1991 Nissan Maxima - Heater Doesn't Work

Open the hood of your Maxima, and try to locate the hoses that bring coolant to the remote mounted heater core inside the body. This may be easier with the engine cold. Once you can determine the correct hoses, Try to concentrate on the hose that goes from the upper section of the engine to the heater core. If you start the engine and allow the engine to warm up, that hose should start to get warm. The other return hose should also get hot. If the hose that brings hot water to the heater is not getting hot, you have some kind of blockage in the hoses. There may be a heater control valve in the hose somewhere to limit heat when using the air conditioning. This valve must be open when asking for heat.

If only one hose gets hot, the heater valve may be located inside the car.

If both hoses get hot, there may be a problem with the heater blend doors not allowing the hot air to reach the interior of the car. Good luck.

Be careful, the coolant temperatures and pressures can severely burn you. If you are not sure of the pressures in the system, squeeze the upper radiator hose. If it is not able to be squeezed closed, DO NOT open the cooling system. Be careful.

I agree with your suggestions. On this model Nissan I have seen several blend issues.

Any ideas of where the heater control valve is located in a 91 maxima?

Probably an airlock if their are no other problems like stuck valves, some cars have special bleeding procedures, often you need to jack up the front of the car (high) safely and run the engine.....but talk to your dealer...