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· Registered
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Discussion Starter · #1 ·
Hey guys I'm currently working on a buddy's 2000 s10 2.2. We just replaced the engine due to the old one being seized. We had to swap intake manifolds because the "new" one didn't have a return fuel line port on the rail. Once installed it cranked and ran about 10 minutes then shut off. Scanned it and got P0178 Fuel composition sensor circuit low. I have searched the entire frame rail and truck for this sensor and I do not see it anywhere. The fuel lines go from the engine straight to the filter then tank. So where is it or is there even one on this truck? If not is it possible it just has bad gas? It sat for almost 2 years prior to the engine swap.

· Registered
2002 S10 2WD (4.3L, AT)
501 Posts
Are you using the 2-year old gas? That might not be your issue, but I would definitely discard the old gas and replace with fresh. Gas does not last forever. Using the old gas may have clogged the injectors or other parts of the fuel system......

· Been there Done it
16,795 Posts
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:

· Been there Done it
16,795 Posts
We need to know if it's a vin 4 or 5 to know what truck he has. As MBP asked yesterday.
The sensor could have gotten unplugged during the swap. Or the engines aren't the same. Or the trucks on it's 3rd engine and no one knows what it's supposed be. Or the friend he's helping isn't telling him the whole story. Or...
One thing is sure, the computer thinks the truck is supposed to have that sensor, but it hasn't heard from it lately. So to protect the engine from damage it shut itself off.
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