Turn on power,15-20 seconds later,get error code 991, abnormal or imperfect combustion. natural gas at 7 on the water column, filter clean. other contributing factors for this code? can hi-limit switch factor in?

Here you are Dorothy

AO Smith code 991 Solution for error code 991
Check the gas type of the water heater. Inspect the environment around the water heater. ... Check if there is grease and/or dirt in the burner (Part #101) and the fan motor (Part #103), especially if the water heater has been installed in a contaminated area.

Error code '991' for type ATI-110C-N by AO Smith. Cause and ...

More help here, but you probably need a service call, especially if it is a gas water heater.

https://www.google.com/search?q=error+code+of+991

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Imperfect Combustion
Clean air inlet filter.
Clean combustion air fan.
Clean heat exchanger fins.
Check for adequate combustion air ventilation openings and clean if necessary.
Check vent system for partial blockage and correct as necessary.

Error 11 is ignition failure, or insufficient gas, or defective sensor, dirty combustion parts.
Do the yearly required maintenance such as cleaning the combustion parts, and error code will probably be solved.
Problem is Rheem does not post service manuals for people to learn how to service their equipment without calling service tech.
Copy following link for Rheem troubleshoot resources
http://waterheatertimer.org/Troubleshoot-Rheem-Tankless-water-heater.html

The cost of yearly maintenance will easily eat away the expected yearly savings of $100 for typical family of 4.

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

Before calling service technician, open following link for troubleshoot resources include Rheem chat, error codes, and regular maintenance manual, and possible product manual:
http://waterheatertimer.org/Troubleshoot-Rheem-Tankless-water-heater.html
http://waterheatertimer.org/pdf/Tankless-maintenance.pdf

Clean the water filter as shown in product manual.
Error E1 is not specifically shown for Rheem/Pronto tankless.
1 is usually related to water flow.
E errors are related to temperature, so E1 can be temperature sensor failure on hot water pipe.
E errors can be loose/bad wiring or part failure.

Error codes are considered start-point for troubleshoot.
Each tankless manual recommends cleaning the water filter monthly, deliming heat exchanger yearly, and cleaning combustion chamber and burner parts yearly.
Regular maintenance can prevent most common error codes, but the cost and nuisance will outweigh expected $1-8 monthly savings on gas bill.
The requirement for continual maintenance should signal folks that tankless are not effortless hot water as described in tankless promotion.

Solve most tankless problems same day, yourself, any brand, using minimal tools for about $350.
http://waterheatertimer.org/How-to-install-gas-water-heater.html

Crossover, low hot water flow, low water pressure, sediment in water lines or heater, heat exchanger needs to be de-limed, water filter needs to be cleaned, water heater temperature set too high, incoming water temperature too high or too low depending on model, MUD or PUD districts with pressure spikes, dirty faucet aerators, loose wires, bad sensor, faulty PD-board, bad venting, dirty burner and combustion parts, etc.

Crossover
http://waterheatertimer.org/Crossover.html
Clean water filter monthly, de-lime tankless yearly
http://waterheatertimer.org/pdf/Tankless-maintenance.pdf
http://waterheatertimer.org/pdf/Rheem-De-lime-tankless.pdf
No error code:
http://waterheatertimer.org/pdf/No-Error-Code.pdf
Overcoming fluctuating temperatures:
http://waterheatertimer.org/pdf/Bosch_Overcoming_fluctuating_temps.pdf
Resources:
http://waterheatertimer.org/Troubleshoot-Rheem-Tankless-water-heater.html

http://globalimageserver.com/FetchDocument.aspx?ID=1371e2e6-22f2-45d6-a712-d2679eac9e0c
is common error codes for The Rheem tankless water heaters and has recommend actions. It says that error code is imperfect combustion alarm, which basically tells you that the system is not getting enough air for some reason.

Error 13 is also an air flow problem. http://www.plumbersurplus.com/pdf/05223.pdf?SSAID=389818 So I doubt that reprogramming will do any good.
Rheem products are air flow sensitive and something as simple as storing something against or close to a Rheem will cause trouble.

http://waterheatertimer.org/pdf/Rheem-TanklessParts-Accessories.pdf If you look at Page 10 of that manual you will see that the RTG20074FK is the natural gas chip for sea level. Basically it lets the unit make adjustments for different altitudes. It is NOT programmable! It should be installed on the control board IF you live at sea level.

05 Imperfect Combustion Alarm
Clean air inlet filter.
Clean combustion air fan.
Clean heat exchanger fins.
Check for adequate combustion air ventilation openings and clean if necessary.
Check vent system for partial blockage and correct as necessary.

If this does not help; contact Rheem customer support.

The 59 code is typically caused by insufficient gas supply to the unit. It can also be caused by obstructed exhaust flow, stuck exhaust damper (located directly on top of the unit's exhaust), insufficient makeup air, or improperly installed venting (i.e. too long or too many elbows).

I would recommend initially checking the incoming gas pressure at the gas connection on the unit. The gas pressure should not drop below 5" water column for a natural gas unit, 8" for a propane system (the unit need to be operating at its maximum output).

I had the same problem and called the manufacturer. The error code C-8 was followed by the number 76. They told me to disconnect the thermostat/digital display from the main unit. This was fairly simple to do by removing the front panel. The unit works steady now without shutting down after a few minutes. The help desk told me that the main circuit board needs to be replaced for the unit to work with the display and that they would mail me one asap. At least we have hot water again.

Oakville, Ontario

I had the same problem with my tankless waterheater (flashing c4 76 alternately). I called Rheem
tech support and they had me 1 power down the remotepanel 2 unplug the unit from the wall 3 remove the cover 4 remove the wires for the remote panel from the circuit board and pull the twisted pair that goes to the remote out from the bottom of the unit. 5 reinstall the cover 6 plug the unit back
into the wall and turn on the hot water see if it works. It did in my case and they are sending me a new remote panel. I hope it helps.........