I've been getting acquainted with one of the fuse boxes under my hood to get up to speed with the relays found there. I'm walking through the diagram and the table of relays seems to match up very consistently with blank entries (-) in the table to empty relay slots in the fuse box.

However one relay labeled FMI2 BIFUEL LPG VALVE has a entry in the relay table but no relay in the fuse box. I'll research more about my car but I'm curious as a general question if there is such a thing as optional relays or for that matter optional circuits in a vehicle?

I guess that might be so with this bi-fuel vehicle thing this relay refers to, I'm only running Gas so it's good I don't need that circuit to be active.

A follow up question would be what's the conventions for these fuse box diagrams, an empty entry in the table would suggest I don't have the circuitry or hardware at all for the devices those portions of the fuse box are capable and intended to support. Perhaps some upgrade hardware passed up at the time of original owner vehicle purchase. Maybe the fuse boxes are used to support multiple vehicles?

Overlay of missing relay slot above the relay fuse box diagram

  • 2
    Doing this cuts down on designing another injection mold for a slightly different box, it is cost cutting.
    – Moab
    Sep 1, 2016 at 0:20
  • 1
    It's common, because it's cheaper to manufacture. They may or may not install the wiring behind it, depending on what's cheaper for them at the time of manufacture. In some cases you can see holes for the relay in the moulding, but no socket underneath, so if you actually tried to plug in a relay it would just fall out.
    – Chris H
    Sep 1, 2016 at 12:04
  • Sometimes they're also used for dealer-installed options (e.g. a trailer towing kit). That way the dealer can easily add options that would otherwise require extensive electrical work.
    – TMN
    Sep 1, 2016 at 13:37

1 Answer 1


Yes. You'll often see optional components in manuals and circuit diagrams. That simplifies the documentation process. Often there will be provision for those components in the actual manufacture of the car as well.

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