Bathtub in guest bathroom has no hot water. Sinks right near bathtub have hot water. I took off the cover of the single turn-style moen bathtub faucet and checked, there are no shutoff valves for the hot/cold water adjacent to where the water enters the moen assembly.

Cold water comes out fine, turning the valve all the way to 'full hot' results in a trickle of cold water coming out.

Thanks for the help.

Sounds like the inlet ports on the cartridge's clogged. If there are no screwdriver stops located where the supplies connect to the valve then shut the water off to the house. Remove the insert button (bathroom) to expose screw.
Remove the screw and pull off handle assembly.
Remove retainer clip (copper horseshoe shaped clip) and pull out cartridge with channel locks, pliers, or cartridge puller. Check the inlet ports on the cartridge for trash . And clean if you find anything. Now turn the water back on to flush out the suppliesReinsert retainer clip and screw in handle assembly, followed by the insert button. Turn water back on . If there's still an problem replace the cartridge. Moen has a lifetime warranty so a replacement cartridge will cost you nothing. Good luck, Tom

Tom nailed good, also open the link below will show hot to remove and install the cartridge, as Tom says Moen has life time warranty on the parts call them at 800-289-6636 and tell them that you are the original owner, they will send you a cartridge for free, Good Luck.

John

Installation Help / Animated Tutorial - Installing the Moen 1225 / 1200 Cartridge