Check Engine light SOLID

[kplant]

I have a '98 explorer sport with about 142k miles on it. new tranny last july was put in. my check engine light went on today and is solid. The manual says I could have a bad tank of fuel or water in the gas. It still runs fine, nothing's wrong with it. Could it just be bad gas, or is there more to it?

 

[1998Exp]

There are about 1000 reasons for the 'check engine light' to illuminate. Take it to Autozone or similar auto parts store, and have the codes read.
If you are very lucky, it's just a loose gas cap (which triggers the 'gross evap emissions' code). Tighten it, and disconnect the battery for 15 mins or so (which erases the codes and resets the CEL).

 

[unpredictable1]

I have a '98 explorer sport with about 142k miles on it. new tranny last july was put in. my check engine light went on today and is solid. The manual says I could have a bad tank of fuel or water in the gas. It still runs fine, nothing's wrong with it. Could it just be bad gas, or is there more to it?

When removing and installing a transmission in these rigs, the exhaust has to be removed.

I'm willing to bet there's an O2 sensor not plugged back in when the exhaust was reinstalled.