Rheem water heater 42vp40sf and the fvd interface failure code is flashing. how do i fix this?

You can reset some models.... depending on gas control valve and FVIR design.
Some newer models you replace the sensor located on outside of tank.
http://waterheatertimer.org/How-to-troubleshoot-gas-water-heater.html#intellivent
http://waterheatertimer.org/How-flammable-vapor-sensor-works-Therm-O-Disc-25VS.pdf

Gene

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

Copy following links for Richmond/Rheem FV to identify gas control valve and options for replacimg TRD glass vial:
http://waterheatertimer.org/How-to-troubleshoot-gas-water-heater.html#intellivent
http://waterheatertimer.org/How-to-repair-Rheem-FVIR.html
http://www.thetankatwaterheaterrescue.com/forums/forum3/
Add comment as needed
Gene

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

Tom W must have been 'recommended answer' from past answers, and not live answer.
What did Tom's answer say?
http://waterheatertimer.org/How-to-troubleshoot-gas-water-heater.html

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

Your safety device is probably sensing humidity as vapor. That's what ours seems to do. The replacement FVD is Rheem AP13447-3 Flammable Vapor Detection Sensor. It's down near the floor, can be pried off with a screwdriver, and unplugged. It needs to be replaced, since the heater will not ignite if its unplugged or too high resistance (detecting gas, or, if broken, moisture after a rain).
http://www.amazon.com/gp/product/B007IB22NM

Rheem, Richmond, Ruud, and Paloma are all the same manufacturer. After about 9 months, my tankless RTG-74PVN started giving me error code 11 and cold water!. I fixed it by cleaning the burner, as shown in this video:
www.howtolou.com/clean-a-burner

Before calling service technician, open following links for Rheem tankless resources including error codes and Rheem chat.
Disconnect wires from remote to see if system stabilizes. Unit will heat to default temperature without remote.

Error codes are considered start-point for troubleshooting.
C8 76 can mean PC board failure.
76 can mean communication failure with remote control
http://waterheatertimer.org/Troubleshoot-Rheem-Tankless-water-heater.html
http://waterheatertimer.org/pdf/ErrorcodeTroubleshooting.pdf
http://waterheatertimer.org/pdf/Rheem-error-code-C7-76.pdf

Similar 42vp50 direct vent product appears on following link:
http://www.rheem.com/product.aspx?id=C24E3F52-A6A4-4BC7-AF5B-0DF23A7A0263
sample manual for power vent series:
http://globalimageserver.com/fetchDocument.aspx?id=c0471d58-5e59-4be6-96f7-68361b8fb515

Manual has troubleshoot section:
If this is combustion-shut-off or FVIR or flammable vapor event, then manual says to call qualified technician.
Service call is certainly less expensive than any fire or injury caused by fire.
Other brands of water heater, for example Bradford White, have customer-resettable FVIR on some models.

Another thing that can be causing problems is air intake, especially in dirty environment like a shop, or if shop chemicals are being inhaled by burner. Burner parts are susceptible to chemicals in trace amounts below odor threshold. You might need fresh air intake for water heater, and air intake needs to be cleaned.

'System lockout' shows on troubleshoot page 32:
1. Gas supply is off or too low to operate.
2. Hot surface ignitor not positioned correctly.
3. Low voltage to the water heater.
4. Electric polarity to unit is incorrect - test and correct.
5. Combustion shut-off device tripped, contact service
agency.
Rheem phone number: 1-800-432-8373

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

Is it flashing 2-3? Try this: You can reset the control by unplugging the blower during start up, leave unplugged for at least 5 seconds, plug back in and within 30 seconds cycle temp control from hot to low 6 times.

Hello,
Most likely your flame sensor wiring is partially burn't or has an open circuit fault that's giving you the error message. This wire starts at the gas control and ends next to the pilot assembly. It can easily be replaced using a small wrench to unscrew the end connector from the control and the flame sensor rod at the pilot assembly. Call Rheem and see if the water heater is still under warranty, but even if it isn't they can most likely order the part you need for you. The phone # is on one of the data decals on the side of the water heater tank. They will need the model # and serial # to order the part. Be sure you unplug the unit before working on it! Douglas

It sounds like the Flame vapour sensor has tripped. Have you been painting in the basement recently or have and pait thiner, bleach or any other flamable material in the same room?

You will have to air out the room for at leaset 24hrs to get the fumes out and then try reseting the tank. If this doesn't work you need to call in a service technition to reset the FVIR sensor