Not sure how to to test it, but it only takes about 2 minutes to pull it out and visually inspect. I had the same problem when I bought my wife's car. The rubber diaphram was torn and letting hot water past, and it was like an oven door had opened. But that was all the time, meaning I wasn't getting any cool air. i bought an extra in case it ever happens again. I did see a way to test them, but it seems like you have to pull it to check it, anyway. At least yours works when it's on. Mine's not. And man is it hot and humid here!

__________________
"Knowing is not enough, we must apply.
Willing is not enough, we must do." Goethe

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1951 Chevy 3100

2003 Indian Chief Roadmaster

1983 GMC 1 ton Dually

1982 Chevy 1 ton Dually, service body (sold)

'90 GMC Suburban 6.2 "SS Veg-Burban"
(single tank WVO\diesel conversion) SOLD

'81 300D ~ Mama's car...my job (now my car)(but still my job) SOLD

'83 300sd ~ rescue car SOLD

2005 Ford Taurus (Mama's new car)(NOT my job!)