MIne does exactly the same thing. When it is 80 degrees F my thermometer reads around 58. I thought it was due to being covered with road goo, but I cleaned it off, and this did not make ant difference.

Harbor Freight (1-800-423-2567) sells an inside/outside automotive digital thermometer for $9.99
(item 33080-2JJA)

I wonder if connecting the outside sensor of this unit to the wiring to the dash might work? I suppose that if it didn't, one could reconnect the old sensor with a bit of shrink tubing.

It would be a gamble, but 12 v negative ground is standard, and the risk of harming either device is less than the risk involved in 10 lottery tickets.

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Semibodacious Transmogrifications a Specialty

1990 300D 2.5 Turbo sedan 171K (Rudolf)
1985 300D Turbo TD Wagon 219K (Remuda)

"Time flies like and arrow, yet fruit flies like a banana"
---Marx (Groucho)