Q: I bought a 2002 GMC Sierra 2500 HD two years ago at 150,000 miles. It now has 190,000 miles with no problems. What maintenance steps should I be concerned with at this point?

A: It sounds like your truck has had good maintenance and prudent use to be so trouble-free. If not done previously to your knowledge, I'd look into flushing and renewing brake, power steering and transmission fluids and engine coolant; renewing rear and front differential fluids; and inspecting engine, transmission and exhaust system mounts. These fluids are often neglected and, along with rubber parts, can't be expected to last forever. Front suspension and steering parts such as shock absorbers, ball joints, the idler arm and tie rod ends may also be getting loose or sloppy, affecting tire wear and crisp handling.

At this mileage, it's reasonable to expect some issues on the horizon. If the Sierra is equipped with the 6.0-liter gasoline engine, you may someday encounter leaking intake manifold gaskets or coolant, which could leak externally or into crankcase. A valve job and timing-chain replacement will likely be needed some time down the road; it's best if all are done together at the same time. Keep driving until any of these become an issue.

Sierras of this vintage equipped with the 6.6-liter diesel engine are known for some problems with the fuel injectors. Regular fuel filter replacement and an occasional dose of Sea Foam additive will help keep the injectors happy.

The standard-duty 4L60E transmission is known for a few issues at high mileage, including valve body wear, which causes jarring shifts between first and second gears. This activates the "check engine" light with trouble code P1870; it's moderately inexpensive to repair. Reaction sun shell failure, meaning no reverse, second or fourth gear, is another possible issue and requires transmission overhaul. Drive it until something breaks. The more robust 4L80E transmission should last as long as the rest of the truck.

I'm a fan of vehicles with high mileage for their age. Odd are these are highway miles, and they're by far the easiest on a vehicle. Loafing along mile after mile in fourth gear sure beats the constant shifting, potholes and starts and stops a city vehicle encounters. Let's see if you can hit 300,000 without too much trouble.

Q: I have a 2007 Buick LaCrosse CXL. The door locks cycle off and on at different times. Also, I may lock the car and when I come back to it the doors may be unlocked. Do you have any suggestions?

A: The best way to diagnose this is to connect a professional-grade scan tool and observe the body control module inputs, particularly those of the door lock switches and lock cylinder switches, for erratic activity. It will certainly be difficult to not to blink or nod off at the wrong time, as the fault occurs intermittently. Possible aids include wiggling keys slightly in the locks, and gently massaging the door panel switches.

If no input activity is seen but the door locks continue cycling, another ploy is to temporarily unplug the remote control door lock receiver. If the symptoms subside, this part may be faulty.

Brad Bergholdt is an automotive technology instructor at Evergreen Valley College in San Jose, Calif. Readers may send him email at under-the-hood@earthlink.net; he cannot make personal replies.