I've been getting error codes intermittently on my tankless water heater, which causes the hot water to shut off. I finally decided to get to the bottom of this issue once and for all this past Friday. I found the user manual and a video guide online, but my water heater turned out to be a little different, so I ended up having to feel my way around. This guide explains how to fix the issues that cause error codes 12, 13, and P1.
What you will need is a good Phillips screwdriver, a vacuum or compressed air and some scotchbrite cleaning pads.
First, turn off the heater from the remote control. Then, shut off the gas, and hot and cold water lines to the heater. Then, unplug the heater from the electrical wall outlet.
Open the front cover by unscrewing four screws along the edges: two on the top edge and two on the bottom edge.
Unscrew one of the screws on the bottom of the heater. This screw holds the PCB box in place. I unscrew this screw so I can swivel the PCB box out of the way.
Unscrew the two screws holding the PCB box onto the burner.
Swivel the PCB box so it's angled out. Now, unscrew all the screws attaching the burner cover. Be sure not to unscrew the screws below the burner.
Pull off the burner cover. Be careful, as there's a gasket lining the cover. Try to keep it from ripping. Now, detach the flame rod by unscrewing the screw holding it in place.
Unscrew the screw holding the igniter switch.
Unscrew the six screws holding the fuel rail (two on the top, four on the bottom). Be sure not to unscrew the many screws on the fuel rail itself.
Carefully remove the fuel rail. There may be residual gas. Try not to breathe it in.
Unplug the two plugs for the flame rods from the PCB.
Unplug the connector to the igniter switch.
Detach the flame rod grommet and the igniter switch grommet.
Carefully slide the burner out. Be careful not to bump it against anything that could bend any of the wires or plates.
Be careful that the wires don't catch on anything.
Carefully place the burner on a flat surface and carefully vacuum out any debris. Be especially careful not to touch the flame rods with your bare hands. Rub the scotchbrite pads onto the flame rods to clean them.
Also vacuum the ODS. This is the root cause for error code 13.
Now, assemble everything back in reverse order.
Plug the heater back to the wall outlet, turn on the cold and hot water and the gas lines. Turn the heater on using the remote.
Test the heater by turning on a hot water faucet. You may get error code 12 or P1. For me, I tried a couple of times and waited a few minutes before trying again, and the error codes went away and the heater has been working consistently since then.
If it doesn't work, then double-check that you plugged everything back in. There's a small window on the burner cover that you can use to check whether there's any flame action going on inside.
One other thing to check is whether the venting was installed correctly. For me, the installer improperly installed the heater. It is supposed to use a concentric pipe where the exhaust is vented out through the center and fresh air comes in through the rim. For me, there's only a pipe venting the exhaust outside. The intake was never hooked up, so it is drawing air directly from the garage, where the heater is installed. I added a foam air filter to filter out large particles from getting into the heater, as a compromise solution, given that my heater was improperly installed. This is just another source of problems.
References:
Rheem Tankless Water Heater Code 12 (Flame Failure)- [Solved!]