the temp.

  2. Just moved, one controller next to the bath. mounting screws completely rusted, had to drill it out. Saw some haze on PCB, cleaned with IPA. Still no lights when reconnecting.

    You said it’s an optional controller? The water is barely warm and I can’t bathe my kids because I can’t get the water hot.

    How can I increase the water temperature without it if its just optional please?

    Also how can I test the wires to see if they’re good? Should I check for a voltage across them? Amps? Ohms? And what reading should I get?


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