Quoted from Boslaw:Thanks very much for the info. I tested the transistors the "dirty way" and Q54 reads 0. The others have different non-zero readings.
I found schematics for this board and Q54 looks like it has 12N10L transistor. IRL540 appears to be a replacement for 12N10L.
More googling about IRL540 finds this:
"An FET is not like an ordinary transistor, and the typical tests won't work. Here are some hints. First, the pins. The left pin is the gate. This pin should be electrically isolated from the others. Pin 2 is the source - positive voltage flows in here - and pin 3 is the drain, normally connected to ground.There is an internal diode across pins 2 and 3. This diode protects against kickbacks from coils. Now, here's the problem. The gate is very well insulated. So much so that just touching it can leave enough of a charge that the transistor is turned on. That makes your meter read funny between pins 2 and 3. Sometime one thing, sometimes another.So the only real test that you can do is to look for a connection between pin 1 and pin 2 or pin 3. If you see that, the FET is probably shorted."
There are no connections between the pins on that transistor, no black spots, no obvious damage, etc. Do I trust the multimeter? Once I get a new fuse and solenoid, should I try testing one last time with the new solenoid to confirm the diagnosis or do I risk doing more damage?
If I do replace the transistor, is the fix as simple as desoldering the bad transistor and adding a new one in its place (hopefully without doing any other damage to my board)?
That is your likely culprit even though it’s not 100%. It is as simple as desoldering and adding a new one. The transistor is the path to ground and if your bumper is locking on it has a permanent path to ground via the transistor. The proper way to test is by touching the three legs but even if one tests good it’s not always good. The fact that your other two are giving a non 0 number with the tab method is a pretty good indication that the transistor currently in that spot has degraded and likely failed.
Also if you haven’t done board work before it would not be a bad idea to find someone to help you with it. These boards are expensive and if you are worried about damaging it, it would be worth seeing if anyone in your area was more comfortable with doing it.
I haven’t checked but usually these bumper transistors are TIP120 which was replaced by TIP102. That other one sounds like it might be the pre-driver but it’s been a long while since I’ve had to look at the board on my bride.