What does it mean?
This diagnostic 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.
The P0234 fault code indicates that the Powertrain Control Module (PCM) detects a dangerously high boost pressure from the engine’s forced induction system. Boost levels exceeding recommended levels can compromise the structural integrity of the engine.
Normally, an engine relies on the vacuum produced by the downward movement of the piston to draw the air and fuel charge into the engine. A supercharger or turbocharger is an air compressor used to increase the air and fuel charge entering the engine. This is known as “forced induction” which allows a much smaller, fuel-efficient engine to produce the power normally available with a much larger engine.
The mechanical device used in forced induction is divided into three categories: positive displacement (roots style), centrifugal, and turbo. Roots style and centrifugal chargers are belt-driven while the turbo relies on exhaust pressure to operate.
A positive displacement supercharger is positioned on top of the intake. A centrifugal one looks much like a rotary air conditioning compressor and is located on the driver’s side of the engine. Turbochargers are located in line with the exhaust system.
As boost pressure increases, the pressure on the engine also increases. Your engine has recommended boost pressure limits to avoid the possibility of engine component failure. The P0234 code is set when these limits are exceeded and should be corrected as soon as possible to avoid damaging the engine or transmission.
Turbochargers rely on exhaust pressure to spin the turbine blades fast enough to produce air pressure above atmospheric pressure. However, they have an inherent lag where the exhaust pressure is insufficient to spin the turbo fast enough to create pressure. Depending on the type of unit used, the turbo needs 1,700 to 2,500 RPM before it starts to “spool up.”
Turbos spin around 250,000 RPM at full acceleration. Boost pressure increases with engine RPM. To regulate boost pressure and avoid overboost, a wastegate is installed. Most modern turbos have an internal wastegate and an external actuator. There is a rod from the actuator to the wastegate on the turbo. Air pressure from the intake manifold passes to the top of the wastegate. When boost pressure increases, it begins to exert force on the actuator spring which holds the wastegate closed. The higher the pressure, the more it overcomes the spring, causing the wastegate to open and divert exhaust gases away from the turbo blades, preventing further boost increase.
Regulating the pressure applied to the wastegate controls boost levels at a specific RPM. To do this, the computer uses barometric or MAP sensors, engine and transmission temperature sensors, knock sensors, and intake pressure sensors to determine how much wastegate opening is needed to provide the best boost levels.
To regulate boost levels, the computer uses either a solenoid, a stepper motor, or a pulse modulator. By regulating the pressure to the wastegate actuator, various degrees of boost are possible.
Symptoms
The symptoms displayed for a P0234 code depend on the cause of the overboost condition:
The Service Engine Soon or Check Engine light will illuminate.
You will experience a loss of power.
The engine may show signs of overheating.
The transmission may show signs of overheating and harsh shifting.
Additional codes associated with the condition defined by P0234 may be present to help identify the cause. Codes are available for all electrical components used by the engine management computer for boost level control.
The engine may show signs of pre-ignition in the form of knocking.
The engine may display misfires.
Causes
Potential causes of this DTC include:
In the absence of other fault codes associated with the overboost condition, it would be safe to say the problem is mechanical. The operation of the wastegate is the most likely.
The wastegate is either stuck closed, causing the turbo to spin higher than normal, thus causing overboost.
The rod from the wastegate actuator to the wastegate on the turbo is bent.
A hose has come off the wastegate or the boost controller.
A supply line to the boost controller or from the controller to the wastegate is clogged.
Dodge trucks with a Cummins diesel have a particular issue. They run fine, but the Check Engine light comes on and sets the P0234 code at idle, but the light goes out several minutes later at cruising speed. The digital boost control gauge is connected to the MAP sensor which has an intermittent fault at idle but does not set a code. Replacing the MAP sensor fixes this.