The space shuttle disaster reminds us again, painfully, of how much modern life depends on machines.

We rely on increasingly complex mechanical and electrical devices not only to fly safely, we hope, through sky and space, but also to support most earthbound routines and rituals.

In the cities we inhabit and buildings we occupy, machines have become indispensable. They keep us comfortable, dispel darkness, entertain us, transport us, help us communicate with one another and ease countless daily tasks.

It doesn't take long for machines to be taken for granted after they are put into regular use. Many machines remain in the background. But when they fail, that failure can range from inconvenient to catastrophic, preoccupying us and eclipsing all other concerns. And failures of complicated machines can prove frustratingly difficult to diagnose and costly to remedy.

Consider all the machines and electromechanical systems on which you depend to make your home or workplace habitable.

Essential to modern civilization, but totally out of sight and mind, are electrical power plant generators, switching devices and transformers that supply the regional distribution grids that deliver power through your meter to your electric panel. From your panel with its circuit breakers, power flows through your electrical circuits to your machines. If a critical elements fail anywhere between power plant generators and your circuits, you are without light, heat and air conditioning.

Clocks, microwave ovens, refrigerators, hair dryers, washing machines, audio systems, television sets, cordless telephones and garage door openers all become useless.

Without power, elevators and escalators stop. Photocopiers, fax machines and computers shut down. If a power outage is regional, urban rail transit systems halt and street lights stop working.

But even with uninterrupted electrical power, think of all the engineered components, woven together within each of these appliances and machines, that have to move, switch or rotate, perhaps billions of times. You need spend only a few minutes watching a service technician pull apart your malfunctioning photocopier to realize what is involved optically, mechanically and electrically in making a simple copy.

Still, can anything match a power blackout for reminding us of our machine-dependent existence?

Well, maybe a toilet that won't flush or a car that won't start.

Despite knowing the principles of internal combustion engines, I am still amazed that such engines work, given all the precisely milled, interconnected, rapidly moving parts and split-second synchronization. And never mind a car's many other subsystems, from power steering to anti-lock brakes to airbags. Although automotive malfunctions are common, modern automobiles are nevertheless incredibly reliable when you consider how they are produced, used and abused.

The same is true of commercial aircraft, thanks to rigorous maintenance requirements. Less certain is the reliability of trains, which seem to have chronic problems. Even cruise ships have been plagued lately by their ventilation systems.

When machines break down, finding and fixing the cause can be straightforward, aided by modular assembly and built-in sensors that allow technicians to quickly pinpoint malfunctions and replace defective parts. But sometimes the cause of a mechanical failure is elusive, even when machines are relatively simple.

Years ago my firm designed a low-income, multifamily housing project with a tried-and-true hydronic heating system. Pumps circulated water heated in boilers through loops of copper pipe supplying baseboard heaters in each apartment. During the winter, several apartments were chronically cold. The mechanical subcontractor, the mechanical engineer who had designed the system and other consulting engineers studied the drawings and inspected the system in futile attempts to find the problem.

Larger pumps were installed, but nothing seemed to improve performance.

After two years and much discomfort, another engineer finally figured it out. Air bubbles in piping loops were creating "vapor lock" and preventing hot water from flowing through some of the secondary circuits supplying the baseboards. The problem was solved by installing air bleeding valves that should have been specified and installed at the outset. It was an engineering oversight, but somehow not an obvious one.

No matter how well we think we are designing and making our machines, malfunctions will occur. Yet most machines, even as their sophistication and numbers increase, are very reliable if we use and maintain them properly.

Unfortunately, improper use and maintenance, rather than bad design and construction, are too often the greatest threats to that reliability.

Roger K. Lewis is a practicing architect and a professor of architecture at the University of Maryland.