Geez, over two hours and no replies here.
I know some of these guys can do better than I can.
But I'll tell you what I can.

The '89 (we have 2 of 'em) has "OBD" - on board diagnostics. In other words the computer is supposed to be able to give you helpful information when it detects a problem. Unfortunately it usually only says that the air-fuel mixture is either too lean or too rich (it can't tell which!) - which is not very helpful.

You might throw in a bottle of 'dry-gas', that costs about a dollar. Then fill the tank with high-test or mid-grade and take it for a good drive, as long as it's running well. After ten minutes or so clear the computer as in #1 below. See how it goes.

1)
You can "clear" the trouble signal by disconnecting the computer's power for 20 seconds or so and then hooking it back up. Ignition MUST be OFF so remove the key to be sure. At rear of battery, on the inner fender wall, is a rail with 3 small electrical items, the last one is a white fuse holder. Remove the fuse (pull). Reinsert after 10-20 seconds. Now start the car. If there is no real problem the light won't come back. If the problem is still there, the light will come back - though likely only after driving for a few minutes.

The rest of this is more technical but within the ability of a do-it-yourselfer.

2) Our worst persistent cause for check-engine lights was transmission fluid and engine oil leaks where the fluid ended up landing on the oxygen sensor, located in the exhaust pipe at the catalytic converter. Oil goop on the sensor's wire leads disabled the sensor; apparently it "breathes" where the wires pass through the sensor body and the goop plugged up the breathing passages. So your mechanic should have a look there. Easy to do. If that's the problem you need to fix the leak, not just get a new sensor.

3) The right way to read and clear the check-engine light is in the 700-900 series FAQ on this web site. From nearly any page, click "Select Link" at the top right. Select 700-900 FAQ, scroll down to the index, then select Engine and OBD Diagnostic Codes. Or go directly to
http://www.brickboard.com/FAQ/700-900/EngineOBDCodes.htm#LH24FuelInjectionFault
The '89 has the LH2.4 system.

Good luck!!
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Sven: '89 245, IPD sways, electric rad. fan conversion, e-codes, 28+ mpg - auto tranny. 850 mi/week commute. '89 245 #2 (wifemobile). '90 244 (spare, runs).