check engine

We would need a little more info, to be able to try to help you.

What year truck?
Have you had the codes pulled?

42,000 is very low mileage for C or K truck. What model truck is this?

If you go to a local Autozone, PartsAmerica/Advance, VIP Auto... they will be able to help you pull the codes.

Could be something as simple as an O2 sensor. Any issues with the engine, transmission lately?

'Check Engine' light and 'Service Engine Soon' light are the same thing. It is not a maintanance thing. The computer is sensing that there is something wrong. You need to get an OBD (On-Board Diagnostic) scanner to see what the problem is. For 96 and UP it is OBDII for 95 and lower it's OBDI.

hey thanks guys i'm in the wrong part of this site ....it's a '04 colly crew I-5 auto 4x4 the code is p0506 auto zone pulled it and gave me a read out it was about the idle was too low.checked the maf and cleaned it,was told the air filter oil could be a problem. i'm only getting 16 to16.9 mpg. my mods are stock k&n,magnaflow side exaust,mobil 1 oil and a bed cover. thanks doug