P0039 OBD-II Trouble Code: Turbo Charger or Supercharger Bypass Valve Control Circuit Range and Performance

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P0039 is a diagnostic trouble code (DTC) for "Turbo Charger or Supercharger Bypass Valve Control Circuit Range and Performance". This can happen for multiple reasons and a mechanic needs to diagnose the specific cause for this code to be triggered in your situation. Our certified mobile mechanics can come to your home or office to perform the Check Engine Light diagnostic. Once we are able to diagnose the problem, you will be provided with an upfront quote for the recommended fix and receive $20.00 off as a credit towards the repair. All our repairs are backed by our 12-month / 12,000-mile warranty.

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P0029 Trouble Code definition

P0029 is the code for Turbo Charger or Supercharger Bypass Valve Control Circuit Range and Performance

What the P0029 code means

Please note that each manufacturer has a slightly different definition for this code as it relates to their particular system. The code generally means there is a problem with the bypass system for the turbo charger or supercharger to relieve pressure either completely or partially depending on how the system is designed. For this P0029 code, the boost pressure is out of range on boost pressure, which is normally 9 psi to 14 psi, for proper engine operation and power.

Symptoms may include poor engine acceleration, whining or rattling noises from the turbocharger or turbo pipes, abnormal smoke from the exhaust, spark plug fouling, excessive engine or transmission temperature, hissing noises from the turbocharger wastegate and/or hoses, and an overall lack of engine performance.

Additional codes may include other boost codes, engine misfire codes, or knock sensor codes. Cylinder detonation is another possibility if pressures or engine temperatures are too high. The boost pressure gauge (if so equipped) will exhibit abnormal levels of boost pressure.

What causes the P0029 code?

Several things can cause the Check Engine Light to come on and an OBD-II scanner to indicate this code.

  • Sensor input voltage signals to the Engine Control Module (ECM) that indicate a boost pressure is out of the normal range specified by the manufacturer will cause a code to be stored and the Check Engine Light will come on. The ECU recognizes this incorrect data as an inability to effectively control boost levels.

  • If the boost is more than 14 psi, it can indicate that pressure is not being sent through the bypass valve when it should to relieve pressure.

  • If the boost pressure is too low, then there may be a loose or cracked hose, loose or missing hose clamp, leaking intercooler, or an excessive vacuum leak.

  • The bypass valve is either stuck shut or not functioning properly.

What are some of the symptoms of P0029 code?

The symptom you will most commonly have when there are problems with the turbo or supercharger bypass valve control circuit is low or no boost, which would lead to a loss of power during acceleration. The engine may buck and jerk if the control circuit is erratic causing the boost to be erratic also. The Check Engine Light would then come on once the ECM has seen the boost pressure out of the normal range.

How does a mechanic diagnose the P0029 code?

A mechanic must verify the code with an OBD-II scanner, then he should reset the code and road test the vehicle to determine if the Check Engine Light and the P0029 code returns. Depending on the manufacturer, a boost pressure gauge test will need to be done to determine if the boost pressure is within the manufacturer's recommended range of operation.

If the pressure is too high, an inspection of the bypass valve and a pressure test must be done. Some bypass valves are spring operated, others (especially Asian models) are diaphragm operated with an electric sensor and control system.

If the pressure is too low, there is probably a leak in the intake system that needs to be carefully inspected.

Common mistakes when diagnosing the P0029 code.

The most common mistakes on this code are related to assuming basic components are operating normally and not following test procedure. Often a pressure sensor on the intake may be faulty and indicates high pressure sent to the ECM. Hose and clamp failures are also a common cause of erratic or low pressure. It is also possible the pressure sensor is faulty. Do not replace the bypass valve, turbocharger or supercharger unless it is clearly at fault.

How serious is the P0029 code?

This code requires immediate attention and should be diagnosed as soon as possible. The vehicle will still run but serious damage can occur to the engine if the problem is not repaired, if the pressure is too high engine damage due to a too lean fuel mixture may occur.

Often times, if the Check Engine Light came on immediately at start up, the OBD-II system can be reset and the vehicle will operate normally.

What repairs can fix the P0029 code?

The most common repairs to address the P0029 code are as follows:

  1. Have a certified technician verify the code with a scanner, reset the fault codes, and perform a road test.

  2. If the P0029 code returns, then follow the test procedure.

  3. Do a pressure test on the intake system between the turbocharger and intake throttle valve. This will verify if the pressure is too low or high as specified by the manufacturer. It will also indicate erratic pressure readings fluctuating low to high.

  4. If the pressure is too high, check the bypass valve to determine if it is slightly or completely closed allowing boost pressure to build excessively. This may require changing the bypass valve or the actuator with a diaphragm.

  5. If the boost pressure is too low, then it is likely that there is a leak in the intake due to a cracked, damaged, or loose hose, a vacuum leak, or a gasket failure. The bypass valve may be stuck open if there is no boost.

  6. The system may have a failed turbo charger or supercharger, but that is usually associated with noise coming out of the unit like a failed bearing or impeller being worn.

  7. If the pressure is in the normal range as indicated by the manufacturer, the pressure sensor may be faulty.

Having experienced this problem several times, the most common problem seen is a crack in the lower part of an intake hose that is not normally visible. Why did it crack there? Dirt and fluids tend to collect in the lowest part of the intake system especially if near a very hot engine component like a radiator. This combination makes the hose brittle and it may have a 1-2" crack that does not open much at idle, but under load, will open like a bladder valve allowing pressure to escape. A loose hose clamp can also cause a similar problem as the rubber hose holds normally at idle or low speeds, but a loose hose clamp will allow it to leak at the connection and lose boost pressure under load.

Additional comments for consideration regarding the P0029 code.

Many vehicles with 100,000 miles on it have momentary sensor problems that usually occur during start up or prolonged stress situations on the drive train. If the Check Engine Light comes on and the vehicle seems to be operating normally, the OBD-II system can be reset using the scanner and the problem may not reoccur. This is why it is important to verify the fault and reset it before doing any repairs.

Need help with a P0029 code?

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