[Alan Roehrich]

If you create high resistance across the temperature sensor, you can make the ECM think the car has not warmed up, and it will not go into closed loop. Measure the resistance of the sensor with the engine at ambient temperature, then go get a resistor to match that.

What will happen if it runs continually in open loop cold enrichment mode for a long time is a multi answer question. Eventually you'll have gasoline in the oil, fouled plugs, and other problems of that nature. You can over heat and melt the catalytic converter guts.