2002 Dodge Neon Temperature Gauge Reads Cold

I have recently had transmission work and they added a transmission fluid cooling system so that I don't have the oil and trans fluid mixing as the Neons are famous for doing. But my engine light came on after driving it for about 40 minutes. The whole time I was driving though, the temperature gauge read cold. It does sound different when deaccelerating. Louder, like it's working harder than it should. That and the light are the only symptoms. Please help me, I don't have a lot of money to fix another major problem!