Heater has power but flashes code L.P - Hayward Pool & Spa

Error Code 20 on a Noritz tankless water heater (model EZ98DV) is typically associated with an issue with the "priority" function, which is a feature that allows the water heater to prioritize certain uses of hot water over others.
When the "priority" light is flashing and error code 20 is displayed, it typically means that there is a problem with the priority function and it needs to be reset.
To fix this issue, you can try the following steps:

1. Turn off the power to the water heater by turning off the circuit breaker or removing the fuse.
2. Wait for about 10 seconds and then turn the power back on.
3. Press and hold the "priority" button for about 5 seconds, until the "priority" light turns off.
4. Turn the water heater back on and check to see if the error code has cleared and the "priority" light is no longer flashing.

If the error code 20 still appears and the priority light is flashing, it's best to consult the manual or a professional technician to diagnose the problem.
It's important to note that the error code 20 can also be caused by other issues such as, faulty thermistor, faulty thermistor harness, faulty PCB, etc. So, it's best to get it checked by a professional.

6 flashes is typically found with power vented and flue damper type heaters ... whirlpool never sold a power vent, but they did sell the AO Smith brand flue damper series heater.
The following manual suggest the problem is either gas supply or pilot assembly.
http://waterheatertimer.org/pdf/Power-damper-troubleshoot-parts-manual.pdf

More resources on following link:
http://waterheatertimer.org/How-to-troubleshoot-gas-water-heater.html#honeywell

Troubleshoot by type of gas control valve
http://waterheatertimer.org/How-to-troubleshoot-gas-water-heater.html#intellivent

Repeated 3 flash on power vented honeywell gas control thermostat means system in lockout.
http://waterheatertimer.org/How-to-troubleshoot-gas-water-heater.html

Repeated 3 flash on atmospheric-vent Honeywell gas control valve means temperature sensor failure.
Take apart gas control, and buy temperature sensor if sold separately at AO Smith dealer.
http://waterheatertimer.org/How-to-replace-Honeywell-gas-control-valve.html

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

Sounds like a pressure switch or simular sensing device that is not proving positive, thus a possible vent blockage or defective sensing device.

The N611 will flash 3 times as an AC heater fault. If your AC heaters are ohming out properly (48 ohms of resistance), then you have a bad power board. If the AC heaters are not showing around 48 ohms of resistance, replace the AC heater. It's going to be one of the two. Here is an article on it:

Norcold Fault Codes AC HE or On light flashing 3 times What to do

Error code means that you have electronic gas control valve.

Copy following link to identify gas control valve, read error code meanings for each, and download service manual relevant to each gas control, but not specifically Rheem since rheem does not post service manuals.
http://waterheatertimer.org/How-to-troubleshoot-gas-water-heater.html#intellivent

The fact that water heater restarts, means that water heater is getting gas, getting sufficient air for combustion, thermister is ok, gas pressure is good, etc.
The fact that you unplug and replug water heater and it works, shows that you have power vent model.
Since water heater worked correctly before, 2 flashes with Rheem power vent model could mean that pressure switch has failed.

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

If your lights are flashing, use these codes to pinpoint the error.

These codes are reading from the top neon down.

F01....OFF, OFF, OFF, Flash, OFF.
F02....OFF, OFF, Flash, OFF, OFF.
F03....OFF, OFF, Flash, Flash, OFF.
F04....OFF, Flash, OFF, OFF, OFF.
F05....OFF, Flash, OFF, Flash, OFF.
F06....OFF, Flash, Flash, OFF, OFF.
F07....OFF, Flash, Flash, Flash, OFF.
F08....Flash, OFF, OFF, OFF, OFF.
F09....Flash, OFF, OFF, Flash, OFF.
F10....Flash, OFF, Flash, OFF, OFF.
F11....Flash, OFF, Flash, Flash, OFF.
F12....Flash, Flash, OFF, OFF, OFF.
F13....Flash, Flash, OFF, Flash, OFF.
F14....Flash, Flash, Flash, OFF, OFF.
F15....Flash, Flash, Flash, Flash, OFF.
f16....OFF, OFF, OFF, OFF, Flash.
F17....OFF, OFF, OFF, Flash, Flash.
F18....OFF, OFF, ON, OFF, OFF. Fault on control board, check Eeprom.
these are the codes and this is what they mean

f01 light 4, motor triac
f02 light 3 motor, jammed/ tacho
f03 lights 3&4 Ntc fault
f04 light 2. pressure sw stuck on empty
f05 lights 2&4, pressure sw stuck on full
f06 lights 2&3, N/A?
f07 lights 2&3&4 heater relay
f08 light 1. heater relay
f09 lights 1&4 eprom/setup error
f10 lights 1&3, pressure switch not sensing
f11 lights 1&3&4, pump/draining error
f12 lights 1&2, communication error
f13 lights 1&2&4, dryer ntc/ fan motor
f14 lights 1&2&3, dryer heater
f15 lights 1&2&3&4, dryer heater relay
f16 N/A
f17 light 4, door lock
f18 light 3 comms error (3 phase motor) power board

also try checking this manual
http://imagebank.indesitcompany.co.uk/brochures_online/pdf/Hotpoint/Laundry/WFseries_19504110305_Jan05_book.pdf

High Limit Switch, look for a bimetal disc somewhere near the elements and apparently this is a manual reset. Should be red button in between two wires on a flat round disc.

Always turn power off when attempting electrical

1. Turn off power vent blower and set water heater temp to vacation (lowest temp)
2. Turn on power vent blower
3. Turn the water heater temp from vacation (lowest temp) to hot (highest temp) four times.
4. Set the water heater temp to normal

try interrupting power to reset.if it still doesnt work consult manual for code to flashing light sequence.also make gas supply is on