Fuel Composition Sensor (FCS), or Flex Fuel Sensor
The fuel composition sensor (FCS), or flex fuel sensor (service parts term), measures the ethanol-gasoline ratio of the fuel being used in a flexible fuel vehicle. Flexible fuel vehicles can be operated with a blend of ethanol and gasoline, up to 85 percent ethanol. In order to adjust the ignition timing and the fuel quantity to be injected, the engine management system requires information about the percentage of ethanol in the fuel.

REMOVAL:

1. Relieve the fuel system pressure.
2. Raise and suitably support the vehicle. Refer to Vehicle Lifting.
3. If equipped, remove the transfer case shield.
4. Remove the fuel line retainers.
5. Disconnect the fuel composition sensor fuel pipes.
6. Disconnect the fuel composition sensor electrical connector
7. Remove the fuel composition sensor bracket nuts.
8. Remove the fuel composition sensor.
9. If required, remove the fuel composition sensor bolts and remove the sensor from the bracket.

INSTALLATION:

If required, install the fuel composition sensor to the bracket and install the bolts

There are a couple of reasons: 1. Low-quality, inferior fuel was used in the system, which threw the sensor off 2. the sensor iitself is bad. If you are sure low quality fuel was not used, check all connections going to the Flex Fuel Sensor for loose connections or broken wires. Due to its location, its vunerable to read debris and weather elements.

If you changed the sensor already, i would suspect debris from the fuel tank is getting clogged up in the sensor, which is interfering with the proper operation of the sensor.