1999 GMC Forward Control 6.5 F, StandardFACTORY OEM SERVICE PROCEDURE

Water-In-Fuel Light Diagnosis

  1. Turn ignition on with engine off. If WATER IN FUEL light does not illuminate, go to next step. If WATER IN FUEL light illuminates briefly, go to step  5.
  2. Disconnect water-in-fuel sensor harness connector. Ensure ignition is on with engine off. Using an unpowered test light connected to ground, probe ignition feed circuit (Pink wire) at water-in-fuel sensor harness connector. See Fig 1. If test light illuminates, go to next step. If test light does not illuminate, go to step  6.
  3. Connect test light between ignition feed circuit (Pink wire) and ground circuit (Black wire) at water-in-fuel sensor harness connector. If test light illuminates, go to next step. If test light does not illuminate, go to step  8.
  4. Using test light connected to ground, probe indicator light control circuit (Yellow/Black wire) at water-in-fuel sensor harness connector. See Fig 1. If test light illuminates, go to step  11. If test light does not illuminate, go to step  10.
  5. Operate vehicle under conditions when problem was noted. If system does not operate properly, go to step  2.
  6. Check for open in ignition feed circuit (Pink wire) between underhood fuse/relay center and water-in-fuel sensor. If problem exists, repair as necessary and go to step  12. If problem does not exist, go to next step.
  7. Check ignition feed circuit (Pink wire) for poor connection at water-in-fuel sensor. If problem exists, repair as necessary and go to step  12. If problem does not exist, go to step  11.
  8. Check for open in ground circuit (Black wire) between water-in-fuel sensor and ground connection. If problem exists, repair as necessary and go to step  12. If problem does not exist, go to next step.
  9. Check ground circuit (Black wire) for poor connection at water-in-fuel sensor. If problem exists, repair as necessary and go to step  12. If problem does not exist, go to step  11.
  10. Repair open in indicator light control circuit (Yellow/Black wire) between instrument cluster and water-in-fuel sensor. After repairs, go to step  12.
  11. Replace water-in-fuel sensor. After repairs, go to next step.
  12. Operate vehicle under conditions when problem was noted. If system does not operate properly, go to step  1.
Fig 1: Identifying Water-In-Fuel Sensor Harness Connector Terminals
G98D00870Courtesy of GENERAL MOTORS CORP.
Factory Service Tools

Use the Manual With the Right Hardware

Pair factory procedures with proven DIY tools so the instructions are easier to execute.

Disclosure: Verified tool recommendations may earn an affiliate commission at no cost to you.

Shop Manual-Ready ToolsCompare Repair QuoteQuote Shield Pro

When to See a Mechanic

Stop DIY work and contact a certified mechanic immediately if any of the following apply:

  • • You smell fuel, burning insulation, or see smoke.
  • • Brakes feel soft, pull hard to one side, or make grinding noises.
  • • The engine overheats, stalls repeatedly, or misfires under load.
  • • You are missing required tools, torque specs, or safe lifting equipment.
  • • You are not confident in the next step or safety outcome.

Rather have a pro handle this?

A certified mobile mechanic comes to your home or office. They bring the tools, you stay comfortable. No tow truck, no waiting room, no surprises.

Comes to youSame-day available12-month warranty
Get a Free Quote

No obligation. See pricing before you book.