Force 90 outboard locked in forward gear after impeller replaced

you have removed the leg to replace the water pump impeller
the leg has the selector rod for the forward and reverse action
as the direction selection is a simple dog that moves forward or back to connect the spur gears and it was working before you pulled the leg off , then I am thinking that when the leg was reassembled that the selector was in a wrong position and now you are only moving it in one direction neutral or reverse instead of the 3 positions forward neutral reverse
nothing else can change except where the rod connects in the leg

Hi Rick:
Don't know what unit you are dealing with, but stuck in my memory is the way the old Merc used to work.
There was a rod that rotated, and the shift mechanism slid into a slot. We only tilted removed the bottom end with the shifter in the forward position.
You may need to drop the bottom end to allow the shifter to be placed in the correct position.
I'm hoping that you are using the term "frozen" to describe "blocked", as another total PITA can happen when portions of the assembly become corroded. Loosening them up requires tha delicate balance of heat, penetrating oil, and brute force. Bearing in mind you don't want to set it on fire or break it.
I'd try dropping the bottom end, engage it in forward, place the shifter in forward, and carefully slide the bottom end back on.
Hope this helps.
cheers

Is this motor used in salt water?
That warning buzzer on your motor is really only tied to the temperature. In order to triage sensor or overheat condition, I'd recommend an infrared thermometer to get a reading on the engine as the buzzer is sounding. There is a product that is like a glue stick which can be pressed against the engine which melts at a very specific temperature... not sure if it's around anymore as the remote thermometers are pretty inexpensive now. Is there good water flow out of the ports below the cowling on the back?

Look below lower mountings you will see the shift rod with along nut and a lock nut,undo long nut,do this with motor in reverse gear,some 40s have a stainless plate cover which you have to remove first

sounds like you need to find the shift rod. It will be behind a plate on the exhaust housing or under the carb.

If the gear lever is loose,it's the gear connection rod that you disconnect to do a g/box drop.If its not loose, drop the g/box off the leg and the first thing you find might be the problem.A loose bolt,a jammed shifter.It will probably be a connection problem than a g/box problemHope this helps

Usually you just buy outboard motor gear oil and that does it. (about 70 to 90) I think. Usualy comes in a squeezy tube for injecting it under pressure.
It needs to be injected from the bottom drain hole and you force it in till it starts to come out of the top drain hole.
Most outboards work like that so I assume yours probably will.

Most impellers are in the leg and require the lower oart of the leg to be removed by disconnecting the gear linkage and undoing the bolts that hold the lower half of the leg on, they are just above the water intake.Don't forget the bolt near the prop. Wiggle the leg and slide it down to remove, then place the leg on the bench. The drive shaft protrudes through the impeller housing, remove the bolts that hold the housing on and slide it off. When fitting the new impeller be sure to fold the fins in the correct direction. When re installing the leg be careful to align the cooling pipes correctly.

Cheers, Timo.

With your lower unit off have someone shift the engine into forward while watching the gear shift select shaft's rotation,let gearshift remain in forward,now place your lower unit in forward and reinstall. In the future just put engine in forward when installing a water pump as there won't be a need to move gearshift