- Caution regarding the codes you get from your Volvo OBD I computer. The fact that the Check Engine light is lit and codes are stored tells you that something is wrong, but the odds that the code stored will actually indicate the true source of the problem are less than 50/50. OBD II on later model cars is better, but far from infallable.

You say that your shift light flashes at you. That is triggered by RPM. RPM is one of the inputs that controls the idle air control valve. I think that we need to figure out how an '89 measures its RPM, and check out that system. I suspect that is where your problem lies. I don't have a manual handy to try to look it up.