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 2001 Mitsubishi Diamante owners, lightly cleaned up (personal details are redacted by NHTSA).
I have had my 2001 Mitsubishi Diamante for one year now and have always found that my front window fogged over with a film the defroster would not remove. Last weekend the 5th of February 2005 I noticed the film getting worse and the car filled with a very pungent smell of antifreeze this caused my eyes to burn and made it hard to continue driving. I took it to my dealer who said the heater core had a leak and I needed new gaskets that were $16 dollars a package and that labor would be $492 dollars for 5 hours. I was quite shocked it would cost so much considering I have well maintained my car and that I only had 41,000 miles on it. I am concerned of the health effects if I have been breathing in antifreeze for over a year now. I am also concerned that the leakage May have caused damage to my ECU because of where the leak is (right above the ECU). …
Purchased 2001 Diamante in September 2004. Had approximately 66,000 miles when purchased and approximately 73,000 miles now. At the time I purchased the car, I noticed a maple syrup smell in the interior, but I didn't know that it was an indicator of a problem so I didn't mention it, I thought it might be a cleaner that was used in the car. In December 2004, brought car to Mitsubishi dealer where car was purchased for a routine oil change and tire rotation. Was informed that strong coolant smell in interior (maple syrup smell) was from a coolant leak caused by broken o-ring which I was told by the service manager "happens in almost all diamantes". I told them that the smell was in my vehicle when I purchased it from the dealer in August and I believe they were aware of it at that time because they were able to quickly identify that same odor when I brought the car in for service. …
This is a Mitsubishi Diamante 2001 ls. I'm experiencing pungent smell of anti-freeze inside the car as soon as I turn on heating. The dealer I got the preowned car from won't fix the problem unless I can show them the leak in heater core. Anti-freeze contains ethylene glycon and the fumes are highly toxic. Fixing the the heater core will cost $1000. I am filing this complaint to ascertain that this is a very very common problem in Mitsubishi Diamante and Mitsubishi motors should be aware of three main risk associated with this: (1) the defect exposes customers to unusual level of toxic ethylene glycol which should not exceed 10-25 ppm levels (consult msds for ethylene glycol) (2) the leak can affect the car's microprocessor and poses a severe hazard for drivers. (3) faulty heater core makes the visibility difficult due to fogging of wind shield thus putting the driver at greater risk. …
At approximately 40,000 miles, my 2001 Mitsubishi Diamante heater core started leaking. The first indication was that the windshield started fogging up immediately. This was in the winter (February, minnesota) and the defroster made it worse, driving was difficult at best. I was lucky in that I was close to home. Had I been on the freeway, it could have been more dangerous. A call to one of the local dealers informed me that the cost for the fix was over $1000. Research that I did on the internet showed that this was a very common failure and that most dealerships prefer to charge the customer outrageous bills for fixing what is obviously a manufacturing defect. Not being able to afford a bill of this size, I found a way to fix the problem myself, but it was not easy. I've found Mitsubishi is not easy to deal with. …
Abnormal smell coming from vents, made me, and my two sons feel nauseous, created a headache for me. I had to open windows in order to drive car due to fumes. Brought car to dealer, and heater core was identified as problem and needed to be replaced. Dealer said this is a very common problem with this year and model. I asked if there was a manufacturer recall and he said no, they only had to do a recall if it was a safety problem. This cost $802.37 to repair. This is most definitely a safety issue. Not only health, but can cause an accident due to fogging of windows, etc. I am sure Mitsubishi knows of this problem. Seems as though a class action suit should be filed against them for this issue. My car only has 47K miles, I purchased new and it has had all regular maintenance required. This is obviously a manufacturers defect. …
I have a 2001 Mitsubishi Diamante ls with 66,000 mile and my heater core went out. I noticed my window would fog up and you can smell antifreeze in the car, to have this replaced is going to cost me almost $998.00 for part and labor. Most mechanics I have talked to about this problem said that I shouldn't be having this problem with the heater core so early. I know of numerous complaints of this defect for this (none of which I knew about before I bought the car) I know that it is dangerous for the antifreeze to be inhaled, and with all the complaints about this obvious defect something should be done about it. There is health risks with the heater core going out and releasing the antifreeze not to mention the safety concerns with the heater core being so near important computer components.
I purchased a used Mitsubishi Diamante in October 2003 with 52,000 miles on it. A couple of months later there was a strong odor coming from the vents and the window started fogging up. The transmission started acting strange so I took it back to the dealer and was told that the heater core was leaking and had leaked on the transmissions computer. They told me they could replace the transmission computer under warranty but the heater core was not covered and it would cost me about $668. I had to come up with the money to get the heater core fixed before the could fix the transmission problem. I also noticed smoke coming out of the vents early in the morning and the passenger side mats were also soaked with antifreeze.
My Diamante has about 42,000 miles on it, and I have just been noticing that the front and side windows fog up and the defrost does not correct the problem. I live in colorado, where it can be very cold in the winter months and this is a serious safety issue! I have also noticed that a smoke or vapor comes out of the vents and smells bad. This too is a health concern of mine. I have a little three year old boy that has respitory problems so I am very concered about taking him in the car. I know this is a big problem, because I see many people complaining about it on the web, but they are probably unaware of the NHTSA. I feel that Mitsubishi should stand behind this.
1) smelled antifreeze but could not find a source. 2) heater coil was leaking. 3) leaked on a computer that controlled my transmission. Why would anyone place an electrical part under a unit that hold a liquid? Why would the electrical part not encased in a protective wrap if it had to be placed next to a pressurized liquid carring coil? 4) I was charged $1,125.00 to fix a problem caused by sorry engineering. I learned that misubishi knew of the problem and never notified their customers. There should be a recall and Mitsubishi should have to pay for the repairs. All parts, heater coil, transmission regulating computer were replaced.
When using the defroster the windows increasingly fog up and it does not clear. From investigating I have come to find out that this issue appears to be caused by a documented problem with the heater core of this model of of vehicle, 2001 Mitsubishi Diamante, in this case. I purchased the vehicle used from a local dealer approximately two weeks ago. I guess lucky for us the weather turned cold, forcing us to use the defroster, and thereby discovering this problem. I realize I most likely do not have any recourse in my case I want to file this on behalf of those who might since, as stated above, this is a known issue with the Diamante.
AC & Heater complaints in other Diamante years
Frequently asked questions
How common are ac & heater problems on the 2001 Mitsubishi Diamante?
NHTSA has 55 complaints about the a/c & heater of the 2001 Mitsubishi Diamante, 71% of the model year's 78 complaints. The component named most often is defroster/defogger/hvac system › heater core (51 mentions).
Is there a recall for 2001 Mitsubishi Diamante ac & heater problems?
No. NHTSA lists no a/c & heater recall for the 2001 Mitsubishi Diamante, though it has 2 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 2001 Mitsubishi Diamante ac & heater problems start?
Half of the complaints with a recorded mileage happened before about 58,000 miles.
Is it dangerous?
Owners linked 0 crashes, 0 fires and 3 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.