"I have a Rheem rheem gas furnace model rcfl-hm4821cc, only a few years old. It was working for several days at beginning of heating season, then abruptly quit. It appears to be trying to cycle through the startup process, with the igniter sparking, then quits. Any idea about what could be wrong?

Rheem furnace error code 68 is a mechanical relay system which signals the blower motor to stop because it has not received a signal from the thermocouple.
Jul 20, 2022

Rheem furnace error code 68 - HVAC Contractors

Motor Power And Communications Issues (Fault Code "68")

ManualsLib https://www.manualslib.com > Rheem-96v-Series
Motor Power And Communications Issues (Fault Code "68"); Line Voltage And Low Voltage Connections To The Communicating Motor - Illustrations - Rheem 96V Series ...

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Your water heat is at an age that I would recommend replacing the whole unit. That one cannot be too efficient and most likely has sediment built up on the bottom of it pretty bad.

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

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

Rheem does not post service manuals.
One reason is that water heaters have changed rapidly last few years, and same model might have different features because Rheem experiments to see which products work after they are sold.

Calls to Rheem for DIY help are largely useless since they want you to call local installer or technician... who will sell you a new heater for more money.
Rheem wants to end DIY repairs since doing it yourself saves you money... which means Rheem, and Rheem-loyal service companies make less money.

http://waterheatertimer.org/Which-is-best-gas-water-heater.html
http://waterheatertimer.org/New-government-water-heater-standards.html

Gene

New government water heater standards

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

I presume you're talking about a gas furnace, even tho the title says Rheem gas water heater. Either way, it works the same for both.

Any black coloring on the heat exchanger is *not* from overheating. Black is from a sooty flame caused by a mis-adjusted burner. (In fact, a sooty flame is less hot). There is an adjustable "air intake plate" for air to enter the burner where the gas enters the burner. A proper flame is made by adjusting this air opening so the proper gas/air mixture feeds into the burner. If the air vent is closed too much it produces a sooty flame.
The age of the furnace is the most important aspect, and that helpful to determine a likely condition of the heat exchanger and burners. Other than a badly adjusted flame, the main thing to watch for is a badly rusted out heat exchanger. But black does not mean rust.

Before calling service technician, open following link for Rheem troubleshooting resources including Rheem chat:
Rheem will suggest you catch up on required yearly maintenance which would cover most causes for common error codes, and cover cause of no error code including plumbing problem or loose wire or PC board failure. Combination of problems can also cause failure and these issues would be covered by general maintenance.
http://waterheatertimer.org/Troubleshoot-Rheem-Tankless-water-heater.html
Solve most tankless problems same day for $350
http://waterheatertimer.org/How-to-install-gas-water-heater.html

1) Rheem offers on-line chat, which you rarely see for typical tank-type water heater
http://www.rheem.com/products/tankless_water_heaters/customer_support/
http://www.rheem.com/hrc.aspx

2) Call local service technician, and put that person in your rolodex-wallet for future help.
Ask technician for tips and tricks for keeping unit working and needed maintenance.
They will gladly offer a yearly service.

3) Read manual several times to get accustom to yearly maintenance required for tankless water heaters. Tankless water heaters are maintenance heavy. All costs are energy.

4) Read manual several times and notice word troubleshoot appears many times.
Since tankless are complicated machines, many internal and external factors can affect operation, especially as unit ages.

5) Look at label on unit to identify exact product number. Identify same/similar product at Rheem, and become familiar with Rheem website
http://www.rheem.com/products/tankless_water_heaters/

6) Read other question/answers at fixya about Rheem tankless to become familiar with the tankless era
http://www.fixya.com/tags/rtg_74pvn

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

8) 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.

Well you might have a pinhole leak than periodically puts out pilot light. Or you might have a bad thermocouple. Thermocouple is inexpensive and easy replacement.

Contact Paloma/Rheem customer (800-432-8373) support and tell them what you have; they'll send you the documentation - by regular/email.

Happy Holidays;]
JZ...

Contact Rheem; the Burner control unit has lost comm with the remote...the part is warranteed for up to five years.