I had the vacuum line at the other side of the "y" break and it tripped the CEL and gave a code 8 ( Altitude Pressure signal from EZL Ignition Control Unit ) and 11 ( Air Injection system ).

Fastlane shows the check valve for $14 and some change.

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1993 190E 2.6 135k
1989 Ford Thunderbird SC 5 Spd 79K