My coolant fan only comes on with the A/C

Hello, sadly there is no quick or easy solution; only testing will reveal the problem you are having with your cooling system. The check engine light came on for some reason -- but maybe not because of a thermostat being stuck open. What code did you get? The condenser fan may be working indeed, but the engine cooling fan operation may not be. Again, testing is needed to pin point the failure.

Try checking for power at the engine fan. If it has power when the engine is warmed up and the temp threshold for fan activation has been met, you may have a bad fan. Alex