Disconnect vacuum hose between EVAP 2-way valve and fuel tank pressure sensor at EVAP 2-way valve end. Connect vacuum pump to open end of vacuum hose. Turn ignition on. Measure voltage between PCM 22-pin connector terminals D15 (Green/Yellow wire) and D22 (Green/Black wire). Apply vacuum one stroke at a time. If voltage smoothly drops from 2.5 to 1.5 volts and holds, problem is intermittent. Check for poor connections or loose hoses. If voltage does not smoothly drop from 2.5 to 1.5 volts, replace fuel tank pressure sensor.
CAUTION:
Stop applying vacuum when voltage drops to about 1.5 volts. Damage to fuel tank pressure sensor may occur.