How often do you guys run into resistor that fail open with no external signs? I can't say it occurs all that often for me. So little that I never suspect these components to be faulty unless I see signs of damage. I had been banging my head on this trucks ecu repair for an hour before finding this guy. Removed him, tested and.... Dead as a door nail. Replaced it and Its all back to normal.

 

Google sulfur attack on chip resistors.

 

How often do you guys run into resistor that fail open with no external signs?

Never.

I've had problems on homemade boards using SMT resistors, but only because I didn't apply enough solder paste. I had to do a burn-in on half a dozen boards (with 4 resistors and 4 MOSFETs) and some took more than one repair attempt.

 

Google sulfur attack on chip resistors.

Interesting but this guy looked brand new, no corrosion and was covered in a nice conformal coating. Its from the 90's so I guess just age and use took its toll.

 

That looks like a cold solder joint on both sides of that device.

But you say that you tested it after it was out of the circuit and it tested as an open. So I'm at a loss.

 

That looks like a cold solder joint on both sides of that device.

But you say that you tested it after it was out of the circuit and it tested as an open. So I'm at a loss.

Good eye, the picture is taken after I removed it. Its just sitting on the pads there. The clear stuff is just a drop of IPA for a clearer picture not the original conformal coating.
Yes testing was done out of circuit with two different meters both showing open.

 

Possibly defective to start with, just took some time to fail.