Code Definition

P1109 — Secondary Port Throttle System

System: Powertrain  |  Type: Manufacturer-Specific  |  Make: GM

About P1109

P1109 is a manufacturer-specific powertrain diagnostic trouble code recorded for GM vehicles. It belongs to the P-prefix family of codes, which covers engine, transmission, emissions, or fuel-system faults.

Scanning & clearing: A generic OBD-II scan tool will usually read and clear powertrain (P) codes. For live data, freeze-frame capture, and bi-directional tests you may need a more capable scan tool that supports the vehicle's ECM protocol.

P1109 is a manufacturer-specific code. The same code string on a different make may have a different meaning. Always confirm the code's definition against the vehicle's service information for the specific make and model under test.

Code Information

The Secondary Port Throttle (SPT) system uses a second set of throttle plates in the intake ports that the Powertrain Control Module (PCM) holds closed at low engine speed to increase air velocity and swirl for better low end torque and emissions, then opens at higher speed for maximum airflow. The plates are usually moved by a vacuum actuator controlled by a PCM driven solenoid, with a position switch or sensor confirming the commanded state. DTC P1109 sets when the PCM detects that the SPT system is not responding correctly to its commands, such as the valves failing to move, sticking, or th...

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