Took cover off control panel of Rheem RTG-74PVP tankless hot water heater. A red light blinks 4 times. What does that indicate is wrong?

Two things- Clean the sensor rods or replace(cheap and easy with YouTube instruction video) and at the same time replace the main program chip (also a easy plug in chip). The whole repair takes two hours for a homeowner and costs about 50 bucks for a chip. These are easy an generally the flame sensor rod is the culprit. The code may also be the limit switch but try EZ and cheap first. See You Tube..
https://www.youtube.com/watch?v=npxcVo7RK2c

http://waterheatertimer.org/Troubleshoot-Rheem-Tankless-water-heater.html

Contact service technician
Call Rheem

Gene

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

C-8 error codes could mean anything from the fan not starting, a bad control board, or a blown capacitor. Best bet would be to call Rheem customer service and they'll ship part or suggest licensed contractor to troubleshoot and replace what's necessary.

Open following link to view fixya questions on 1 model of Rheem tankless
Some older questions have information that is useful, and varied
http://www.fixya.com/tags/rtg_74pvn

Open typical manual to review required regular maintenance for tankless on page 29
The word maintenance is used throughout manual, indicating that tankless are a yearly budget nuisance
http://waterheatertimer.org/pdf/Rheem-RTG74IndoorOutdoor_owners.pdf

All costs are money.
You can also add simple tank-type water heater that can be turned on when tankless is on the blink.
http://waterheatertimer.org/How-to-install-gas-water-heater.html

RTG-95XP Item Name: Outdoor Rheem Tankless Water Heater RTGL-74XP Item Price: $910-$1100 Gas Type: Propane Venting: No Item SKU: Rheem RTG-95XP replaces RTG-74XP Product Description: Rheem Tankless Water Heater RTG-95XP

Be aware of this --- http://www.cpsc.gov/cpscpub/prerel/prhtml07/07108.html

And this ---

Rheem RTG-95XP replaces RTG-74XP

This makes getting information hard to come by. Please use the link below for information.

http://www.rheemtanklessonline.com/contact.php

Rheem Tankless Water Heater Selection and Installation Videos

• Introduction - time 04:11
• Pre-Qualifying the installation - time 01:13
• Selecting the Right Product - time 01:48
• Key Differences Between Tankless & Storage Heater- time 01:27
• Choosing the Location - time 01:20
• Installation: Major Components - time 00:58
• Installation: Tools Required - time 00:48
• Installation: Mounting the Heater - time 01:15
• Installation: Plumbing & Gas Connections - time 04:29
• Installation: Side Wall Vent Termination - time 02:41
• Outdoor Installation - time 01:21
• Connecting the Remote Control - time 01:42
• Turning on the Water Heater - time 01:00
• Commercial vs. Residential Products - time 01:47
• Manifolding Multiple Units - time 01:43
• Troubleshooting & Additional Support - time 01:01

need to contact rheem what you can do until then is remove the control wire it will run until parts ship out

contact rheem you probably have a circuit board problemb

This is something you will have to contact rheem about due to the fact you don't have a fault code

Sounds like you may have the wrong type gas going to this? Is this an LP gas or natural gas unit? will say on label. Natural gas units run only on natural gas ditto for LP. Could be issue with manifold pressure settings. Could be obstruction in exchanger or vent. Have a authorized technician check over

Has it ever run? Is cold water going to cold water inlet? Hot out to hot water piping? Power ON? Control pad on? gas pressure correct?