I just purchased my 04 Trailblazer a week ago. There was no check engine light when I purchased the truck, and drove it a day before the light came on. Check engine light came on the 2nd day that I owned the truck, but the light went out after 15 minutes of driving the truck ( two or three start up cycles). I have ran the truck for another 6 days with no check engine light but when I left work it came back on. Drove another 15 minutes ( two start up cycles) and the light went off. Put autozones code reader on both days the light came on and both times it came back PH0016 ( Crank/Cam shaft sensor). I have read that there is a Camshaft Position Actuator Solenoid. I have also read of people running a engine cleaner then changing the oil. I was going to do a oil change in the next day or two with full synthetic oil. Does anyone have any input. Thanks for the advice.