Gauging accuracy: How accurate is that gauge?
Whether it's the bathroom scale or the car's gas gauge, we tend to believe what we see. Like it or not.

And more than 99 percent of the time, the gauges in our cars and trucks are accurate. They should be. Automotive gauges must meet strict standards set by the federal government, standards that can take years to develop.

However parts wear out. Electronic module programming is flawed. We mis-read an indicator. In other words, things can go wrong.

Naperville, Ill., resident Mark Bergren tells of a family van he "inherited" from his wife, Sue, when the heating function for the driver's seat no longer worked.

"I began using it and the van immediately developed additional problems," Bergren says. The battery mysteriously drained, the speedometer stopped working, and the gas gauge had issues.

"It would only decrement half way to empty, no matter how much fuel was used," Bergren says of the fuel gauge. The frustrated commuter bought his own jump-start "brick," which he carried with him and used frequently.

Long story short, the battery drain problem suddenly reversed and the speedometer started working again. The fuel gauge never recovered.

Some gauge problems used to be mechanical, such as a speedometer; now they are all electrical or electronic and share data with the vehicle's computers, says Allen Day of Washtenaw Community College's automotive tech program in Ann Arbor, Mich.

In the past, Day says, each gauge was connected to its component. Now all information, including vehicle speed and fuel level, goes through an electronic bus. Each module or sensor is individually programmed by a person.

"Programming defects are possible, but not common," Day says. "A communication error can cause gauge malfunctions as well as a module or sensor failure."

The gauge cluster is a module in its own right, he explains. "It talks on the 'bus' and uses the data for its display," says Day.

Generally speaking, since 1996, auto technicians have used scan tools to chase down problems, Day says.

Typically consumers aren't aware of the extensive role of electronics in vehicle operation and management, he said. And repairs can be expensive.

"Most consumers, when they realize the complexities involved, assume that the dealer is the only choice for these types of repairs," he says. Sometimes they are right. Though most independent shops have scan tools, staff may lack the ability to program the new modules when that is needed.

Newer requirements

Since 2007, tire pressure sensors have been required in new vehicles sold in the U.S. Perhaps not quite as unnerving as an errant gas gauge, a light indicating low tire pressure is worthy of immediate attention.

The automakers test and re-test this kind of sensor well before a vehicle goes to market and continue testing it once regular production begins.

New car and truck models have to be validated by the automaker and test results maintained on file as required by the federal government, says Douglas Kidd, General Motors lead engineer for tire pressure monitoring.

While the automaker answers to the consumer for significant problems with a gauge, the manufacturer works closely with its suppliers. And there are many of them behind the instrument panel that the factory fits into the assembled vehicles.