P0034 OBD-II Trouble Code: Turbocharger or Supercharger Bypass Valve Control Circuit Low

P0034 trouble code definition

P0035 is the code for Turbocharger or Supercharger Bypass Valve Control Circuit Low

What the P0034 code means

Note that each manufacturer has a slightly different definition for this code as it is relative to their particular system. This usually means there is a problem with the bypass system for the turbocharger or supercharger to relieve intake air pressure, either completely or partially, depending on how the system is designed.

For this P0035 code, the boost pressure is too low for proper engine operation and power.

What causes the P0034 code?

A variety of things can cause the Check Engine Light to come on and an OBD-II scanner to indicate this P0034 code.

• Sensor input voltage signals to the engine control module (ECM) that there is an indication of a boost level that is lower than 9 psi or as specified by the manufacturer which will cause a code to be stored and an Check Engine Light lamp to be illuminated. The ECM recognizes the data as an out of specification signal and prevents the ECM to properly control boost levels.

• If the boost is less than 9 psi, it can indicate that pressure is not enough and the system possible has an intake air leak which prevents proper operation and power.

• The turbo charger bypass valve be partially or completely open preventing any boost.

• Intake air hoses, pipes, intercooler or clamps may be defective, damaged, or loose.

What are the symptoms of the P0034 code?

• The ECM detects the bypass valve control circuit has a low current condition or open circuit.

• The ECM cannot control the bypass valve for the turbo or supercharger.

• The engine may have loss of power.

• The Check Engine Light will come on once the ECM detects the failure.

How does a mechanic diagnose the P0034 code?

• A mechanic will verify the code with an OBD-II scanner, then the mechanic should reset the code and road test the vehicle to determine if the Check Engine Light comes back on and if the P0034 code returns.

• Depending on the manufacturer, a boost pressure test will need to be determined to see if the boost pressure is within the manufacturer's recommended range.

• If the pressure is too low, a pressure test must be done to verify the pressure is low and out of manufacturer specification.

• If the pressure is too low, then an inspection of the bypass valve should be done. The bypass valves are operated differently depending on the manufacturer. Some are spring operated and others, especially Asian models, are diaphragm operated with an electric sensor and control system.

Common mistakes when diagnosing the P0034 code

The most common mistake for this code is related to assuming basic components are operating normally.

• The problem is usually related to intake leaks from cracked or damaged hoses, hose clamps loose or missing, or the bypass valve not functioning correctly to release pressure.

• If the vehicle has an intercooler, that should be checked for leaks if the pressure gauge indicates low output.

• Sometimes the boost pressure hose is off or leaking which prevents the valve from working.

• Both the spring and diaphragm type valves must be checked as they can be cleaned or repaired with service kits.

• It is also possible that the turbocharger or supercharger may be bad due to worn bearings, vanes, or is operating out of specification.

How serious is the P0034 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, especially if the pressure is too low. Engine damage, poor performance, and excessive fuel consumption 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 P0034 code?

The most common repairs to address the P0034 code are as follows:

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

If the P0034 code returns, then follow the test procedure:

• Do a pressure test on the intake system between the turbocharger and intake throttle valve. This will verify that the pressure is too low as specified by the manufacturer.

• If the pressure is too low, check the intake hoses, pipes, intercooler and clamps for cracks, being loose or damaged. If they are correct, then check the bypass valve to determine if it is slightly or completely open allowing boost pressure to escape.

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

I have found that this low pressure code P0035 is most commonly caused by intake hoses, pipes, and clamps not being tight and in good condition causing leaks in pressure. It is also possible that the intercooler may be damaged and leaking. Cracked hoses are hard to find and are usually not visible, as they are on the bottom of the hose in a low area which allows debris and fluids to collect, causing the hose to become brittle and crack which you can often not feel.

In one scenario, I’ve had to start the engine and check the hoses and intercooler by hand, just to to feel the air pressure coming out. Sometimes, the pressure under normal operation will blow off a hose completely causing a total loss of boost which is rather shocking when it happens.

Additional comments for consideration regarding the P0035 code

Many vehicles with mileage over 100,000 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 P0035 code?

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