P0234 – Engine boost condition -limit exceeded

By Reinier (Contact Me)
Last Updated 2017-01-08
Automobile Repair Shop Owner

Table of Contents

1. What Does Code P0234 Mean?
2. What are the common causes of code P0234 ?
3. What are the symptoms of code P0234 ?
4. How do you troubleshoot code P0234 ?
5. Codes Related to P0234
6. Get Help with P0234

What Does Code P0234 Mean?

SPECIAL NOTES: Code P0234 is solely concerned with boost control issues on OEM turbochargers, and therefore this guide does NOT apply to stock applications that employ superchargers, which is an entirely different technology that requires boost control techniques and mechanisms that bear no relation to the boost control methods used on turbochargers. Superchargers are also relatively rare on stock applications, being used mostly on Mercedes-Benz products and a few other imported European applications. END OF SPECIAL NOTES.

OBD II fault code P0234 is a generic trouble code that is defined as “Engine boost condition -limit exceeded”, and is set when the PCM (Powertrain Control Module) detects a level of boost pressure being delivered to the engine by a forced-induction device that matches, or exceeds the maximum boost pressure limit set by the manufacturer for that application.

Forced induction devices in the form of turbochargers are used by engine manufacturers to increase the performance of their engines by forcing compressed air into the inlet tract, and from there into the cylinders. The rationale behind the technology is the fact that more air can be mixed with more fuel, while still maintaining an air/fuel mixture that is close to the stoichiometric point for the fuel used on that application. For instance, the stoichiometric ratio for gasoline is 14.7 parts of air to one part of fuel; at this ratio, all the fuel is combusted using all the available air.

NOTE: For diesel engines, the issue is a little more complicated. Since these engines are not throttled and almost always run with excess air, the ideal air/fuel ratio can vary from anywhere between about 14.6 parts of air to one part of fuel, to as much as 40 parts (or more) of air to one part of fuel, depending on the application, as well as the engine speed and load.

However, even on stock applications that are designed for forced induction, the technology places extreme loads and stresses on engines. Thus, to increase engine life car makers use devices known as “waste gates” to dump, or relieve excess drive pressure both as a means to extend engine life, and to strike a balance between the increased power delivery, and the overall durability, reliability, and running/maintenance costs of their engines. To accomplish this, most stock turbochargers are fitted with internal waste gates (aka “dump valves”) to reduce the drive pressure, and therefore the speed of the turbine wheel.

In practice, turbochargers are driven by the exhaust gas that exits the engine, hence the term “drive pressure”. The exhaust gas drives a turbine wheel, which in its turn, drives a compressor wheel that is connected to the turbine wheel via a shaft that passes through the internal wall that divides the turbocharger assembly into two halves. The compressor wheel is fed with air through the inlet ducting that starts at the air filter box: the intake air is then compressed by the rapidly spinning compressor wheel before being fed to the engine through the inlet manifold, sometimes passing through an intercooler on the way to the engine to reduce the compressed air’s temperature.

NOTE: Since compressed air gains heat in the process of being compressed, it expands, which reduces the volume of air that is available to the engine. Cooling the air by passing it through a heat exchanger (aka “intercooler”) causes the air to contract, which increases its density, which means more cool air can be squeezed into the same volume. As a practical matter though, the boost level that the turbocharger ultimately delivers to the engine depends on the design and diameter of the turbine and compressor wheels, the volume, flow rate, and pressure of the exhaust gas that drives the turbine wheel, the length and volume of both the inlet ducting and exhaust systems, as well as on whether or not the compressed air is cooled before being fed into the engine.

If car engines always ran at constant speeds, forced induction systems would have been largely self-regulating. However, car engines do not run at constant speeds, and once a turbocharger is spooled up and rotating at 250 000 RPM (or sometimes more) and the throttle is suddenly closed even partially, the boost pressure being developed by the still-spinning compressor wheel can cause severe engine damage, because the engine cannot “process” the large volume of highly compressed air at that reduced throttle setting. Thus, if the waste gate fails, excessive boost pressures can cause fatal engine damage (even over relatively short time periods) if that pressure cannot be dumped, or prevented from building up in the first place.

To get around this problem, the turbocharger is fitted with a waste gate in the turbine wheel housing that if it is opened, allows some of the drive pressure (exhaust gas) to escape into the exhaust system. This has the practical advantage of limiting the amount of exhaust gas that is available to drive the turbine wheel, and since the action of compressing the inlet air exerts a braking force on the compressor wheel, the rotational speed of the turbine wheel can be controlled effectively, while still maintaining maximum design boost pressure (albeit with a reduction in the drive pressure) since not all the exhaust gas that exits the engine can escape through the waste gate.

In terms of operation on most stock applications, the waste gate is opened by a vacuum actuator when the PCM receives a signal voltage from the MAP (Manifold Absolute Pressure) sensor (among others) that the maximum allowable boost pressure has been reached. Upon receiving the pressure signal from the MAP sensor, the PCM opens a vacuum solenoid/valve to allow engine vacuum to act on the waste gate actuator, which is connected to the waste gate proper with a connecting rod.

On a fully functional system, the PCM also adapts the fuel delivery strategy, ignition timing, and other affected engine management systems to maintain maximum engine performance. When the PCM deems it safe to close the waste gate to restore full drive pressure to the turbine wheel, it will close the vacuum solenoid/valve. Spring pressure in the actuator then acts on the pushrod, which closes the waste gate, and keeps it closed until the PCM receives the next signal to open the waste gate.

While the opening and closing cycles of the waste gate occurs automatically and in a generally seamless fashion any malfunction in, or failure of, any component that controls and/or monitors the function and operation of the waste gate will cause the PCM to set code P0234, and illuminate a warning light.

NOTE #1: While most stock applications employ internal waste gates, some imported applications use external dumping mechanisms. These are known, as the name suggests, as “external waste gates”, and while they work just as well or better than the internal variety, they require additional ducting, and are therefore not popular among American car manufacturers. Although the basic operating principles of these devices are similar to the internal variety, external waste gates are more sensitive to variations in the strength of the compression spring that keeps them closed than internal waste gates are. Refer to the manual for the application for detailed information on troubleshooting issues with external waste gates.

NOTE #2: There exists another variety of boost control mechanism known as a “blow-off valve”, although it is not commonly found on stock applications in the American domestic market. With this design, the valve is located on the inlet tract, as opposed to inside the turbocharger. With this design, boost is controlled by “blowing off” some compressed intake air, instead of allowing some of the drive pressure (exhaust gas) to be bled off into the exhaust system through the internal waste gate.

The image below shows a typical waste gate (shown in the closed position in this image) on a typical OEM turbocharger. Note the vacuum actuator, (circled in red) that is attached to the waste gate with an adjustable push rod. Also, note the black vacuum hose that is connected to the engine vacuum system. It is via this hose that the engine vacuum acts on the actuator diaphragm.

What are the common causes of code P0234 ?

Some typical causes of code P0234 could include the following-

• Defective MAP (Manifold Absolute Pressure) sensor
• Damaged, burnt, shorted, disconnected, or corroded wiring and/or connectors in the MAP sensor control circuit
• Damaged, split, cracked, or dislodged vacuum lines
• Defective waste gate actuator
• Mechanical failure(s) of the waste gate, or its linkage to the vacuum actuator
• Binding or sticking waste gate spindle where it passes into the turbocharger casing. Note that this is more likely to happen on vehicles that spend long periods in storage, or on vehicles that are not driven regularly
• Ill-considered modifications to the boost control system, or the use of aftermarket parts that could include so-called “performance parts” that are intended to alter the boost characteristics of a stock turbocharger
• Ill-considered, or illegal modifications to a stock exhaust system

What are the symptoms of code P0234 ?

Apart from a stored trouble code and an illuminated warning light, the symptoms of code P0234 are much the same across all applications, and these could include the following-

• Loss of power. This can manifest in varying degrees, but on applications where sections of the inlet ducting are made from rubber or silicon, excessive boost pressure can cause these sections to rupture or separate from metal sections of the inlet tract. When this happens, all the boost pressure is lost, which causes severe power loss.
• Depending on the degree of over boost, most applications will develop detonation noises that can resemble those of a bearing knock, and especially upon acceleration. Note that detonation noises indicate a serious condition that can potentially destroy an engine in very short order.
• Even slight to moderate over boost conditions can cause the engine to overheat. Note that depending on the application and the actual degree of over boost, engine overheating can cause secondary symptoms that can range from misfires due to cylinder head gasket failure, to fatal engine damage. In some cases, engine over heating can cause the transmission to overheat as well.

How do you troubleshoot code P0234 ?

NOTE #1: Apart from a digital multimeter and a repair manual for the application being worked on, a graduated vacuum pump will be most helpful in diagnosing this code. If the application is not fitted with a factory-installed boost gauge, a suitable pressure gauge will also be required.

NOTE #2: Be aware that on some applications, the terms MAP (Manifold Absolute Pressure) sensor, and “Turbocharger Boost Sensor” is used interchangeably. However, to avoid confusion, refer to the manual for the application being worked on for details on the terminology used by that manufacturer to describe various parts and components.

Step 1

Record all fault codes present, as well as all available freeze frame data. This information can be of use should an intermittent fault be diagnosed later on.

NOTE: Over-boost conditions can sometimes set off a number of other codes along with P0234, but in some cases, the possible cause(s) of an over boost condition can be indicated by codes other than P0234. Thus, if other codes are present, note the order in which they were stored; for instance, if MAP (Manifold Absolute Pressure) sensor related codes were stored before P0234, it is possible that the over boost condition is the direct result of a failure of the MAP sensor and/or its control circuit. Similarly, codes that follow P0234 are the result of the over boost condition.

Step 2

Make sure the engine is cold, and refer to the manual to locate all sensors, vacuum lines, wiring / connectors, and other components that are relevant to the boost-pressure control system. Be aware though that on some applications, it may be necessary to remove protective covers and shields over the engine to gain full access to all components.

Step 3

MAP sensor failure is a common cause of this code, so start the diagnostic procedure by locating the sensor. Perform a thorough visual inspection of its wiring; look for damaged, burnt, shorted, disconnected, or corroded wiring and/or connectors. Make repair as required.

If no visible damage is found, consult the manual to determine the function of each wire, and follow the directions provided in the manual (KOER/KOEO) to test the wiring for continuity, reference voltage, and resistance. In many cases, the PCM supplies the ground for the MAP sensor, so be sure to check this circuit as well. Compare all obtained readings to the values stated in the manual, and make repairs as required to ensure that all electrical values fall within the manufacturer’s specifications.

NOTE: The MAP sensor itself forms part of the control circuit, so be sure to follow the directions provided in the manual to test the operation of the sensor as well. Replace the sensor if any deviations from specified reference data are found.

Step 4

If all electrical values check out and the MA sensor is serviceable, perform a thorough visual inspection of all associated vacuum lines. Check for cracked, split, damaged, or dislodged vacuum lines, especially in the vacuum circuit that connects the turbocharger waste gate actuator with the engine vacuum. Replace all vacuum lines that are in a less-than-perfect condition.

Step 5

If the vacuum and electrical systems check out, attach the vacuum pump to the actuator at the point where the engine vacuum is normally connected. Refer to the manual for details on the strength of the vacuum required to open the waste gate, and apply the correct vacuum to the actuator. There is little point in applying a stronger vacuum, since doing so will only result in an inaccurate conclusion as to the serviceability (or otherwise) of the actuator diaphragm.

Observe the push rod as the vacuum is applied. If the diaphragm is not perforated and the waste gate is not sticking or jammed, the push rod will move smoothly until the mechanism is in the fully open position. Check this by attempting to move the rod further when the full, required vacuum is applied- if the rod can be moved some more correct the rods’ adjustment. Follow the directions provided in the manual to adjust the mechanism to the manufacturer’s specifications.

If the push rod does not react when the vacuum is applied, remove the actuator retaining bolts/screws and attempt to rotate the waste gate manually. If the mechanism moves freely, replace the actuator. Note though that if the vacuum causes the waste to open fully, the motion must reverse when the vacuum is removed. If it does not, the spring in the actuator is likely broken, which means the actuator must be replaced.

NOTE: Bear in mind that if the waste gate cannot be rotated manually, or if an inordinate amount of force is required to rotate it, the remedy could involve removal and disassembly of the turbocharger. However, one trick to free the mechanism is to apply a liberal amount of penetrating lubricant to the spindle. Wait a few minutes for the lubricant to act, and try to move the mechanism again. If the lubricant frees the mechanism, great- but if not, be aware that removing a turbocharger from an engine requires skills and equipment that most average non-professional mechanics do not own. In these cases, the better option by far is to refer the vehicle for professional diagnosis and repair.

Step 6

If the push rod cannot be moved further (implying that the waste gate is in the fully open position) when the required vacuum is applied to the actuator, and the vacuum holds steady on the gauge for at least a couple of minutes, refer to the manual to determine exactly how vacuum is supplied to the actuator, since the method of supply varies between applications. Thoroughly inspect this part of the boost control system, and perform all repairs and/or replacement of parts and components in strict accordance with the instructions provided in the manual.

Step 7

The diagnostic/repair steps up to this point will resolve over boost conditions nine times out of every ten: however, to verify that the problem has indeed been resolved, clear all codes, and operate the vehicle for at least one complete drive cycle with a scanner connected to record the operation of the turbocharger and the boost control system in real time.

If the code does not return, the repair can be considered to have been successful, but if the code and symptoms do return, the only other likely causes is an intermittent fault that affects the operation of the waste gate on the one hand, or a restricted exhaust system that impedes the effective dumping of excess drive pressure, on the other.

One way to check for restrictions in the exhaust system is to attach a boost gauge to the inlet at the point between the turbocharger and the inlet manifold that most manufacturers provide for this purpose. Once the boost gauge is attached securely, start the engine, and raise the engine speed to between 2500 and 3000 RPM to allow the turbocharger to spool up to full speed, but be sure to keep a close watch both on the reading on the boost gauge, as well as on the waste gate actuator while the boost pressure rises.

If the exhaust system is NOT restricted, the boost pressure will rise until it reaches the specified value, and assuming that the waste gate works as intended the boost pressure will remain close to this value when the throttle is closed suddenly, since the excess drive pressure (exhaust gas) will simply pass through the open waste gate, and into the exhaust system. Note however that the boost pressure will decrease when the engine is allowed to return to idling speed; this is normal, and to be expected.

If however, the boost pressure exceeds the specified value for that application while the engine is running at a steady speed (2500 – 3000 RPM) even though the waste gate is seen to be opening, the exhaust system is restricted since the drive pressure cannot be vented or relieved effectively. The same is true if the waste gate is seen to be opening, but the boost pressure spikes when the throttle is closed suddenly.

NOTE: If the application being worked on has a factory-fitted boost gauge, use this gauge during Step 7 instead of attaching a pressure gauge to the inlet tract, but enlist the services of an assistant to monitor either the boost gauge, or the operation of the waste gate actuator.

Step 8

Bear in mind that not all applications are equipped to indicate the increases in exhaust gas temperatures that come with a restricted exhaust system. So if it is suspected that a restriction in the exhaust system is causing the over boost condition but there are no codes present that indicate this possibility, refer the vehicle to a specialist exhaust-fitment shop for professional diagnosis and repair.

If on the other hand, an intermittent fault elsewhere in the boost control system is suspected, be aware that this type of problem can sometimes be extremely challenging and time consuming to find and repair. In fact, in some cases, it might be necessary to allow the fault to worsen considerably before an accurate diagnosis and definitive repair can be made.

Codes Related to P0234

Note that while the generic codes listed below are not strictly related to P0234 – “Engine Over- boost Condition – Limit Exceeded”, any of the codes below could potentially cause code P0234, or contribute to code P0234 being set depending on the application, and how the relationship between P0234 and each individual code listed here affects any particular application. Therefore, always refer to the manual for the application being worked for details of the codes below whenever one or more of the codes listed here is present along with P0234 in order to ensure a definitive and reliable repair of code P0234.

• P0235 – Relates to “Turbocharger Boost Sensor A Circuit Malfunction”
• P0236 – Relates to “Turbocharger Boost Sensor A Circuit Range/Performance”
• P0237 – Relates to “Turbocharger Boost Sensor A Circuit Low”
• P0238 – Relates to “Turbocharger Boost Sensor A Circuit High”
• P0239 – Relates to “Turbocharger Boost Sensor B Circuit Malfunction”
• P0240 – Relates to “Turbocharger Boost Sensor B Circuit Range/Performance”
• P0241 – Relates to “Turbocharger Boost Sensor B Circuit Low”
• P0242 – Relates to “Turbocharger Boost Sensor B Circuit High”
• P0243 – Relates to “Turbocharger Waste-gate Solenoid A Malfunction”
• P0244 – Relates to “Turbocharger Waste-gate Solenoid A Range/Performance”
• P0245 – Relates to “Turbocharger Waste-gate Solenoid A Low”
• P0246 – Relates to “Turbocharger Waste-gate Solenoid A High”
• P0247 – Relates to “Turbocharger Waste-gate Solenoid B Malfunction”
• P0248 – Relates to “Turbocharger Waste-gate Solenoid B Range/Performance”
• P0249 – Relates to “Turbocharger Waste-gate Solenoid B Low”
• P0250 – Relates to “Turbocharger Waste-gate Solenoid B High”

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