When diagnosing hot-wire air flow meters (A/F meters) on Subarus and similarly equipped Asian vehicles, technicians may find that, while the meter passes all recommended test procedures, the engine exhibits driveability conditions associated with lean air/fuel mixture ratios. In these cases, it's best to form a diagnostic strategy based on the relatively simple operating principles of the modern hot-wire A/F meter.

Unless otherwise noted, we'll use diagnostic charts, trouble codes and component configurations that apply to a 1990 Subaru Loyale that is equipped with a 1.8L, throttle body fuel-injected engine and manual transmission. In most cases, these general diagnostic procedures can be used to troubleshoot suspected air flow meter faults on other nameplates equipped with hot-wire air flow sensors.

In order to diagnose fuel-related problems caused by faulty A/F meters, it's very important to understand how the meter itself operates. The operating principles of hot-wire mass A/F meters are, on the surface, very simple. In the hot-wire system, an electrical current passes through a wire suspended in the engine's intake air stream. As the engine runs, the wire is cooled by air flowing into the engine. As the wire cools, electrical resistance is reduced, which increases current flow through the wire. The A/F meter actually measures the increased current flowing through the wire and converts this measurement into a voltage...