There's an electrical connector under the dash of my 2000 Subaru Forester that is apparently instrumental in the diagnosis of some OBD-II trouble codes. It's a pair of bladed terminals like any other on the car:

Specifically, some of the diagnostic procedures in the factory service manual call for testing several evap system valves in turn like so:

1) Connect test mode connector. 2) Turn ignition switch to ON. 3) Operate drain valve.*

It was a little scary, actually, the first time I connected it and switched the key on -- the fans spun up every second or so, I could hear relays clicking all over the car, and the MIL started flashing.

But I'm not sure exactly what I should be expecting to happen. Is there an implicit sub-step to item 3 above -- "Operate drain valve" -- that I don't know about?

Or is the valve being operated just by putting the car into test mode? I applied a stethoscope to one of these valves that's easy to get to and it seemed to be clicking open and closed, and passing air. Is that all there is to it?

If so, which valves are supposed to be operated in this mode? Drain valve? Shut valve? Purge control valve? Pressure control valve? Literally every electrical component on the car? I can't really say "this valve is dead" without knowing for sure that it should be switching. Is there a list in my FSM somewhere I haven't found?

My car's a 2.5 liter, SOHC, manual transmission, USDM.

*For the curious, this "drain valve" is PN 42084FA120, and everywhere except my manual calls it "canister ventilator valve".

  • 1
    One of my points of confusion here is also whether this is specific to Subarus (which I doubt) or applies to any OBD-II car. Feel free to edit out the Subaru mentions if the latter is true.
    – jscs
    Commented Dec 17, 2013 at 20:38
  • Several makes of vehicle before OBD1 and OBD2 had a WOW function. The last one I know of was on Fords. The tip counter button was pushed when turning on the ignition. Doing this made all the dash lights operate, gauges went full scale. Any faults on the vehicle could then be displayed by repeatedly pushing the trip button, the fault codes appearing on the dash. All OBD2 faults are accessed by the 16 pin DLC plug. Commented Dec 17, 2013 at 21:15
  • There's a test mode for my Mitsubishi Eclipse as well, but it just forces the timing to a set value and zeros the IAV (Idle Air Valve) to assist in making BISS (Base Idle Set Screw) adjustments. Commented Dec 17, 2013 at 21:26
  • Where did you get this test connector from? Did it come with the car? Commented Dec 18, 2013 at 2:16
  • 2
    @Larry: Yes, it's an integral part of the car: google.com/… It's two normal plastic electrical terminals/plugs, hanging down from a pigtail under the dash, near the fuse box.
    – jscs
    Commented Dec 18, 2013 at 7:49

1 Answer 1


The test connectors are common on Subaru vehicles. They put the car is diagnostic mode and allow for things like the ECU to be reprogrammed. I actually have a Saabaru and had to connect these to use a Cobb Accessport. While it was connected I also had fans and other items turning on and off randomly.

Would need some context for the instructions in the manual as to explain which drain valve, but that should be a separate step.

  • Ditto on my Subarus
    – Bob Cross
    Commented Dec 18, 2013 at 12:43
  • The context here is the diagnostic procedure in the FSM for a P0440. The drain valve I mentioned is P/N 42084FA120, called "canister ventilator valve" everywhere except in my manual. The diagnostic procedure also calls for checking two other valves in the EVAP system, and the instructions for those are identical: connect connector, key on, operate valve. That's why, for this and future diagnostics, I'm trying to find a detailed description of what "test mode" entails, i.e., what components are "tested".
    – jscs
    Commented Dec 18, 2013 at 20:24

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