One of the most common obd2 fault codes on a 2003 mitsubishi eclipse is p0181, which translates to an issue with the fuel temperature sensor. when the eclipse throws this code, you'll see your check engine light illuminate, and it's best to take a look at it as soon as possible. if this is your issue -- or you've got a similar one -- you'll need to first know where the sensor is located, before you dive into a repair. unfortunately, it's not simple to get to. the sensor is located inside the fuel tank, as it detects the ambient temperature of the fuel there. in order to gain access, you'll need to drop the fuel tank, which can be a time consuming process, especially if you don't have a lift. it's generally a recommendation that you take your eclipse to a certified mechanic for this repair.
On the 2003 mitsubishi eclipse the thermostat is located under the housing near the water pump on the cylinder head.