P1257

DTC P1257 Boost Control Problem Diagnostic Chart:

Boost Control Solenoid Wiring Schematic:

CIRCUIT DESCRIPTION
Under most conditions, the Powertrain Control Module (PCM) commands the boost control solenoid to operate at a 100% duty cycle "ON" to allow full intake boost pressure upon demand. However, if reverse gear is selected, the PCM detects rapid deceleration, or engine load is extremely high, reduced boost pressure is desired. Under these conditions, the PCM commands the boost control solenoid to operate at a 0% duty cycle "OFF", which opens the bypass valve to reduce boost pressure by recirculating it back through the supercharger inlet.


DIAGNOSTIC TROUBLE CODE (DTC) P1257 WILL SET WHEN
^ Intake air temperature greater than -18°C (0°F).
^ Boost duty cycle less than 100%.
^ Engine torque exceeds a calibrated maximum torque value. This value varies depending on commanded gear and engine RPM.
^ Above conditions for at least 6 seconds.



ACTION TAKEN (PCM WILL DEFAULT TO)
If the PCM detects a malfunction in the boost control system that causes engine torque to be too high (bypass valve not opening), it will illuminate the Malfunction Indicator Lamp (MIL) and set DTC P1257.

DTC CHART TEST DESCRIPTION
Number(s) below refer to circled number(s) on the diagnostic chart.
1. CKT 1724 shorted to ground will keep the bypass valve closed when the PCM is commanding it open. possibly causing an overboost condition during high engine load situations.
2. No inlet vacuum to the bypass valve actuator may cause the bypass valve to remain closed during deceleration. This condition may be perceived as a sail-on, possibly accompanied by a rough idle.
3. Checks for a sticking boost control solenoid.

DIAGNOSTIC AIDS
An intermittent may be caused by rubbed through wire insulation.

Check for:
^ CKT 1724 shorted to ground.
^ Intermittent test. - Disconnect PCM blue connector and install a Digital Voltmeter (DVM) to monitor voltage between PCM harness terminal "BE10" and ground. With the key "ON," observe voltage while manipulating related connectors and wiring harness. If the failure is induced, the voltage display will change.