P1691

What Does Code P1691 Mean?

DTC P1691 signifies an electrical malfunction within the circuit controlling the Turbocharger Pressure Control Solenoid. This code is generated by the Engine Control Module (ECM), also known as the Powertrain Control Module (PCM), when it detects that the electrical parameters of the turbocharger boost control solenoid circuit are outside the manufacturer’s specified range. The ECM actively monitors the voltage, current, or resistance of this circuit to ensure proper functionality of the boost pressure regulation system. A P1691 typically indicates an open circuit, a short to ground, a short to voltage, or an out-of-range resistance reading within the solenoid or its wiring. This prevents the ECM from accurately controlling the turbocharger’s wastegate or variable geometry mechanism, leading to either an overboost or underboost condition, and significantly impacting engine performance and efficiency.

Common Symptoms

• Illumination of the Check Engine Light (MIL): This is almost always the first and most obvious symptom.
• Reduced Engine Power (Limp Mode): The ECM may limit engine power to prevent damage from improper boost pressure, often resulting in a noticeable lack of acceleration.
• Poor Fuel Economy: Inefficient engine operation due to incorrect boost pressure can lead to increased fuel consumption.
• Engine Hesitation or Surging: Inconsistent boost control can cause the engine to hesitate under acceleration or surge erratically.
• Unusual Turbocharger Noises: A wastegate stuck open or closed due to a non-functioning solenoid might produce an audible whistling, hissing, or rattling sound.
• Engine Overheating: While less direct, sustained high engine loads with improper boost can contribute to increased engine temperatures.

What Causes the Code P1691?

• Faulty Turbocharger Boost Control Solenoid: The most common cause, involving an internal electrical failure (e.g., an open or shorted coil winding) within the solenoid itself.
• Damaged Wiring Harness:
  • Open circuit in the power supply or ground wire to the solenoid.
  • Short to ground or short to voltage in the solenoid’s control circuit.
  • Frayed, chafed, or severed wiring due to vibration, heat, or rodent damage.
• Corroded or Loose Electrical Connections: Poor contact at the solenoid connector or within the ECM/PCM connector can disrupt circuit integrity.
• Blown Fuse: A fuse protecting the turbo pressure control solenoid circuit, or a shared circuit, may have blown due to an overcurrent condition.
• Failed Engine Control Module (ECM/PCM): Although rare, an internal driver circuit failure within the ECM responsible for controlling the solenoid can set this code.

How to Diagnose and Troubleshoot

Diagnosing P1691 requires a systematic approach, combining visual inspection with precise electrical testing:

1. Initial Scan Tool Analysis:
   • Connect an OBD-II scan tool to confirm P1691 and check for any related or accompanying Diagnostic Trouble Codes (DTCs), such as P0234 (Overboost) or P0299 (Underboost).
   • Review freeze frame data, which captures engine conditions (RPM, load, throttle position) at the time the code was set. This can provide valuable clues for replication or understanding operating conditions.
2. Thorough Visual Inspection:
   • Locate the turbocharger pressure control solenoid. Inspect its electrical connector for signs of corrosion, damage, bent pins, or loose connections.
   • Carefully trace the wiring harness from the solenoid back to the ECM/PCM. Look for any visible damage, chafing, cuts, or signs of heat exposure that could compromise wire integrity.
   • Ensure all vacuum or pressure lines (if applicable to the specific solenoid design) are securely connected and free from cracks or leaks, though the code is primarily electrical.
3. Electrical System Testing with a Digital Multimeter (DMM):
   • Solenoid Resistance Test: Disconnect the solenoid’s electrical connector. Using a DMM set to ohms, measure the internal resistance across the solenoid’s terminals. Compare this reading to the manufacturer’s specifications (typically found in a service manual, often ranging from 10 to 50 ohms). An “OL” (open circuit) reading or a resistance significantly outside the specified range indicates a faulty solenoid.
   • Power Supply Test: With the ignition in the “ON” position (engine off), measure the voltage at the power supply wire of the disconnected solenoid connector. You should typically read battery voltage (approx. 12V). If no voltage is present, check for a blown fuse or an open circuit in the power supply wiring.
   • Control Circuit Test: This circuit is typically controlled by the ECM/PCM.
   • Continuity to Ground/ECM: With the ignition OFF and the ECM/PCM connector disconnected, test the continuity of the control wire from the solenoid connector to the appropriate pin at the ECM/PCM connector. There should be good continuity (low resistance).
   • Short to Ground/Voltage: With the solenoid connector disconnected, check for a short to ground or battery voltage on the control wire using the DMM.
   • ECM Driver Function (Advanced): For ECM-controlled ground circuits, if the wiring is confirmed good, an oscilloscope can be used to observe the pulsed ground signal from the ECM while commanding the solenoid with a bi-directional scan tool (if available) or during engine operation. A missing or incorrect signal points towards an ECM issue.
   • Wiring Harness Continuity: Disconnect both the solenoid and ECM/PCM connectors. Test for continuity of each wire in the solenoid circuit from end to end. Also, check each wire for shorts to adjacent wires and to vehicle ground.
4. Bi-directional Control Test (If Available):
   • Utilize a professional scan tool with bi-directional control capabilities to directly command the turbo pressure control solenoid ON and OFF. While commanding, monitor related live data streams, such as boost pressure readings, to observe the system’s response. This can help confirm solenoid functionality and ECM control.

Recommended Repairs and Solutions

Based on the diagnostic findings, the following repairs are typically recommended:

• Replace the Turbocharger Boost Control Solenoid: If electrical tests confirm an internal fault within the solenoid (open, shorted, or out-of-spec resistance), replacement is the definitive solution. Always ensure the replacement part is an OEM equivalent or meets OEM specifications for proper function and longevity.
• Repair or Replace Damaged Wiring: If the wiring harness is found to be open, shorted, or severely damaged, repair the affected section using proper automotive wiring repair techniques (soldering, heat shrink tubing, appropriate gauge wire) or replace the entire harness segment if damage is extensive. Ensure proper routing to prevent future damage from heat, abrasion, or vibration.
• Clean or Replace Corroded Connectors: If corrosion is present at the solenoid or ECM/PCM connectors, carefully clean the terminals using electrical contact cleaner and a small brush. Apply dielectric grease upon reassembly to prevent future corrosion. If pins are severely bent or corroded beyond repair, the connector may need to be replaced.
• Replace Blown Fuses: If a fuse was identified as the culprit, replace it with a fuse of the correct amperage rating. Investigate the underlying cause of the fuse blowing (e.g., a short in the circuit) to prevent immediate recurrence.
• ECM/PCM Replacement (Last Resort): If all other components (solenoid, wiring, fuses) have been thoroughly tested and confirmed good, and specific tests indicate an internal ECM driver fault, then ECM replacement may be necessary. This is a complex and often expensive repair that usually requires programming and coding the new module to the vehicle. Ensure absolutely all other possibilities have been exhausted before considering ECM replacement.

Important Mechanics’ Tips:

• Always disconnect the battery’s negative terminal before performing any electrical work to prevent accidental shorts or damage to sensitive electronic components.
• Refer to the vehicle-specific service manual for exact wiring diagrams, component locations, pinouts, and manufacturer-specified resistance values for the turbo control solenoid.
• After completing any repair, clear the DTCs from the ECM/PCM and perform a thorough test drive under various driving conditions to ensure the code does not return and that the turbocharger boost control system is operating correctly. Monitor live data for desired vs. actual boost pressure readings.
• Be mindful that some vehicles integrate the boost control solenoid into the turbocharger assembly, making replacement more involved.