I just had my furnace cleaned it is a gas rheem. The service guy said it was overheated because of a#3 tube he said the furnace heat exchange is black. What can be do regarding this

Rheem fault code 11 - No ignition. Check to make sure there is gas to the unit and that the pressure is correct. Ensure the ignitor is operational before calling for service. Rheem fault code 16 - This is an over temperature warning which could indicate a possible clogged heat exchanger.
https://www.horizonservice.net/how-to-troubleshoot-tankless-water-heater-error-codes/
https://www.youtube.com/watch?v=QsmIyYexRzM

1) Rheem does not post service manuals.
Call Rheem and ask why, since the other big manufacturers, AO Smith and Bradford White do post service manuals.

2) You need new propane gas control valve, and matching combustion parts to fit existing tank.
http://waterheatertimer.org/pdf/Rheem-Water-Heater-parts-catalog.pdf
http://waterheatertimer.org/images/IMG_1826-LP-burner-450.jpg

3) Rheem does not recommend changing gas type, especially from natural gas to LPG.
LPG is higher pressure, and burns much hotter and requires cast iron burner, unlike the lighter-weight metal used for Natural gas burner.
The manifold tube and thermocouple must match the gas control valve, and gas-type and have correct thread for propane.
The combustion chamber and heat transfer surface can also be heavier metal for propane heaters.

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

Before calling service technician, or Rheem, copy following link for troubleshoot resources:
http://waterheatertimer.org/Troubleshoot-Rheem-Tankless-water-heater.html

Error 14 is low manifold pressure, cracked heat exchanger, restricted air flow.
C2 can be dirty combustion parts, bad gas solenoid valve, bad circuit board.

Combination of these two error codes implies possible PC board failure, which is similar to failed motherboard on computer.
Suggest that service technician perform the required yearly maintenance to see it that fixes error code.
http://waterheatertimer.org/pdf/Rheem-CondensingTankless-maintenance-2012.pdf

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

The OHL is not for sale due to a couple of reasons. If the heat exchanger has a hot spot it will melt the OHL making it useless. The OHL is just a large fuse. Rheem will completely warranty (if within date range) if the OHL or heat exchanger has problems. Bypassing the OHL is very dangerous and could result in a house fire since the appliance is mounted to a wall. I would suggest scrapping the copper heat exchanger for cash. That about all its worth in the condition its in.

Before calling Rheem or service technician, open following link for troubleshoot resources including error codes, Rheem chat, typical required maintenance manual, and possible product manual:
http://waterheatertimer.org/Troubleshoot-Rheem-Tankless-water-heater.html
http://waterheatertimer.org/pdf/Tankless-maintenance.pdf

Error codes are start point for troubleshoot.
Error 14 is 'overheat wrap fault.'
This can include venting problem, failed heat exchanger, bad safety circuit, gas problem.
Rheem does not have a service manual posted for this specific problem.

Additional information: Tankless water heaters consume large amounts of gas during combustion. The combustion burns away hydrogen and the remaining poisonous byproduct is vented into the atmosphere at each home day and night.
Byproduct includes acidic water vapor that condenses along the vent pipe and drips back down vent.
Each tankless vent pipe has a drain line that directs acidic water into household drain.
If this drain line becomes clogged, or if vent was not installed correctly, then water drips down into heat exchanger and can erode or crack exchanger.
The typical maintenance manual above shows steps that can solve many tankless problems before they happen.

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

Error codes are start point for troubleshoot.
C2 11 is 'no ignition' or 'gas shut off vale is not open'
Gas shut off valve handle must sit parallel with gas line.
This kind of error code can be air in gas line, improper grounding at electrical outlet, bad igniter, shorted solenoid, loose connections, etc.
Most tankless manuals recommend yearly service, and that type of work usually covers cause for most common error codes: for example checking gas pressure, gas line, air in gas line, clean and check burner and combustion parts and igniter, inspect that vent condensate is properly draining and vent has good draft, clean water filter, clean air intake, check for loose wires, delime heat exchanger, test outlet with circuit analyzer etc.

ok code 11 is a gas issue... i would take a guess that you dont have a 3/4 inch gas feed, from the meter to the unit, and if you do..you home gas furnace is close to the unit. I will also take a guess, that you did not have the problem during the summer
am I close.....

1 clean flame sensor
2 clean burner YouTube video
3 clean heat exchanger
Start with 1,ect
Rheem website has instructions or call they will e-mail info.

   By sealed unit you mean that the burner air comes from outside of the house?      You have checked the heat exchanger for cracks by removing the burners and looking with a mirror and flashlight, correct, or had a service tech so?
The furnace burners have enough air and are burning clean (blue) with just a tip of orange at the end? You bought the house new and the fireplace has never been used?  If it has been used to clean smoke for the house for whatwever reason I woudl start by havign the ducts cleaned.   Another possibility I have seen is insolation inside of ductwork breaking down.