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 2002 Mitsubishi Diamante owners, lightly cleaned up (personal details are redacted by NHTSA).
'02 Mitsubishi Diamante; heater core failure; 2nd owner eff. 2/05; bought from same dealer that leased to 1 person; found many other owners suffering same issue on google to site: http://www.diamanteowners.com/forum/index.php?s=918c701e5465c87381ccae0aa22c20a3&showtopic=3615 I have been quoted $1200 by 2 dealers and $900 by a mechanic/independent. The dealer that leased the car to '1st owner' and sold it to me at end of lease told me that they are working on two diamantes per week for heater core replacement! Why is there no recall yet??? This leak - per brad/sale and service dealer states that if I fail to fix immediately - 10 hours of labor - radiator coolant will leak into passenger foot well/flood and the leakage goes over my computer that runs the car! 2 repairs a week is unacceptable!!! Please help!!! …
The first time my 5 year old son drove in this car, he vomitted. Everytime he rides in it he becomes nauseous. The car's windshield fogs up and a sickly sweet smell comes out of the vents. The windshield is covered in a permanent haze. We noticed these symptoms but had not given it a second thought since the car was new and had no reason to suspect major problems. The car was recently in the shop for 1 week, when we picked it up we really noticed the sweet smell and alerted the shop. After inspection the shop diagnozed it as a heater core problem, a leak that is vaporizing and condensating on the windshields. The anti-freeze fumes are noxious, and after some research we have found that Mitsubishi has been aware of the problem and has even issued 2 TSB about the problem. Mitsubishi fails to correct the problem by fixing the car. …
This is the second time I am complaining about the same problem. The end of June 2006 I drove to work and reached down to pick some stuff up off the passenger side floor mat. It ended up being soaked. At first I thought some water had come in because the past few days our area had torrential rains. When I got home the next day I took up the floor mat to dry it out and notice all the liquid under it was greenish yellow. In my mind I went crazy because the same thing had happened to me a year and 2 weeks earlier. Once again the heater core became faulty and poured the engine coolant into the passenger spaces. Wonderful, since my wife was 8 months pregnant, great for her to be breathing those chemicals. I have to say I was fortunate that it didn't leak on any of the computer components, like has happened to others, especially while I was driving. …
02 Diamante with approx 42, 000 ml - upon starting the car the front window begins fogging up and the entire car then fills with steam that is sweet but overwhelming. It gets so bad it is nearly impossible to see out the front windshield and then the smell overwhelms you within about 5-10 minutes forcing the driver to pull over or drive with the windows down (not easy at 18 deg outside and 2 kids under 5 in the back). Symptoms indicate that this is a heater core problem that dealers are aware of and is a very common problem that they see approx 2-3 times a month with a charge of $1000.00 - $1400.00 to fix and that if not fixed then the main computer for the car would short out. So just looking at 1 dealer low end numbers: 2 heater cores per month * $1200= $2400*12months = $28,800 a year per dealer for a known problem. …
The heater core is an extremely dangerous problem. The windows in the car fog up so bad I couldnt drive the car...had to pull over and wipe the windows down. But the thing is...I didnt know what the problems was...until I realized the windows werent just being fogged by the cold, ect it was a toxic mist from a heater core problem...I pulled over 2 times because I got sick...I noticed a mist coming out of the vents....so all in all I called Mitsubishi and they refuse to do anything about this.this heater core problem can lead to accidents, sickness, and othe expensive problems in the car! The fumes are extremely toxic!!! There are 1000's of complaints and blogs about this problem...I'm sure there are 100,000+ complaints abpout this problem. And Mitsubishi refuses to help or do anything about this. …
I purchased 2002 mitusbishi Diamante ls with 35000 miles from the Mitsubishi dealer in san bernardino, california. After driving off I noticed a smell of maple syrup. I also noticed that my eyes burned and itched when the heater was on. This was likely due to the white cloud that came from the heater duct. It was also causing cloudy windows that made safe navigation impossible. The carpet I starting to puddle with antifreeze on the passenger side. I took it in for a routine oil change, and had the dealer mechanic check the smell. They informed me that the heater core was leaking, and would need to be replaced. Their quote was $1060 and was not covered by my bumper to bumper warranty. This is a health hazard and the leaky heater core could cause massive damage to the computer that controls the engine. They will not correct the problem even thoug it wa a problem at drive off.
Fall of 2007, noticed increased fogging of windows that did not go away with defroster, fresh air. Began to smell sweet, syrupy smell. November 07, heat refused to work, checked antifreeze level, very low. Windows continuing to not be cleared. Oily film covering windows. Currently, I have not had complete failure of heater or computer module. I have researched this problem on the Diamante owners web site. My problems mirror exactly as fellow posters. November 29, 2007, I pulled back the carpet to inspect computer module and heater core area, there is gunky green ooze on the floor and on the computer module. I am currently in the position of researching the best way to have problem fixed, whether at the dealer, another auto shop, or self repair.
Tl contact owns a 2002 Mitsubishi Diamante. While driving 45 MPH, the contact noticed leakage on the inside of the vehicle. In addition, she was unable to see out of the windshield due to the fog. The dealer stated that there was an A/C evaporator was cracked and they made the replacement. The contact stated that the failure occurred again immediately after the replacement. The dealer stated that the heater core caused the failure and they have replaced it on five occasions however, the failure continued to occur. As a result, the vents would disperse anti-freeze inside the vehicle when the heater or defroster was activated. As of January 2, 2008, the dealer had not repaired the vehicle. The failure mileage was 100 and current mileage was 65,000.
The vehicle is a 2002 Mitsubishi Diamante ls. The windows in the vehicle have been constantly foggy in a dry environment. When wiping windows down, I noticed that the condensation was oily. I have also noticed a syrup smell in the vehicle. The defroster has not been working properly, so I investigated online to find out if it was a common failure. What I found out was that the syrup smell and oily condensation was most likely due to a leaky heater core. I took the vehicle to a local mechanic who found that the o-ring seals for the heater core were failing. They pressured tested the core to be sure it was not failing also. The old parts are not available. The defroster was not related and only sparked my investigation into the oily condensation.
2002 Mitsubishi Diamante 65K miles. Failure of heater core/ heater core gaskets/ o-rings. Windshield will not defog, strong coolant odor and residue on windshield and side windows. Resulted in dangerously low visibility while driving. Windshield too foggy for safe driving. Strong smell of antifreeze in passenger compt. Had to open windows in sub freezing temp to get fresh air to passenger compt while driving. Cost to repair estimated $800-$1100. Problem appears to be widespread based on complaints filed at this NHTSA site as some complaints are categorized under different component names but have same problem descriptions. Greater than 60 complaints by my count for 2002 model year alone on relatively low number of vehicles sold in us.
AC & Heater complaints in other Diamante years
Frequently asked questions
How common are ac & heater problems on the 2002 Mitsubishi Diamante?
NHTSA has 69 complaints about the a/c & heater of the 2002 Mitsubishi Diamante, 56% of the model year's 123 complaints. The component named most often is defroster/defogger/hvac system › heater core (62 mentions).
Is there a recall for 2002 Mitsubishi Diamante ac & heater problems?
No. NHTSA lists no a/c & heater recall for the 2002 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 2002 Mitsubishi Diamante ac & heater problems start?
Half of the complaints with a recorded mileage happened before about 60,000 miles, and 6% before 25,000 miles.
Is it dangerous?
Owners linked 0 crashes, 0 fires and 6 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.