Audi > A4, S4 > 2002-2007
Fuel Supply System
20 - Fuel tank, vehicles with all wheel drive up to 06.03
Fuel Tank Lock Switch F314 , checking The Fuel Tank Lock Switch F314 consists of a reed contact on fuel filler tube and a fuel tank cap with an embedded magnet. If fuel tank cap is not fully screwed on, the reed contact switches on an indicator in the instrument cluster.
Special tools, testers and auxiliary items required
Portable multimeter V.A.G 1526 C
Connector test set V.A.G 1594 C
If indicator is lit, check the following points in order:
Fuel tank cap is completely screwed on.
Correct fuel tank cap (Identification: Threads are gray in color).
Switch - arrow - at fuel filler tube is fully inserted.
Electrical malfunction in switch.
Wiring connection to instrument cluster is faulty (check according to schematics).
Fuel Tank Lock Switch F314 , checking electrical
Remove rear seat bench Body Interior - Repair Group 72 .
Unbolt cover for right locking flange - arrows - .
Disconnect electrical connector - 2 - .
Ignore - 1 - .
Connect hand multimeter V.A.G 1526 B between terminal - 1 - and - 2 - for resistance measurement.
Fuel tank cap Specified value
Screwed on tight ? ?
Not or incorrectly screwed on less than 1 ?
If specifications are not obtained:
Remove right rear wheel housing liner Body Exterior - Repair Group 66 .
Replace Fuel Tank Lock Switch F314 - arrow - , thereby unclipping and clipping in the switch and electrical wiring at fuel filler tube.
2004 A4 1.8t USP Quattro 6MT
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I think the "gas cap" warning light is actually a condition when the fuel system leak detection....
fails the self check. There is a leak detection pump that runs and either creates a vacuum or a low pressure on the fuel tank (I forget now, I have to review the lit.) and tank ventilation fuel vapor recovery plumbing. If the system doesn't hold the specified differential pressure/vacuum, then the "gas cap" warning light is turned on. I don't think there is an electrical cap sensor.
I can't find the reed switch for the tank cap status anywhere in the wiring diagrams....
The description for the function is also incomplete. For example, the magnet in the gas cap, when placed in close proximity with the reed switch, can either close the reed switch contacts together and establish circuit continuity,(a N.O. or normally open switch,) or pull the contacts open and break circuit continuity at the reed switch. (A N.C. or normally closed switch). Depending on the design of the monitoring device, either switch or circuit condition as "open" or "closed" circuit can be used to infer the status as "cap installed" or "cap not installed". Therefore, testing the circuit continuity at the IC end, may be inconclusive if continuity does not exist with the gas cap installed.
This doesn't help much, I know, but is a basic description of the functional aspects involved, or not. :>/ Lacking clear documentation of the circuit/reed switch setup, as a practical matter, it can be assumed that the reed switch is N.O.. As such, if the circuit between the reed switch is broken or "open" anywhere from the IC to the switch, or the reed switch itself is faultily, will exhibit the flashing gas cap warning light with the cap installed.