My Takagi T-M1 water heater is not functioning. After less than a minute of operating it stops and the red light in the front is blinking.

Please reach out to your local plumber.

Generally tankless must be maintained on yearly basis.
Error codes are usually stored in the computer memory until they reach a threshold and then the error code is displayed... it is possible the control board is bad.. or the sensor that feeds information to the control board is bad.
Contact manufacturer or installer.
Contact local service technician who is qualified on that specific brand.

Gene
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This indicates a reduced supply of water to the heater. Check water flow through the tap has not been reduced If so, check filter area

checked for long time final find out the sensors socket has some water in it and I dry it out and the heater fire up right away "hope this can help other people " good luck.

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)

Code 541 means the 2 way valve is not turning, that could be the valve has build up calcium due to hard water and the attached motor is not able to turn it, or the motor is broke or finally the controller (Compute Board is defective).
The 2 wau valve sits behind another valve when looking at the unit on the left hand corner, it must be taken out first to gain access to the 2 way valve.
The 2 way valve motor can be seperated from the valve by two screws and the valve can be manually opened but turning it to the horizantal position, there is a pin on that shaft.
Turn kon the heater then but leave the motor connected, if the motor turns for short time then the PCB is most likely ok. This usually indicated that there is too much of build up inside the valve. If the tankless was properly with bypass valves (outside of the unit) installed then this can be easily fixed.
Turn the unit off and run with an external pump and bucket with vinegar for about 30 minutes by using the bypass valves. This will clean the 2 way valve from inside and should turn then easily and the motor will be able to control it. This is usually beyond the DIY scope, call a real pro for that.
http://www.justtankless.com

to clear the 991 Takagi code, simultaneously press the bottom 2 grey buttons of the four just to the left of the flashing LED light. When the LED stops flashing, release and then press the reset button on the black panel about 8 inches above that

explain your problem.
Takagi is a great tankless heater, very few problems...

need more info

diaphragm valve or micro switch best of luck

Pilot must heat the thermocoupler a couple of minutes before letting go of the button. The coupler controls the main valve.