DTC P1639: 5-Volt Reference Circuit "B" Or No. 2 (Grand PRIX, Impala, LESABRE, Monte CARLO & Park Avenue): Description
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Powertrain Control Module (PCM) uses 5-volt reference circuit as a sensor feed for A/C refrigerant pressure sensor. PCM monitors voltage on 5-volt reference circuit. DTC sets when voltage is out of tolerance.