Rinnai Avenger 25 ,will light and start, then go out It starts fine and stays on for approx. 30 seconds then turns itself off and flashes code 12 and start stop indicator also flashes red.

https://www.justanswer.com/plumbing/ag6cf-rinnai-avenger-25-gas-heater-keep-getting.html

since you tried to by-pass security may have done more damage to a simply worn key problem

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.

list of error codes

http://www.manualslib.com/manual/618685/Rinnai-Avenger-25.html?page=21

According to the owner/service manual I found for you, code 18 means "Temperature sensor faulty -- Service call." You can download the entire manual at the link below, the information about this code is on page 17.

Note: the manual includes information for 3-4 models. http://www.rinnai.com.au/media/748133/CI_Avenger_Blaze_Dynamo_Enduro__1_.pdf

Did you get any error codes? Press on/off button twice to reset heater or
press both the Up and Down buttons simultaneously for 3 seconds

It can also be a faulty temperature sensor, this would need a service call...cheers

The Key Symbol means the controls have been locked. Either by accident or intentionally. If you don't have the code to unlock the controls. Turn the Master ON/OFF Switch to OFF and unplug the unit from the wall outlet. Wait 30 minutes and plug the unit back in, turn the Master ON/OFF Switch to ON. The heater should have reverted back to the original factory settings. Now, you can change the settings to whatever you prefer. Just stay away from the Lock-Out button.

Hop this helped you solve the problem. Please let me know. Thanks.

thermocouple high cut out on convection fan rinnai fitters job , have to wait sorry , it is in the manual under fault codes .

Temperature sensor is faulty.
Ring them. its under warranty.