P0033

What Does Code P0033 Mean?

The diagnostic trouble code P0033 signifies an issue within the electrical control circuit for the turbocharger/supercharger bypass valve, designated as ‘A’. The “control circuit low” aspect indicates that the Engine Control Module (ECM) or Powertrain Control Module (PCM) has detected a voltage or current level in this circuit that is consistently below its calibrated operating threshold. This typically points to an open circuit, a short to ground, or a malfunction within the solenoid itself causing excessive resistance or insufficient current draw, which prevents proper actuation of the bypass valve. The turbocharger/supercharger bypass valve (often referred to as a diverter valve on gasoline engines) is a critical component in the boost management system. Its primary function is to recirculate excess boost pressure from the compressor outlet back into the compressor inlet during periods of low engine load or rapid throttle closure. This action prevents compressor surge, which is a condition where pressurized air has no place to go, leading to violent pressure fluctuations that can damage the compressor wheel and shaft. By opening, the bypass valve protects the turbocharger and helps maintain optimal throttle response by keeping the turbo spooled. When the control circuit is faulty, the valve may not operate, leading to incorrect boost pressure regulation and potentially causing over-boosting or under-boosting conditions that directly impact engine performance and longevity.

Common Symptoms

• Reduced engine power or “limp mode” activation
• Noticeable turbo lag or delayed throttle response
• Abnormal “whooshing” or “fluttering” noises from the turbocharger area, especially during gear shifts or throttle lift-off
• Check Engine Light (CEL) illumination
• Potential for excessive exhaust smoke (diesel applications)
• Engine hesitation or stumble under acceleration

What Causes the Code P0033?

• Faulty turbocharger/supercharger bypass valve solenoid/actuator
• Open or short to ground in the bypass valve control circuit wiring
• Corroded, loose, or damaged electrical connectors at the bypass valve or PCM
• Failed or malfunctioning Engine Control Module (ECM) or Powertrain Control Module (PCM) (less common)
• Vacuum leaks in the lines controlling the bypass valve (if vacuum-actuated)

How to Diagnose and Troubleshoot

Diagnosing P0033 requires methodical electrical testing and a thorough understanding of the boost control system. Follow these steps:

1. Verify Code and Gather Data: Connect an OBD-II scanner and confirm P0033 is the only active code. Review Freeze Frame Data to understand the engine operating conditions (RPM, load, throttle position, boost pressure) when the fault occurred. Check for any related boost pressure or sensor codes that might indicate a broader issue.
2. Visual Inspection:
   • Visually inspect the turbocharger/supercharger bypass valve and its electrical connector for any signs of damage, corrosion, or loose connections.
   • Trace the wiring harness leading to the bypass valve. Look for chafing, cuts, pinches, or signs of heat damage.
   • If the bypass valve is vacuum-actuated, inspect all associated vacuum lines for cracks, leaks, or disconnections. Check the integrity of the vacuum reservoir and one-way valves.
3. Electrical Circuit Testing (with DMM):
   • Power Supply Check: With the ignition ON and engine OFF, disconnect the bypass valve solenoid connector. Using a Digital Multimeter (DMM) set to DC voltage, check for battery voltage (typically 12V) at the power supply terminal of the bypass valve connector (refer to wiring diagrams for specific pinouts). If no voltage is present, troubleshoot the power supply circuit, including fuses and relays.
   • Ground Circuit Check: Check for continuity to ground on the ground terminal of the bypass valve connector. Resistance should be very low (close to 0 ohms). If no good ground is present, trace and repair the ground circuit.
   • Control Circuit Integrity (Open/Short): Reconnect the bypass valve solenoid. Backprobe the control wire at the bypass valve connector and then at the PCM connector (with PCM disconnected). Check for continuity between these two points. There should be very low resistance. Check for a short to ground or a short to voltage on this control wire.
   • Solenoid Resistance Check: Disconnect the bypass valve solenoid connector. Measure the internal resistance of the solenoid coil using the DMM. Compare this reading to manufacturer specifications (typically 10-30 ohms). An open circuit (infinite resistance) or a short circuit (very low resistance, near 0 ohms) indicates a faulty solenoid.
4. Bypass Valve Actuation Test: If available, use a bi-directional scan tool to command the bypass valve open and closed. Listen for an audible click or observe the valve’s physical movement. Monitor boost pressure PIDs during this test. If the valve does not respond or acts erratically, it indicates a mechanical or electrical fault within the valve assembly or its control.
5. Vacuum System Test (if applicable): If the bypass valve is vacuum-controlled, apply vacuum directly to the valve using a hand-held vacuum pump. The valve should operate smoothly and hold vacuum. Test the vacuum solenoid that controls this vacuum for proper operation and electrical integrity.
6. PCM Input/Output Verification: If all other tests pass, and the wiring and solenoid are confirmed good, it may be necessary to verify the PCM’s output signal to the bypass valve using an oscilloscope. This confirms the PCM is attempting to control the valve correctly. This is usually a last resort, as PCM failures are rare.

Recommended Repairs and Solutions

Based on the diagnostic findings, the following repairs are commonly indicated:

• Replace the Turbocharger/Supercharger Bypass Valve Solenoid/Actuator: This is the most common repair for P0033, especially if the internal resistance test failed, or the valve did not respond to actuation commands. Ensure the replacement part is an OEM equivalent or meets manufacturer specifications to ensure proper boost regulation.
• Repair or Replace Damaged Wiring/Connectors: If the diagnostic steps identified an open circuit, short to ground, or poor connection in the wiring harness or connector, repair or replace the affected section. Use appropriate electrical repair techniques, including soldering and heat-shrink tubing, for durable repairs.
• Address Vacuum Leaks: For vacuum-actuated bypass valves, replace any cracked, brittle, or disconnected vacuum lines. Ensure the vacuum reservoir and associated check valves are functioning correctly.
• PCM Replacement (Rare): Only consider PCM replacement after thoroughly ruling out all other possibilities, as it is an expensive and complex repair that often requires reprogramming.

Mechanic’s Tips:

After any repair, clear the DTCs from the ECM/PCM and perform a road test under various driving conditions, including periods of high and low engine load, to confirm the repair. Monitor real-time data for boost pressure and bypass valve operation to ensure the system is functioning as intended. Some vehicles may require an adaptive relearn procedure after replacing boost control components. Always consult the vehicle-specific service information for proper procedures.