Q: Check engine light on and got the codes P0700 and P0731

Diagnostic trouble code P0700 and code P0731 indicates that there is a fault in your car’s "automatic transmission control system". Although there could be a faulty "part," (such as a shift solenoid or speed sensor), there are some preliminary checks that you can perform just in case it is resolvable (or at least, "diagnosable") without the assistance of a mechanic.

For example, on Neon’s, there are reports of the heat exchanger in the radiator (the heat exchanger cools circulating transmission fluid) leaking and then letting the engine coolant mix with transmission fluid.

So you want to check and be sure that the transmission fluid is not "contaminated" with coolant because that sort of contamination could "potentially" cause these codes to set. It is also possible for a low (or even too high) transmission fluid level to cause hydraulic malfunctions that in turn will set the codes you are seeing.

So those are two things you can check right away. If those initial checks of fluid quantity and quality do not reveal anything, then you should have an expert evaluate the following mechanical and electrical components in the transmission: the transmission control module, transmission valve body, shift solenoids (typically in valve body); and a check for a possible open or short in the wiring TCM wiring harness. Typically, the problem will be found within the foregoing components without having to remove the transmission. But if not, there is an internal mechanical problem that will require further evaluation. If you would like to get a precise diagnosis, a certified professional from YourMechanic can come to your car’s location to inspect the Check Engine Light and follow through with the necessary repairs.