Inside the FPR is a rubber diaphragm/valve, at idle when vacuum is highest and fuel demand lowest the vacuum in the hose pulls open the diaphragm allowing fuel to flow into the Return Fuel line and back to the fuel tank.
If that rubber diaphragm leaks.......................fuel is sucked into intake via the vacuum hose.
Fuel injected engines can't use a Choke Plate but a cold engine still needs extra fuel, advanced spark and high idle to run until it warms up.
Computer uses an ECT(engine coolant temp) sensor to decide if engine needs to be "choked" and runs engine that way until it warms up.
ECT is a 2 wire Sensor, engine will also have a 1 wire Sender used for dash board temp gauge, these 2 separate devices are usually located near each other on lower intake by thermostat housing.
When you start engine cold idle should go up to 1,500+ RPMs, the drop down to about 1,100 RPMs.
As engine warms up idle should start going down to warm engine target RPM of 750 with automatic, 650 with manual trans.
If that happens then ECT sensor is working OK.
When in choke mode the computer ignores the O2 sensor, it is running engine rich on purpose, and O2 sensor won't work until it gets up to 650degF in any case.
This is called Open Loop operation for the computer, it has no feedback to adjust fuel mix.
After engine warms up computer switches to Closed Loop operation, it is getting feedback from O2 sensor, so you have a closed loop, maf>>computer>>O2>>computer
On the upper intake the is a 2 wire IAT(intake air temp) sensor(also called ACT sensor), colder air is denser than warmer air so requires more fuel per liter of air, IAT sensor determines air temp in intake and computer adjusts fuel mix accordingly.
If it is reporting colder than it should you would run slightly richer, BUT............O2 sensor should correct for this, it is the final word on Lean or Rich.
Good site here to read up on sensors and testing them: http://oldfuelinjection.packrad.net/page10.html