Q: Check engine light came on, stayed 2-3 days, then went off.

If the check engine light is off, the engine operating temperature is fully within the normal range (not too cold and not too hot) and there are no outward signs of engine malfunction, you need do nothing at this time. Any component on a vehicle, including the thermostat, can potentially operate erratically and/or intermittently particularly as the component nears the end of its normal service life (no part lasts forever). Thermostats are very important and indeed are often considered a maintenance item. In particular, if the water pump is replaced, the engine re-built, or other major engine work is done, most knowledgeable and professional mechanics will replace the thermostat automatically. In your circumstance, you will probably have to eventually replace the thermostat but that could be tomorrow or 6 months from tomorrow. Keep a CLOSE watch on engine temperature. At the first indication of any out of range reading from what you normally see, reconsider the thermostat as failure of that part can cause overheating. If the engine is too cold (due to a stuck open thermostat) that causes problems such as oil dilution and degraded fuel efficiency so you have to be wary of that failure mode, too. You can request thermostat replacement on a mobile basis from YourMechanic and you will save quite a bit, maybe enough to make the repair affordable now. If you have further questions or concerns, do not hesitate to re-contact YourMechanic because we want you to make the most of your repair dollars and help you to get the best possible results.