This can be caused by a number of things, the most likely being a failure in the oxygen sensor or it's wiring, like a broken or corroded harness.
P1131 = Insufficient switching O2 Bank 1 Sensor 1 Sensor/lean. Could be a B1 S1 O2 sensor going bad.
were is the iac locate on a 1999 mercury mystique
Yes , the ELECTRIC fuel pump is inside the gas tank on a 1999 Mercury Mystique
According to the 1999 Mercury Mystique Owner Guide : ( 58.5 liters / 15.4 U.S. gallons )
The thread on cap on your engine coolant reservoir is the pressure cap / radiator cap on a 1999 Mercury Mystique
According to the 1999 Mercury Mystique Owner Guide : ( 58.5 liters / 15.4 U.S. gallons )
According to the 1999 Mercury Mystique Owner Guide : ( 5W-30 ) certified for gasoline engines ( 2.0 and 2.5 liter )
The 1999 Mercury Mystique OBD 2 port is under driver side dash next to hood release
IN the Middle!¬
In the drivers footwell behind the kick panel ( there is an access hole in the panel ) according to the 1999 Mercury Mystique Owner Guide
The thermostat for a 1999 Mercury Mystique is inside of the top water inlet. Remove the top radiator hose from the engine, and then inlet can be removed to get to the thermostat.
Personally, I get about 23 - 25 mpg. on my 1999 mercury mystique. I haven't counted high way mileage yet because I don't get on the highway alot.