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 2003 Mitsubishi Diamante owners, lightly cleaned up (personal details are redacted by NHTSA).
Steering wheel began to shake and visible fumes began entering vehicle through air vents. There was a strong stench of anti-freeze. Check engine light was on and vehicle became hard to control. After taking car to local dealership, I was immediately told, even before an inspection was done, that the heater core on vehicle had a slow leak that over the course of "several months" that eventually leaked on to the transmission electric box and shorted out the entire system. It seemed obvious that this was a familiar issue with this particular vehicle. I was originally given an estimate of $3,300, being that the vehicle was past its warranty period. After several calls to owner of dealership and doing research on this vehicles complaint history, the price was reduced 3 separate times, finally costing me $880. …
For the past two years our car has had a very pungent fowl smelling odor when the air conditioner or heater is engaged that has gotten progressively worse. I took the car for repairs prior to the termination of the warranty and was told (by the dealership) that they did not notice a smell, and there was no problem. Six moths ago the car began to run hot and I took the car to our neighborhood mechanic. Under pressure he noticed a small leak in the coolant piping and replaced a section of pipe ($150.00). I asked if this could be causing the odor (as mentioned earlier and he stated maybe, or it could be the heater core).within the past two weeks the 'check engine soon' light has come on and upon checking the message and inspection of my car I've been told the heater core needs to be replaced. …
I noticed the smell of maple syrup about two weeks ago. Today, the inside windshield and side windows of the Diamante fogged up with a grease film that would not clear. I went to my local mobile 1 and had the coolant system flushed. When I drove off it was actually worse. It got to the point that I could not see. I have suffered severe headaches for the last two weeks as well as nausea. I immediately drove to my Mitsubishi dealer. After analysis, it was determined that the heater core went out. As well, an oil leak had formed and the timing belt was going bad. They said that coolant was leaking in the right passenger floor , I felt it and never once did it feel damp, and they needed to clean the carpet and replace the flux for $237.00. The cost to repair the heater core and oil leak, $1300.00. The timing belt repair is $700.00. Thank god I bought the extended warranty. …
This is a Mitsubishi Diamante 2003 es. I'm experiencing a sweet smell of anti-freeze inside the car as soon as I turn on heating. Anti-freeze contains ethylene glycon and the fumes are highly toxic. Fixing the heater core will cost $1300. 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 windshield thus putting the driver at greater risk. I will eventually fix the problem by spending $1300 if Mitsubishi refuses to issue a recall. …
2003 Mitsubishi Diamante ls 58,000 miles. I noticed a 'maple syrup' smell in the cabin when the air conditioner and heater were running. I took it to the dealership when I began having transmission problems as well. No transmission problems were found, but the dealer advised that the heater core needed to be replaced at a cost of over $1,000. They will not know for several days if the leaky heater core has affected the transmission or engine controls. The heater core was leaking potentially toxic coolant into my passenger compartment in the car. Dealer advised me that they expect every Diamante from 1997-2003 will need this due to a manufacturer's defect. Mitsubishi also released a service bulletin to all dealers regarding this problem. …
2003 Mitsubishi Diamante with 65000 miles. Noticed fog coming out of vents around the radio. It smelled like anti-freeze and created a fog on my windows that was bad enough to render the car undriveable. I was also getting an error message of p-1751. This refers to a problem with the transmission relay control. I took the car to a transmission shop and after diagnosing for two days, the mechanic told me that he had discovered a leaking heater coil that May or May not have damaged the PCM. Worst case scenario, $1600. It will at least cost $500, best case scenario. I uncovered numerous complaints from Diamante owners with models from a wide range of years who were experiencing the same problems. This is criminal negligence and I personally request an investigation to research how mistubishi has been getting away with this, for so long. …
I own a 2003 Mitsubishi Diamante ls with 70,000. I purchased the car from the dealership brand new. In November 2007, there was a strong smell of antifreeze in the car. After some investigation, it was discovered there was a leak in the heating core, which was causing the antifreeze to leak out of the compartment and into the cabin of the car...needless to say, the car is undriveable because of the health risks associated with breathing antifreeze. Estimated repairs are around $1500. This is completely ridiculous!!!! I am outraged to see that so many 2003 Diamante owners are having the same problem. I believe an investigation needs to be conducted, and Mitsubishi motors should be held accountable for this!!!
Heater core issue with Mitsubishi Diamante 2003 summary: notice consistent and very noticeable foul odor when heater is on. . Fogging of front windshield. Have had this problem identified as consistent with heater cores issues by other Diamante owners who have had their car serviced at a Mitsubishi dealer. Have not replaced anything yet because the parts and labor quotes have been anywhere from $600-$1000. Car has 36,000 miles on it.
The car only has 63,000 miles. An antifreeze odor was noticed initially and began to intensify with time. When the defroster is running the windshield will fog up and the anti-freeze smell is so intense that it nauseates my wife and kids. I have also noticed that the carpet on the passenger side is damp. It appears that this is a very common problem with the Diamante due to a failure with the heater core and its connections.
2003 Mitsubishi Diamante coolant leak inside the car. Steam fogging the windows and strong chemical odor leaving residue on windows. Quoted $1000 to repair, they say it needs a new heater core. It May not be the heater core, but just seals. I have bypassed the heater hoses and fortunately live in a warm area.
AC & Heater complaints in other Diamante years
Frequently asked questions
How common are ac & heater problems on the 2003 Mitsubishi Diamante?
NHTSA has 27 complaints about the a/c & heater of the 2003 Mitsubishi Diamante, 48% of the model year's 56 complaints. The component named most often is defroster/defogger/hvac system › heater core (24 mentions).
Is there a recall for 2003 Mitsubishi Diamante ac & heater problems?
No. NHTSA lists no a/c & heater recall for the 2003 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 2003 Mitsubishi Diamante ac & heater problems start?
Half of the complaints with a recorded mileage happened before about 60,000 miles, and 10% before 25,000 miles.
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.