Question

2005 Toyota Corolla CE gets code P0171 in cold weather.
P0171 MIL Fuel system too lean.
Fuel system 1: Open Loop
Fuel system 2:N/A
Load value : 0.0%
The engine runs smoothly and gas mileage is excellent.

What is wrong?
Thank you

Answer 1

When you see fuel system 1 open loop, this is for the upstream 02 sensor, when the 02 sensor  cycles from lean to rich as it is suppose to the ECM see this as closed loop. In this case it does not even see the 02 sensor at all . there is  2 approach for this a proper working thermostat and a truly functioning upstream  02 sensor.