Q: The fuel gauge on my 2005 TrailBlazer is not working properly. I fill the tank, but the "low fuel" light comes on and the gauge reads empty for the first 20 miles. Then the light goes out, the gauge reads empty to one-quarter and "low fuel" chimes whenever it wants to. What's going on?

A: Instrumentation in modern motor vehicles isn't quite as simple as it used to be. Before computerized systems, most instruments operated by providing battery voltage though the instrument to the sending unit which grounded the circuit, varying the instrument reading. Today, the sending unit supplies a signal to the powertrain control module (PCM), which then communicates to the instrument panel cluster, which then displays the information. A scan tool can troubleshoot the system by identifying any related fault codes. Because of the consistency of the inaccurate fuel level, I'd look for an issue with the sending unit.

Q: I own a 2003 Oldsmobile Bravada with 51,000 miles that has given me excellent service except for one very frustrating problem: ignition failure. The ignition switch has been replaced three times in seven years. The dealer tells me there are no fault codes, and they do not know what's causing the failure.

A: My Alldata automotive database pulled up GM bulletin 04-06-03-002, dated February 2004, that suggests checking and reindexing the ignition switch in the steering column. Apparently, if the switch is not indexed properly, certain circuits can either remain on after turning the key off or fail to connect when the key is turned on. The positioning tab on the lock cylinder must align with and lock into the tab slot on the lock module.

Q: We have a 2007 GMC Yukon. Sometimes it acts as if the battery is dead and will not start, and jump-starting doesn't help. Several different dealers haven't found the trouble. The vehicle only has 30,000 miles on it, so we don't want to trade it off.

A: The fact that you can't jump-start it with a booster battery is a good clue. GM bulletin 07-08-49-018, dated September 2007, identified the possibility of the parking brake assembly chafing or cutting wiring in the instrument panel harness. Ask the dealer to check the five points of potential contact as outlined in the bulletin.

Paul Brand, author of "How to Repair Your Car," is an automotive troubleshooter, driving instructor and former race-car driver. Write to him at: Star Tribune, 425 Portland Ave. S., Minneapolis, Minn. 55488 or via email at paulbrand@startribune.com.