Technical Description of OBD-II Trouble Code
Malfunction in the Circuit of Turbocharger Boost Sensor A
What does that signify?
This diagnostic trouble code (DTC) is a generic powertrain code. It is considered generic because it applies to all makes and models of vehicles (1996-newer), although specific repair steps may be slightly different depending on the model (Nissan, Honda, Renault, etc.).
Code P0235 is a generic code because it refers to a problem in the turbocharger boost sensor A circuit. Although it is generic, it is not identical for all vehicles.
OBD codes do not pinpoint a specific item, but rather indicate an area for the technician to investigate for potential causes of a problem within this circuit, which could have multiple possibilities.
All vehicles with code P0235 have one thing in common. It refers to an unacceptable difference between the ECM (electronic control module) programmed percentage of turbo boost at a specific RPM and the sensor’s signal value. These two values must closely match.
How Turbocharging (forced induction) Enhances Performance
Turbochargers force a significantly larger amount of air into the engine than the engine can handle under normal conditions. The increased volume of air, along with more fuel, results in greater power.
Typically, a turbocharger can increase horsepower by 35 to 50 percent in an engine specifically designed for turbocharging. However, a conventional engine’s components cannot handle the stress caused by forced induction.
Turbochargers provide high horsepower gains with minimal or no negative impact on fuel economy. They utilize exhaust velocity to drive the turbocharger, essentially providing free horsepower. This is the advantage. The disadvantage is that they are prone to failure for various reasons and can fail unexpectedly. When there is an indication of a turbocharger problem, it should be addressed as soon as possible. A turbocharged engine magnifies engine problems due to the compressed air mass.
Never attempt to tighten the wastegate or make any modifications to a stock turbocharged engine in an effort to increase boost pressures. Most engines’ fuel and timing curve cannot adapt to higher than normal boost pressures, and it will result in engine damage.
Note: This DTC is essentially the same as P0239, which refers to the “B” turbocharger.
Symptoms
Symptoms of a P0235 diagnostic trouble code may include:
The code P0235 indicates that there is a problem somewhere in this circuit, preventing proper boost control. Additional codes related to this malfunction may be set, each pertaining to a specific section of the circuit.
The engine may lack acceleration.
The boost pressure gauge will show less than 9 pounds of boost or over 14 pounds of boost, both of which are outside the normal range.
Unusual whining noises or rattling from the turbocharger or piping.
Engine knock sensor code may appear, indicating detonation due to high cylinder head temperatures.
The engine may exhibit an overall lack of power.
Smoke from the exhaust.
Fouled spark plugs.
Engine temperature unusually high at cruise speeds.
Hissing noises at the wastegate.
Causes
Turbos typically spin at an incredible 100,000 to 150,000 RPM. They are not tolerant to out-of-balance conditions or lack of clean oil to the bearing. Potential causes for this DTC include:
Vacuum leak at the intake manifold.
Dirty air cleaner.
Defective wastegate – either stuck open, closed, or leaking.
Insufficient oil supply to the main shaft bearing – obstruction in the oil feed or return line.
Bearing failure causing low spin-up due to drag.
Wobble in the bearing causing the turbine blades to strike the turbo housing.
Nicked, bent, or missing turbine blades causing an out-of-balance situation.
Oil seal leak on the compressor side of the turbo, indicated by oil in the turbo and fouled plugs.
Excessive end play in the turbo shaft.
Defective intercooler.
Loose connections at intake pipe to throttle body.
Cracks in the turbo housing.
Loose exhaust manifold to turbo bolts.
Poor electrical connection at Turbo boost sensor.
Short or open in the sensor harness between the sensor and ECM.
Defective sensor or ECM 5 volt reference driver.
Diagnostic Steps & Possible Solutions
In my experience, the diagnostic process starts with the most common turbo problems and systematically progresses to the least likely ones. Simple tools such as a vacuum gauge and dial indicator are needed.
Ensure that the engine is running properly with no misfiring plugs and no codes related to a failed knock sensor.
Inspect the hose clamps at the turbo outlet, intercooler, and throttle body for tightness when the engine is cold.
Attempt to move the turbo on the exhaust flange to check if it is tight.
Check the intake manifold for any leaks, including vacuum hoses.
Remove the actuating arm from the wastegate and manually operate the valve to check for a sticking valve that could cause a drop in boost.
Find a non-ported vacuum in the intake manifold and install a vacuum gauge. Start the engine and check that the engine has between 16 to 22 inches of vacuum at idle. If it has less than 16, the catalytic converter is faulty and will not allow boost to build.
Quickly rev the engine to 5000 RPM and release the throttle while monitoring the vacuum gauge to observe the boost pressure. If the boost pressure exceeds 19 pounds, the wastegate is faulty. If the boost fails to rise between 14 to 19 pounds, there is a problem with the turbo itself.
Shut down the engine and let it cool. Remove the turbo outlet hose and inspect the turbo to see if the blades are striking the sides of the housing. Look for bent or missing blades or oil in the turbo. Spin the blades by hand and check for grinding or resistance, which indicates a faulty turbo.
Inspect the oil lines from the engine block to the center turbo bearing and the return line from the bearing to the oil pan for any leaks.
Attach the dial indicator to the outlet turbine nose and move the turbo shaft in and out. If there is more than 0.003 of endplay, the center bearing is faulty.
If the turbo passes these tests, it is functioning properly. Use the service manual to test the boost sensor and harness using a volt/ohmmeter. Locate the 5-volt reference from the ECM to the sensor and check the voltage. If there is no voltage, there is an open or short in the harness or a faulty ECM.
Locate the proper reference signal from the boost sensor to the ECM and confirm that the voltage varies as the RPM increases. If there is no voltage climb, the sensor is faulty.
