Arduino not functioning outdoors

Let me know if this is in the wrong section.

I’m working on a project that will heat a greenhouse using solar energy absorbed from heat collectors. I am using an arduino to monitor and record 4 temperatures using I2C, running 2 12V computer fans for air flow, an SD shield to record the temp data, and an I2C LCD shield for a real-time display. This is all running off of a 12V car battery. My problem is I get the code working when I am inside, but when I try the system outside the whole system freezes.
The first test I did over the weekend went well, but the second weekend test froze after one day. The greenhouse does get hot (about 45 C or 115 F), but even if I test when it is cool, the arduino is unresponsive. Any ideas?

CanSystem12.ino (5.44 KB)

Its unlikely to be a problem with your code: you may get more help if you post your wiring diagram (hand-drawn and photographed if nec)

What is different between indoor and outdoor tests? For example are you using the same car battery to power the inside and outside tests? Do you have all of the fans hooked up for both tests? Is it so humid outside that water is condensing- that may be causing electrical problems? Do you run the indoor tests for prolonged periods?

When I test the arduino code, I power it off of my laptop; the car battery is only for outside. Today I'm going to ground the battery with a stake in the ground. Yes the fan systems are connected on both. The arduino is in a box (not water/airtight yet) but condensation would only be a long term problem when it is left out.

Well, turns out that the problem only occurs when the arduino is INSIDE the greenhouse; it works normally outside. Strange solution and I still don't know why it freezes only when it is surrounded by aluminum and polycarbonate sheets.

Temperature and humidity are likely different inside the greenhouse: Perhaps pack your arduino with some silica gel and see if it lasts longer?