Our showers have been cold for a day now. the main unit flashes and says c7 or c4. i can't find my manual, any idea as to what this means?

I just went through lots of troubleshooting the same problem. Most searches will tell you its a communication error with the remote. After I was able to rule out the wiring, I bought a new remote ($70), hooked it up and immediately got the same error. Called Rheem Tech Support at 866-720-2076. They said code 76 meant I needed a new control board and chip. My unit was out of warranty but they sent the needed parts and detailed install instructions the same day, via FedEx overnight! It was a very simple task to replace. I happen to own 4 different units and am very glad I chose Rheem brand. Its not often I encounter a company willing to stand behind their products, even after the warranty period. Oh, and if you disconnect the remote control wires from the main unit (unplug unit, then remove face panel and they will be labeled, hooked on 2 small phillips head screws, replace panel and plug back in) your unit should still (more often than not) kick on and heat to about 100F by default...a do-able shower temp :) hope this helps!

1) Before calling service technician who can test parts, open following link for Rheem resources including error codes, Rheem chat and phone number, possible manual:
If local technician cannot solve problem, Rheem can probably recommend a person.
http://waterheatertimer.org/Troubleshoot-Rheem-Tankless-water-heater.html

2) Error codes are start-point for troubleshooting.

3) Multiple error codes can indicate PC board failure.
If your computer motherboard fails, most people feel the computer is compromised even after motherboard replacement.

4) Error code C7-76 is communication error with remote, and can include bad remote, bad wire to remote, loose wire, or bad PC board.
http://waterheatertimer.org/pdf/Rheem-error-code-C7-76.pdf

5) Error C4-13 can include part failure, vent problem, oxygen depletion, and can also include PC board failure.
http://waterheatertimer.org/pdf/Error-code-13.pdf

6) Tankless routine maintenance manual:
http://waterheatertimer.org/pdf/Tankless-maintenance.pdf

7) Some posts about tankless concede that qualified technician was unable to repair unit.
Solve most tankless problems yourself, same day, for about $350.
http://waterheatertimer.org/How-to-install-gas-water-heater.html

8) 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.

Before calling service technician, open following link for Rheem troubleshoot resources including error codes and typical manuals.
http://waterheatertimer.org/Troubleshoot-Rheem-Tankless-water-heater.html
C7 76 is problem with remote control. Following link might help.
http://waterheatertimer.org/pdf/Rheem-error-code-C7-76.pdf

Read manual cover-to-cover to make sure your tankless water heater receives yearly maintenance required for unit to continue working.
Tankless are maintenance heavy which can quickly eliminate any promised energy savings unless you de-lime and troubleshoot and read tips-and-tricks others have written and glean information from service technician who visits your house. Mostly glean how much tankless repairs and parts cost vrs buying whole new tank-type gas water heater at local box store, since all costs are energy.

C7 is communication fault with remote.
C4 is reportedly oxygen sensor problem.

Open following link for Rheem/Paloma/Richmond/Ruud tankless troubleshoot resources including error codes, self help, manuals, parts links, tips, and contact information.
http://waterheatertimer.org/Troubleshoot-Rheem-Tankless-water-heater.html

Add a comment with your experience so others with same-similar problem can benefit.

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.

I am a free answerer, and I specialize in giving you resources for free
The paid answerers may have a person who knows your product and the error codes
The manual will also have the error codes.
The person who installed the unit will also know the error codes (or should know)
Each day I see 20-25 unanswered questions about tankless water heaters.
The number of questions shows the complexity of the machines, and the inability of the manufactures to take care of their customers.
The manufacturer will refer you to the installer.

http://www.thetankatwaterheaterrescue.com/forums/forum3/

http://www.rheem.com/products/tankless_water_heaters/
The link above goes to Rheem tankless water heaters
Click on a product and it opens a page showing many tankless models
Click 'more information' and another page opens with spec sheets and other pdf
Not surprisingly there are no troubleshooting steps or troubleshooting flowcharts
There are plenty of ways to buy a tankless
There are not many ways a do-it-yourselfer can fix a tankless

flush it with vinegar

I had the same problem with my Paloma a year and a half after installation...contacted Rheem customer service (800-432-8373) and they recommened a local installer in my area. The technican diagnosed the problem as the "flow control valve"; he managed to get Rheem/Paloma to cover the part under warranty...I only had to shell out the money for the install.

check the connections between the controller. try disconnecting it to see if that makes a difference. 76 means that it's either the PVC board of the connection between the controller and the tankless

actually, does it give you a big rumble? if it does, then your problem is that the meter's btu's are not high enough and there's too litle gas getting to the heater so it's blowing itself out, basically. you'll need to call your gas provider and ask for them to bring in a higher btu meter. around 220,000 btu meter is best, so your furnace and gas stove(if you have one) dont take to much gas themselves, meaning dont get a 180,000 btu meter, because it would only be able to supply the heater.

Error code 13 is "oxygen depletion sensor" activated.
The first number is a sequence indicator.
If you need the training manual I have it and can send it to you.
I got Rheem to send it to me a while back and it is a big help when working with these units.
It covers th RTG-42, 53, 66 and 74.