Uploaded September 2016 | Updated September 2026, 3 weeks ago
In this video we discuss the most common cause of a GMC or Chevy flex fuel equipped vehicle with a check engine light illuminated and a stored code P0172. These GM produced Flex Fuel systems use a PCM based virtual flex fuel sensor (V-FFS)algorithm in order to calculate fuel metering. The ethanol level calculation occurs with the engine running after a refueling event has been detected via a measured change in the fuel level sender output. The V-FFS algorithm temporarily closes the canister purge valve for a few seconds and monitors information from the closed loop fuel trim system to calculate the ethanol content. This logic executes several times until the ethanol calculation is deemed to be stable. This can take several minutes under low fuel flow conditions such as idle, or a shorter time during higher fuel flow, off-idle conditions. Earlier PCM software versions tend to error on the side of higher fuel alcohol content over time requiring a reset of those learned values to bring LT fuel trims into range. Watch this helpful video for more information.
We mentioned a video outlining testing for verifying fuel alcohol percentage. Watch that video here: youtube.com/watch?v=d8pV-x9uLiE
Please subscribe now to The Car doctor Channel for more helpful automotive related videos. Thanks for watching!
In this video we discuss the most common cause of a GMC or Chevy flex fuel equipped vehicle with a check engine light illuminated and a stored code P0172. These GM produced Flex Fuel systems use a PCM based virtual flex fuel sensor (V-FFS)algorithm in order to calculate fuel metering. The ethanol level calculation occurs with the engine running after a refueling event has been detected via a measured change in the fuel level sender output. The V-FFS algorithm temporarily closes the canister purge valve for a few seconds and monitors information from the closed loop fuel trim system to calculate the ethanol content. This logic executes several times until the ethanol calculation is deemed to be stable. This can take several minutes under low fuel flow conditions such as idle, or a shorter time during higher fuel flow, off-idle conditions. Earlier PCM software versions tend to error on the side of higher fuel alcohol content over time requiring a reset of those learned values to bring LT fuel trims into range. Watch this helpful video for more information.
We mentioned a video outlining testing for verifying fuel alcohol percentage. Watch that video here: youtube.com/watch?v=d8pV-x9uLiE
Please subscribe now to The Car doctor Channel for more helpful automotive related videos. Thanks for watching!
