My 1989 245 (130k miles) suddenly started running roughly at idle and off idle, and the "check engine" light came on. It runs fine at cruise.

After checking the "check engine" diagnostic codes, I replaced the oxygen sensor, to no avail. Per the suggestions of others on this forum, I cleaned the TPS and its connectors, and the AMM connectors, and all the grounds I could find.

This morning I took it to a friend's SAAB shop, and he measured the emissions and said it was running much too lean. Pulling the wire off the new oxygen sensor made it run leaner, so at least we know the new O2 sensor is working. He had me check the voltage at the injectors, and it was good: 13.9 volts. He checked for an intake leak by spraying the manifold-to-cylinder-head gasket, and there is no apparent leak. He says the ignition system is fine.

My friend then disconnected the cylinder head temperature sensor (the one for the fuel injection system, under intake port 3.) According to him, this should have made the computer think the engine was cold and made the mixture very rich, but it made no difference: the engine still ran far too lean.

My friend thinks the computer has gone bad, but I don't want to replace it without confirmation. Is there any way to test it? Could something else be causing the problem?