Positive air shutoff systems—often abbreviated as PASS—are safety devices that protect diesel-powered equipment from a phenomenon known as diesel engine runaway. This occurs when a diesel engine ingests a hydrocarbon vapor and uses it as an uncontrolled fuel source. Once runaway begins, the only effective way to shut down the engine is by removing the air supply. A positive air shutoff valve accomplishes this by closing off the air supply entering through the intake.

When a diesel engine begins to overspeed due to ingesting combustible vapors, traditional shutdown procedures fail because the engine is no longer running on fuel from the tank. A positive air shutoff valve installed in the engine's air intake path closes completely when activated, blocking airflow and stopping combustion. Without oxygen, the engine halts regardless of any external fuel source.

Positive air shutoff systems can be activated in two ways. Automatically activated systems recognize when a diesel engine begins to overspeed and activate automatically, requiring no action from the operator. Manual systems require the operator or a bystander to activate the shutdown. In either case, when the system activates, it closes off the air supply and shuts down the engine.

In many jurisdictions, positive air shutdown systems are required for diesel equipment entering sites where flammable gases, vapors, or dust are present. Oil and gas locations such as well sites and refineries frequently require PASS as a condition of entry, and site-safety personnel will inspect equipment before granting access.