What does the LF code mean?

Thermostat is probably mechanical and has welded contacts meaning it is on all of the time or the sensor, if electronic, is faulty or not in the right spot to detect the temperature.

PDF]Operation and Installation Manual Outdoor Tankless Water ...

https://www.rinnai.us/.../U287-1932(00)_Outdoor_...

Rinnai America Corp.Temperature controlled continuous flow gas hot water system. .... This water heaterwill attempt to provide hot water at the temperature setting even when the ...

Hot water system temperatures and the code ' Plumbing ...

plumbingengineer.com/content/hot-water-system-temperatures-and-code

Feb 1, 2015 - There have also been incidents where the circulating pump stopped ...before it connects back to the cold water inlet to the water heater. .... Is there a maximum temperature limit for domestic water heaters? ... a means to limit the maximum setting at the valve to 120°F (49°C) or ... Some do and some don't.

[PDF]User's Operation Manual - contents - Navien

If this is a new hook up some one has got the two water pipes hooked up on the wrong sides If it works that way might be as simple to use a black marker then remark each side. Save a lot of work as well someone won't make a mistake some day and get into trouble.

I'm guessing about model. I Goggled Eco failure. For reheme this site and discussion may help. http://terrylove.com/forums/index.php?threads/ge-rheem-power-vent-eco-failure-code-gg50s06rvj.43646/
but since it's gas, and an expensive unit I'd call the installer or his service.

Simply means hot water heater called for heat for 1 hour and never rose in temperature or reach stepping, unit automatically goes into lockout. I would reset power

Most Larger water heaters are 220 volt ac single phase. This means you need 3 wires, two hot wires one neutral or ground. You will need to come from the breaker box off of a dual breaker of 30 amps or better. See the manual with the new heater. In your breaker box you have two sides or in coming service. The right side and the left each side is hot 110volts ac. you need to pull one leg (hot) from each side into the dual breaker out to the heater connections. The ground or neutral wire is un-insulated copper. Run these wires out to the heater. One black hot at 110 vac, one white hot at 110 vac and one plain copper ground.

Before calling service technician, open following link for Rheem troubleshoot resources including Rheem chat:
http://waterheatertimer.org/Troubleshoot-Rheem-Tankless-water-heater.html

Error codes are considered start point for troubleshooting, and can sometimes mislead.
Error P1 is 'not enough water to operate the unit:'
http://waterheatertimer.org/pdf/Rheem-Error-code-P1.pdf

This problem can be caused by plumbing problems such as crossover, or clogged water lines, or dirty water filter, or clogged water softener, or temperature of unit set too high. Or combination of these problems.
Remember if temperature is set very high, then more cold water is used at the faucet. This means less hot water is drawn through the tankless water heater, so the flow rate may drop below activation threshold.
http://waterheatertimer.org/Crossover.html
http://waterheatertimer.org/Low-hot-water-pressure.html

Before calling service technician, open following link for Rheem troubleshooting resources.
http://waterheatertimer.org/Troubleshoot-Rheem-Tankless-water-heater.html

No error code can mean variety of problems:
http://waterheatertimer.org/pdf/No-Error-Code.pdf
http://waterheatertimer.org/Low-hot-water-pressure.html
http://waterheatertimer.org/Crossover.html

If thermostat is set high that can also cause unit not to turn on.

It runs off 220v, meaning there are two "hots" and a ground. The 190tc uses TWO sets of 220v at 40amps each. Instead of only having one dbl throw breaker, you need 2. Having said that... the white wire "should" be the same as the black wire and can be connected which ever way you like