Hi there,

The ECU will make small adjustments to fuel trim over time and varied driving - figure about 10 miles worth. As it encounters problems in this adjustment process it will set some codes for diagnostics that don't light the CEL. When it can no longer compensate for an issue, such as a leaking injector, manifold gasket, exhaust leak, or fouled plug, just to name a few examples, it will turn on the CEL and run a rich fuel mix to "get you to the garage".

Should that problem it couldn't compensate for be intermittent, or clear up independently, the ECU will not reset on its own. Yes, you can clear the codes by using the OBD button sequence, but the reset only occurs when you remove power from the computer. In your 92, that can be done by lifting fuse 6 for a moment - a more graceful act than disconnecting the battery as it leaves your radio alone.

So now, after having reset the computer, you may be enjoying good fuel mileage once again and will continue to until the original fault that set the CEL returns. In some cases, I've done a reset, and have yet to see whatever set the light come back. Since it seems to be an intermittent electrical "gremlin", flaky ground connections are often cited as suspects. In particular, those at the fuel rail mounting screws even had Volvo writing service bulletins.

Maybe there won't be a next time, but if the light comes on again, get all the codes from the OBD box, and then do a complete reset by lifting fuse 6 for a moment. Others reading this: 89-90 the fuse is the 25A blade fuse under the hood. 91-early 93 - fuse 6 inside. Late 93, back under the hood again at the battery terminal.
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Art Benstein near Baltimore

Coca-Cola was originally green.