ECO Smart ECO 18 tankless water heater. Newly installed and the display shows an E5 code but there are no codes listed in the manual.

copy following links for rheem/eco-sense error codes, manuals, required yearly maintenance, support number and resources.
http://waterheatertimer.org/Troubleshoot-Rheem-Tankless-water-heater.html
Probably bad PC board, or bad remote control

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

Copy following link for general troubleshoot and error codes:
http://waterheatertimer.org/Troubleshoot-Rheem-Tankless-water-heater.html

Error code 36 does not show in any resource, including 2008 service manual, or 2012 product manual.
Check that remote control display is illustrating the correct number.
For example there is error code 35.

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

Error code 36 does not show in Rheem tankless manuals.

Copy following link for general troubleshoot and error codes, and Rheem contact information:
http://waterheatertimer.org/Troubleshoot-Rheem-Tankless-water-heater.html

Error code 36 does not show in any resource, including 2008 service manual, or 2012 product manual.
Check that remote control display is illustrating the correct number.
For example there is error code 35.

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

http://waterheatertimer.org/How-to-install-gas-water-heater.html

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

P1 error code is not enough water flow to operate the unit: increase water flow to unit.
http://waterheatertimer.org/pdf/Rheem-Error-code-P1.pdf
http://waterheatertimer.org/Troubleshoot-Rheem-Tankless-water-heater.html

Error codes are considered start-point for troubleshooting.
Page 18 of maintenance manual shows how to clean water filter monthly.
http://waterheatertimer.org/pdf/Tankless-maintenance.pdf

Other causes can include faulty sensor, clogged in-line filter, clogged water softener, sediment in pipe, sediment at elbow, bad check valve, water leak, etc.

Take a moment and rate answer:
And take advantage of fixya expert assistance live.
For a price, expert works with you while you work on water heater or any do-it-yourself project.
Fixya is always less expensive than a service call.

Multiple error codes can mean PC board failure.
C2 11 is no ignition.
Error 24 is malfunction of operational switch: "press max or mim button on PC board, or replace PC board." On some models, max and mim button can be located just above dip switches, and rotates with screwdriver. See diagram in manual.

Before calling service technician, open following links for troubleshoot resources including error codes, Rheem chat, and possible manual.
http://waterheatertimer.org/Troubleshoot-Rheem-Tankless-water-heater.html
http://waterheatertimer.org/pdf/ErrorcodeTroubleshooting.pdf

1) BOSCH website shows error codes.
FO does not appear:
http://www.boschhotwater.com/HelpfulResources/TechnicalSupport/BoschAquaStarModels/2400ESTechSupport/Troubleshooting/tabid/1295/Default.aspx

2) I do not see FO error code listed on Internet search
Other FO questions at fixya seem to have no answer:
http://www.fixya.com/tags/error_f0_bosch

3) At this point, I would suggest calling a service technician.
While waiting for service, read manual cover to cover several times.

4) Your manual is valuable resource, and shows the word 'maintenance' and 'troubleshoot' frequently.
Tankless are maintenance heavy, and should be de-limed each year.
U-tube search "delime tankless water heater."

5) Also explore Bosch website for your manual and other valuable information.

6) When service tech arrives, ask that person for tips and tricks about your unit that will help you maintain your water heater.

7) All costs are energy. Install DIY tank-type gas water heater and avoid the whole headache.
http://waterheatertimer.org/How-to-install-gas-water-heater.html

BOSCH error codes:
http://www.boschhotwater.com/HelpfulResources/TechnicalSupport/BoschAquaStarModels/2400ESTechSupport/Troubleshooting/tabid/1295/Default.aspx

Best place to start is troubleshoot section in manual. Page 24
http://www.boschhotwater.com/Portals/7/TechManuals/250SX_English.pdf

Problems range from air supply, gas supply, venting, part failure or combination of listed things.
Don't forget, tankless require yearly maintenance, outlined in manual, and cannot be exposed to more than 11 grains hardness in water, and should be de-limed each year.

All costs are energy.
I suggest reading entire manual several times, so you are pro about your tankless water heater.
Even with this knowledge, trained service technician will have to be consulted some day, and tankless should never be installed if factory trained technician not available in area.

Bosch has online tech support form:
http://forms.boschhotwater.com/Home/BoschTechSupportForm/tabid/614/Default.aspx

Install tank type water heater that can be put online when Bosch is not working
http://waterheatertimer.org/How-to-install-gas-water-heater.html

Add a comment for more free help.
Also take advantage of fixya expert assistance live.
For a price, expert works with you while you work on water heater or any do-it-yourself project.
Fixya is always less expensive than a service call.

IL code is: water heater has lime deposits.
Tankless should be de-limed each year using white vinegar..
Search U-tube for "de-lime tankless heater" to see how to do this yourself, or call service technician.
There are special fittings that can be added to tank at the hot-n-cold ports that let you easily de-lime your water heater each year.

Typical manual shows error IL on page 33:
http://waterheatertimer.org/pdf/Rheem-RTG74IndoorOutdoor_owners.pdf

Typical manual also shows regular maintenance on page 29.
The word maintenance is used throughout the manual.

Research shows that regular de-liming will not restore full efficiency to tankless water heaters:
http://waterheatertimer.org/pdf/Battelle_Final_Report.pdf