Conditions for Setting the DTC

WARNING: This page is about a different car, the 2003 GMC Sonoma, 2003 GMC Jimmy, 2003 Chevrolet S10 Pickup, and 2003 Chevrolet Blazer. However, it is still accessible from the selected car via links, so may be relevant.

During the ignition ON test, the PCM detects a PWM signal is out of range.
During the RUN test, the PCM detects a PWM signal less than 5 percent for more than 6 seconds.

Fig 1: DTC P1638 Specification Chart

Courtesy of GENERAL MOTORS CORP.

Fig 2: DTC P1638 Specification Chart

Courtesy of GENERAL MOTORS CORP.