Heater core mitsibishi Diamante

check that the hot water can flow though the heater core and that the water control (tap) is working

Heater core is like a radiator. Blower motor blows through the heater core and heats your car up.

I'd steer clear of this one. The starter problem could be anything, regardless of what he 'thinks'. It could be a starter, it could be wiring, it could be the ignition switch, or it could even be the relay (the cheapest and easiest to replace, which is why I suspect he 'thinks' it's the problem).
The heater core is your greater concern. There are no o-rings in the heater core itself. If the heater core is leaking it must be replaced. There is a heater shut off valve under the dash and there may be o-rings where that is plumbed in. The actuator for the heater shut off valve is reportedly prone to failure. In either case, heater core or shut off valve, it's a pain to get to, requiring dismantling or removing the dash, and will probably be quite expensive to replace unless you're really handy and mechanically inclined.

You have to remove the entire dash to access the HVAC box. I promise is not a easy or fun job. Everything is really tight and hard to get to. Hope this helps.

does the windshield fog up? is there a sweet smell in the car? is the color of the fluid the same color of the coolant? is your car loosing coolant? then yes if it
the heater box has a drain for water that gets plugged up

Symptoms of a damaged heater core is moisture and the odor of coolant in the cabin as well as the possibility of coolant puddled in the passenger floorboard. The heater core is inside the dash and requires removal of the dashboard assembly in most vehicles. Unless you are comfortable with a large amount of work disassembling the dash, I would refer this to a competent mechanic.

1. check thermosat
2. both hoses near the heater core hot
3. watre circulating thats good
4. thermosat what it souds like

Why do you think you need a heater? Has the fan stopped blowing or are you referring to the need to replace the leaking gasket in the heater core?
If you take it to a mechanic for the heater core gasket, you can expect $1200-$1400 just to replace a $0.80 part. It is very labor intensive but can be done yourself if you have about 6 hours.

I think the steps to replace the 'heater core' are a few too many to list here- sry and it is a pain to do in most vehicles- im not familiar on the exact steps on the particular veh. Best thing to do is get a repair manual and follow its guidance on removal and reinstallation of the heater core. Good Luck

2002 diamante heater core sits in center of the dashboard just over the on board computer for the car. To access it you need to take out the console between the seats and the entire dashboard to get to it. The only thing wrong with the heater core is the leaking gaskets (about $10 in parts). Mitsubishi knows about the problem and when the dash is taken apart you will see a plastic sheet over the onboard computer to somewhat protect it from the leaking heater core gaskets. So the expense in the repair is in the labor to get it all apart and back together again. The dealers seem to replace entire heater core when they fix this problem. I did just the gaskets and that solved the problem. I have read others who did it themselves only replaced the gaskets and that solved it for them as well. Maybe the dealers think you will feel better replacing a larger part for so much labor.