Code 53, Ground Circuit (Ca Only) Circuit Open

On California vehicles, an additional ground is supplied to the ECM. The ECM supplies 12 volts to circuit No. 450 through a buffer. The ECM looks for the voltage to be low. This code will set if voltage is high.

NOTE: Test numbers refer to test numbers on diagnostic chart.

This verifies Code 53 is not intermittent.
With test light to battery positive, probing terminal "Y4" checks the wire to engine block for an open circuit.

Fig 1: Code 53, Circuit Wiring Diagram & Flow Chart

Courtesy of GENERAL MOTORS CORP.