My grandfather sold me his 88' DeVille sedan a few years back.

It had had some throttle issues (it would open wide and not shut), so I parked it in my garage for two years and didn't touch it at all after the throttle was repaired.

In retrospect, this was probably a rather stupid move.

I found myself needing it recently, and after replacing the oil and battery, the SERVICE SOON light is still on. Would anyone know offhand just what could possibly be causing it?

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After checking the ECM, I see error codes of 22 and 55. 22 is easy enough - the throttle position sensor. That's just a matter of money. 55 is the ECM/oxygen sensor - I have NO idea about that. Someone mind clueing me in on these?

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Both codes could be ECM related. Clear codes at the climate control center and see if they set again. If they are hard codes then proceed with the tests. Disconnect TPS connector, turn ignition on, connect a voltmeter to TPS harness PIN C and ground. It will read either 5.0 volts or 0 volts. Then report back. For code 55 , disconnect oxygen sensor and note "check engine light" with engine running for less than one minute. It will be either on or off. Report back as this will narrow the problem down to O2 sensor or ECM.