Here's an explanation written by @Hucko
17 years ago.
I did not post the link to thread because it turns into a 10 year running argument about nothing, but this first post is very clear and concise:
What is Flex Fuel?
A flexible fueled vehicle (FFV) has a single fuel tank, fuel system, and engine. The vehicle is designed to run on unleaded gasoline and an alcohol fuel (usually ethanol) in any mixture. The engine and fuel system in a flex-fuel vehicle must be adapted slightly to run on alcohol fuels because they are corrosive. There must also be a special sensor in the fuel line to analyze the fuel mixture and control the fuel injection and timing to adjust for different fuel compositions.
What Service Engine Soon codes does it throw?
P0178 and P0179 are directly related to the sensor its self
P0178 - Fuel Composition Sensor. Sensor Circuit Low Volts
P0179 - Fuel Composition Sensor. Sensor Circuit High Volts
Can this really effect anything?
Yes, It can cause your truck to run richer then it should.
In my case I was also throwing an HO2 Sensor (P0141), this went away after I replaced the Flex Fuel sensor.
How much does this sensor cost and what is the part number?
There are 2 part numbers (both are the same part):
The first one: 12568450 (older part number)
The second one: 12570260 (most recent)
From a GM dealer this part will run $613 Note: 2005 prices
From GMPartsDirect.com it runs $350
dealer will charge around $900-1100 to fix! Back then
PM ME for details on finding the older part number cheaper.
Where is the sensor located and what does it look like?
Passenger side, Front frame rail. Right behind the wheel well.
It has 2 fuel lines coming out of it and 1 power.
What kind of tools are needed to replace this myself?
Size 13mm Socket
Fuel Line disconnects ($5 - $10)
How long does it take and is it easy?
Yes it is easy, and it takes about 2-3 hours depending on how you go about getting to it. Take your time and be careful!
(a little bit of gas will come out of those lines)
I pulled off the passenger side wheel and went thru the wheel well with a friend under the truck helping.
I hope I was of some help! PM ME with your questions! (His words, not mine)
Lots more info here, 100 links or so: