Rinnai tankless water heater

Push breaker(s) fully off and then back on.
... then open the tankless and carefully test across the terminals where wires connect to heater...
Tape tester leads to wood sticks to keep hands away from power
Following link has wiring for tankless electric
http://waterheatertimer.org/How-to-wire-Tankless-electric-water-heater-new.html

Code 12 is a flame failure.
First step in diagnosing the root of the issue is to check for any separation, obstruction, or failure in the vent system.
Second step is to use the isolator kit under the heater to flush the heat exchanger with vinegar or rectorseal calci-free.
These two things are a great starting point to working on any tankless water heater. Flame failure can be as simple as the heat exchanger being clogged not allowing the sensors to work properly or the exhaust being mixed with the air intake. If the problem persists I would suggest having a Rinnai Certified Tech look at the heater.

Go to Rinnai and get a manual with the error codes... for example a shorted element will trip breaker... wiring problems can cause breaker to trip... anything that causes heat on wire to exceed amp rating of breaker will cause it to trip...
http://waterheatertimer.org/Water-heater-is-tripping-breaker.html

If you need further help, I’m available over the phone at https://www.6ya.com/expert/gene_9f0ef4df2f9897e7

Usually, Breaker tripping indicates a hard short ... probably the heating element is shorted and needs replacement. Possible bad C/B but don't think so.
Good Luck! handy Henry.

If you need further help, I’m available over the phone at https://www.6ya.com/expert/henry_239f63a136de60f0

Check the circuit breaker?

One thing they don't tell you about tankless heaters is that when they turn on, they may cause the lights in the house to flicker. Make sure that you have a good breaker panel (one that is relatively new with plenty of capacity) and run a circuit specifically for the tankless heater.

The rinnai is a computer that heats water. It is not ordinary water heater most people are familiar with, and repairs are not do-it-yourself, requiring a trained technician qualified in that brand who can diagnose both computer and water heater problems and plumbing problems.
Call rinnai for recommended local technician. Click on support tab. Note their webpage is very short on repair details, unlike companies that sell tank-type heaters... because tankless units are not do-it-yourself.
http://www.rinnai.us/tankless-water-heater
Problem might be simple blown glass fuse on circuit board or complete failure of computer and sensors.
Here is page written for Rheem tankless, that explains general complexity of tankless computers, and showing required maintenance, such as deliming each year:
http://waterheatertimer.org/Troubleshoot-Rheem-Tankless-water-heater.html
To save problems with tankless computers plus anticipated expense of maintenance and repairs, I recommend homeowner install tank-type gas water heater:
http://waterheatertimer.org/How-to-install-gas-water-heater.html
Also install one-time-use whole house surge protector, and install grounding rods to prevent further damage... lightning frequently likes the same places over and again:
http://waterheatertimer.org/How-to-wire-Whole-house-surge-protector.html
Gene
h

If you need further help, I’m available over the phone at https://www.6ya.com/expert/gene_9f0ef4df2f9897e7

Rinnai RU98e is gas water heater with .4 gallon per minute activation.
This tankless gas water heater plugs into ordinary 120Volt 60 Hz outlet and consumes 63 watts when running.

For electric tankless:
Copy following links for illustrated walk-thru and wire-breaker size calculation for electric:
http://waterheatertimer.org/How-to-wire-Tankless-electric-water-heater.html

If you need further help, I’m available over the phone at https://www.6ya.com/expert/gene_9f0ef4df2f9897e7

check the error codes on it by pressing the on off button for 2 seconds then while holding the on/off button press the up button this should display the last 9 error codes let me know what they are