Error Code 541 indicates that the Two Way Valve is malfunctioning. This assembly consists of a small motor and a rotating ball valve that is operated by the motor. The replacement part is only sold as a unit (Motor and valve) for about $200. It is located in the lower left of the unit, behind the flow valve. (i.e. it is hard to reach in order to service it). You basically have to take out the exhaust fan, take out the flow valve in order to replace the 2-way valve. It's doable as a DIY, but it's not trivial.
The valve could be crusty from deposits, so before taking everything apart, it would be wise to flush the heater for 45 minutes with anti-scaling and a recirculating pump to see if anything breaks loose. (using the service valves that should be mounted on the bottom of the unit)
The descaling procedure freed up the valve and got it operating properly. Try this FIRST. I tried to take out the fan and it is not possible to do with the T-M1 installed. The bottom of the fan housing is in a recess at the bottom of the heater case. Removing the fan is a MAJOR disassembly process. Removing the flow valve and two-way valve requires several new gaskets for re-installation as well. Refer to the IPB for the T-M1. It's not a DYI project unless you have plenty of other parts on hand and ready to go.
And I should add that you have plenty of time (hours) that you can do without hot water to field strip your heater to replace the two-way valve. I could not see where the valve can be "adjusted" in place to manually open it. To remove the valve, the entire hot water output line system has to be disassembled plus removal of electrical components.
I did a 1.5 hr vinegar flush. Bought a 1/6 hp submersible pump and a 5 gal orange bucket from the orange big box store. I had an extra set of washing machine hoses on hand. Used 2 1/2 gal of white vinegar as per the descaling procedures.
I am glad to hear that you got the unit working again with just a flush. I was not as fortunate. I was able to disassemble everything with the TM-1 installed. Getting the fan out is a little tricky, but not impossible - after all, the factory has to be able to get the fan in/out! Yes, some new gaskets are necessary. By the time I ordered parts and did the work, our hot water was out for five days.
I should add that an Error 541 (stuck 2-way valve) might have fried the main computer board with an over-current draw from the stuck 2-way valve motor. Takagi admitted that the Error 541 often requires a replacement of the motherboard. So it's an expensive problem if this occurs (for me, I had to replace the motherboard). My advice? Be diligent about flushing your heater with de-scaling solution at least once per year, more if you live where the water is hard with minerals.
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541 is the 2 way valve error, could be the computer, or the valve itself. The valve has a motor that could be bad. As a temp solution you can unscrew the motor from the valve and turn the valve manually but leave the motor connected, That should fix the problem, the water temp could flactuate a bit since the valve is not being controlled by the computer.
You can also try to descale the unit, that will most likely make the valve move freely and if the motor and the computer are working then you are done, problem fixed.
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