Which parts fail
Components named in complaints (air conditioning, heater, defroster, HVAC controls).
Mileage when the problem started
Owner reports
Selected complaints filed with NHTSA by 2000 Mitsubishi Diamante owners, lightly cleaned up (personal details are redacted by NHTSA).
For several weeks it was becoming increasingly difficult for me to clear the windshield on my car and I attributed it to hot, humid weather. Coming back on the freeway one evening it suddenly became much worse and my husband and I tried many different combinations of cool/warm/fresh air to be able to see out of the window, but it never fully cleared. It was dangerous to drive under those conditions, but we made it home. There was a whitish film beginning to form on the window, visible in daylight. What was in the fumes that formed the film I am afraid to imagine. Several days later as I was driving my son to school we both noticed a white, cloudy smoke coming from the vents directly in front of us. We kept the windows down to avoid breathing in the fumes. The next day I took my car to the dealership and was told the gaskets adjacent to the heater core had failed. …
For over 2 years now I have had problems with an odor in my 2000 diamanti and the defogger never seemed to work on the windows. A couple of weeks ago I saw something leak onto the passenger side rug but didn't' t guess that this could have something to do with the heater core. Upon taking it to my dealer I was told this was hole in the heater core that had to be fixed immediately because it was dangerous and hazardous to my health to breath the fumes that were coming out of the core. The cost would be approximately $1200. I noticed I often had headaches when I drove long distances but was very concerned when my grandchild became very car sick (which had never happened before) upon riding in my car. I still didn't associate the heater with our problems. I think this is very dangerous and something should be recalled on these. …
The heater core has failed for the second time. I purchased the car from a dealer with 54,000 miles on it and after 500 miles the heatercore is fried. The worst part is that when I went to the Mitsubishi dealership to get my remote reprogrammed I got the printout of all the previous service that was done on it. The heater core was already replaced at 24,000 miles!!! Does this mean that I will have to spend around $1,000 every 25K miles that I drive the car??? Mitsubishi, please address this problem! Regarding to the problem itself... I got all the usual symptoms - the windshield was fogging up, there was a strong smell of the antifreeze in the car and now there's a pool of antifreeze by the passenger's seat. This cannot be good for my respiratory system... Antifreeze is a poison. …
Windows fog constantly and I can not get them defrosted. Smoke and smell of antifreeze comes into cabin of car. Took to dealer a year ago and was told nothing was wrong, car now has 98,000 miles and they are telling me it will cost 672.00 to replace the heater core. I have found several complaints about the same issues. I became ill in January of this year, the doctors (4 of them and three hospitals, ct scans, etc) could not figure out what it was. Turns out it was breathing the fumes in my car. At that time, the smoke only appeared a couple of times, but now it has increased considerably. I think it's sad that Mitsubishi will not repair this problem, nor have they corrected it.
I purchased the car used from a Mitsubishi dealer, a month or so later I noticed a maple syrup type smell. The proceeding months the problem got worse and noticed that my windshield kept fogging up. I took it to my local repair guy who gave me an estimate of one thousand dollars to replace the heater core and informed me that if I dont I could have a computer failure because it is right under the heater core which in turn May cause an accident due to loss of control. My passenger side carpet is now soaked with coolant and the smell is horrible coming from the vents. I have heard that the coolant is toxic and can cause problems in your respiratory system which concerns me even more.
Vehicle check engine light came on and locked into third gear. Noticed smell of coolant inside drivers compartment and hazing of the windshield. Brought vehicle to dealer to have problem improperly diagnosed and had to have it re-tested two weeks later. Heater core leakage has caused damage to the ECU and now both have to be replaced. Total cost of repair for both was $3,357.62. Coolant fumes are very toxic and cause a significant health risk and ECU failure could easily result in a very serious accident on the highway or elsewhere. The "fogging" of the windshield is a visibility hazard as well caused by the hot coolant in the defrost vents.
Dealer replaced the heater core at 37,000 miles, and at 80,000 miles the heather core failed again. Consumer found two technical service bulletins pertaining to this problem. In both incidents, the windshield became foggy on the inside and the defrost fail to clear it. Mechanic stated the o-ring failed. When this occurred, antifreeze leaked into the engine parts. The anti freeze got heated and evaporated. It then seeks through the vents onto the windshield. Consumer noted this posed two safety hazards. Repairing dealer stated they alone has seen this problem several times.
About two months after buying a 2000 Diamante; in oct, 2004, I started to notice thick fog on the windsheild that wouldn't go away by using the de-fogger and when using the heater, I would also notice strong smell on anit-freeze comming out from the vent. Upon inspection, the dealership described the problem as faulty o-ring in the heater core. The fix was costly, around $1100.00 however the car has been problem free since then.
Windows were not clearing properly and smell of anti-freeze was very strong. Upon calling the Mitsubishi dealer, I was told, it's probably the heater core. I ask if this is normal for a car less than three years olds with approximately 40,000 miles. The service manager told me, well to give you and idea, I just replaced one last week. I was told vehicle is out of warranty, cost to repair $650.
An anti freeze and smoke odor was coming into the cabin of the vehicle through the air vents. Dealership indicated this was caused by a problem with the heater core gasket, and needed to be replaced. The entire dashboard has to be taken apart to replace the part. This was a common problem with these vehicles.
AC & Heater complaints in other Diamante years
Frequently asked questions
How common are ac & heater problems on the 2000 Mitsubishi Diamante?
NHTSA has 24 complaints about the a/c & heater of the 2000 Mitsubishi Diamante, 47% of the model year's 51 complaints. The component named most often is defroster/defogger/hvac system › heater core (20 mentions).
Is there a recall for 2000 Mitsubishi Diamante ac & heater problems?
No. NHTSA lists no a/c & heater recall for the 2000 Mitsubishi Diamante, though it has 3 recalls for other components. If the problem is widespread, a repair may still be covered by an extended warranty or a manufacturer service bulletin — ask a dealer.
At what mileage do 2000 Mitsubishi Diamante ac & heater problems start?
Half of the complaints with a recorded mileage happened before about 54,000 miles.
Is it dangerous?
Owners linked 0 crashes, 0 fires and 4 injuries to a/c & heater problems on this model year. If you notice symptoms, have the car checked before driving further.
Source: U.S. National Highway Traffic Safety Administration (NHTSA) — owner complaints through October 20, 2026, recalls and defect investigations; fuel economy from the EPA. Complaints are unverified reports and are not proof of a defect. How we rate model years.