2003 Mitsubishi Diamante Problems

What owners report

Owners of the 2003 Mitsubishi Diamante most often describe coolant leaking into the cabin, usually from the heater core, with a sweet odor from the vents and a film that fogs the windshield. Other frequent reports include brake shaking when braking, and no-start or limp-mode conditions tied to a transmission relay, with some linked to coolant reaching electronic parts. Where mileage is given, most heater core reports fall between about 40,000 and 90,000 miles, though some are lower or near 164,000 miles.

1. 1
   Engine cooling

   Coolant leaks into the cabin, fogged windows and odor

   • 37 complaints (66%)
   • median 60,000 mi at 4.7 yrs old (30 state mileage)
   • owner-quoted repair $1,167 (median of 13 quotes)
   • filed 2005–2011

   Owners describe coolant leaking into the cabin, often traced to the heater core, with a sweet, syrupy, or chemical smell from the vents and a film that fogs the windshield and side windows. Some say the film was greasy or green, that they felt nauseous or had headaches, or that the carpet or floor was damp with coolant. Dealers quoted a heater core replacement at several hundred dollars or more, and some owners did the work themselves or said the warranty had ended. Some report coolant dripping onto electronic modules, which owners say led to a transmission relay or computer failure. Where mileage is given, most fall between about 40,000 and 90,000 miles, with some lower and a report near 164,000, and many say the odor or fog appears when the heat or air conditioning is on.

   What to check: Check the carpets and footwells for dampness or residue, and notice whether there is a sweet coolant smell with the heater running. Ask whether the heater core and related hoses were replaced, and whether the transmission relay and computer were checked for coolant damage, since complaints describe leaks reaching electronic modules.

   2 excerpts from owner complaints in NHTSA records

   “I purchased a used 2003 Mitsubishi Diamante from bankston dodge in 2007. It was clean and ran well. There was an unusual odor inside, but smelled as if it could have been what the dealership used to clean the inside. It turned out to be anti-freeze. The vehicle had a heater core leak prior to our purchase. Recently the gas gauge has stopped working.”

   NHTSA complaint #10276190, filed 2009-07, 50,000 miles

   “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.”

   NHTSA complaint #10183423, filed 2007-02, 62,500 miles
2. 2
   Brakes

   Shaking or vibration when braking

   • 7 complaints (13%)
   • median 8,500 mi at 1.5 yrs old (6 state mileage, few)
   • filed 2004–2006

   Owners describe the vehicle shaking, shuddering, or vibrating when the brakes are applied, sometimes so strongly that they say it is hard to control. Several say the rotors or brake parts were serviced or replaced, but the shaking returned. A mechanic said some rotors were too damaged to resurface, and according to complaints a dealer employee said brake problems were common because cars sit a long time before sale. Steering wheel vibration and brake noise are also described in some complaints. Where mileage is given, several reports come from vehicles with under 35,000 miles, and some say the vibration began soon after purchase.

   What to check: On a test drive, note any pulsing in the brake pedal or steering wheel when braking from moderate speed, and ask for the brake and rotor repair history. Ask whether the front rotors were measured for thickness and runout, since complaints describe repeat repairs that did not stop the shaking.

   2 excerpts from owner complaints in NHTSA records

   “When I apply the brakes, the car shakes and jerks uncontrollably. I have had work done on the rotors as well as the brakes. I have four new tires, but problem still occurs. I am afraid that one day while driving this car the brakes will fail and cause my family and I serious injury . This is a 2003 Mitsubishi Diamante, and the incidents have occurred daily 3 months after I purchased the vehicle.”

   NHTSA complaint #10114500, filed 2005-03, 30,000 miles

   “When I apply the brakes, the car shakes and jerks uncontrollably. I have had work done on the rotors as well as the brakes. I have less than 20k miles on the car. It is highly unusual to have 4 warped rotors without some type of manufacturing defect.”

   NHTSA complaint #10134166, filed 2005-08, 15,000 miles
3. 3
   Transmission and drivetrain

   No-start or limp mode tied to transmission relay

   • 9 complaints (16%)
   • median 60,000 mi at 5.5 yrs old (9 state mileage, few)
   • filed 2006–2013

   Owners describe intermittent no-start conditions, a flashing neutral indicator, or a limited safe mode in which the car runs only in second gear. Several link these events to the transmission control relay, with trouble code P1751 or a relay failure, and some say coolant dripped onto the computer before the problem began. A complaint describes a gear shifter that could not be released from third gear, and another describes the car losing drive until the engine was restarted. Where mileage is given, these complaints range from about 60,000 to 164,000 miles, and each describes a coolant leak present at the same time.

   What to check: During a test drive, watch for a flashing neutral light, a failure to start, or a limited second-gear mode, and ask for any stored codes. Ask whether the transmission relay and the computer were checked after any coolant leak, since these complaints link the two.

   2 excerpts from owner complaints in NHTSA records

   “I own a 2003 Mitsubishi Diamante, around 60k miles I started noticing a sweet smell from the vents and a constant fog when the heat or air was turned on in the car. After taking my car to the mechanic I was told that my heater core was busted. My car also sdriving in safe mode meaning I can only run in second gear which is burning a tank of gas every three day, I love 10 miles from work. My car flashed in neutral will in drive. …”

   NHTSA complaint #10240783, filed 2008-09, 60,000 miles

   “My transmission relay went out due to a leak heater core. The core has been leaking for some time and has fogged my windows so bad I could not see at times, having to pull over and wipe off the film. This is a serious problem with these cars.”

   NHTSA complaint #10226354, filed 2008-05, 114,132 miles

Buying note. A buyer considering a 2003 Diamante could check the carpets and windshield for coolant residue with the heater on, and ask whether the heater core and transmission relay have been replaced or checked. Brake shaking is also reported, so asking for brake and rotor repair history before buying is a sensible step.

Summary written with AI from a sample of 52 NHTSA complaints for this model year (picked at random, with complaints that state a mileage and recent ones taken first), then checked automatically against the source records. Where a problem shows a complaint count, every complaint for this model year was matched with fixed keyword rules (written with AI and tested against the sample), and the mileage, age and repair figures are calculated from those complaints; owners' figures are as stated, not verified. Other counts refer to the sample. The excerpts come from NHTSA's public complaint records and are unverified; capitalisation is normalised, NHTSA staff notes are removed, and shortened text ends with “…”. Something wrong? Request a correction.