the only cel i have had was from bad o2 sensors but if you are wanting to do some fairley inexpensive preventative maintaince you can change all the fluids, grease your drivetrain nipples, replace the pcv valve, idle control valve, throttle position sensor, and fuel pressure regulator - then a little more expensive is mass air flow sensor and egr valve -again, both easy to do yourself - the fluids and greasing of nipples made a huge difference with the way my vx drove - well, that and buying a set of decent new tires