Operation of a typical in-tank fuel level sending unit is based on variable resistance. As
the float moves up or down according to fuel level, the resistance in the sending unit varies. This varying resistance is processed by the engine control module (ECM) and the resulting information is sent to the instrument panel cluster (IPC) for display on the fuel gauge.

In the past, the sending unit was set up so that high resistance indicated high fuel level, and low resistance indicated low fuel level.

Sulfur contaminants found in some of today's gasolines can form deposits on the sending unit, resulting in higher-than-intended resistance.
The high resistance from the deposits could be misinterpreted as a high fuel level, when in fact the fuel level is low. The driver may run out of fuel, even though the fuel gauge is not indicating low or empty.

To offset this effect, engineering has begun "flipping" fuel senders, reversing the circuitry.
This means that high resistance now indicates low fuel level and low resistance indicates high fuel level. So, if deposit build-up adds to sender resistance, the gauge will err in the direction of indicating less fuel than is actually in the tank.

The "flipping" process began in the 2004 model year continued through model years 2005-06.

The remaining vehicles will have their fuel level sending units "flipped" for the 2007 model year.

TIP: When diagnosing fuel sender operation, it's critical to refer to the appropriate model year information in SI, to be sure which resistance specifications apply.

TIP: GMSPO offers GM Fuel System Treatment PLUS p/n 88861011 (88861012 in Canada) which helps protect fuel sending units from contaminating deposits.

More proof of damage to the float sender due to Ethanol eating mask off card