|Trouble Code||Fault Location||Probable Cause|
|P0238||Manifold absolute pressure (MAP) sensor A, TC system high input||Wiring short to positive, MAP sensor, ECM|
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What Does Code P0238 Mean?
OBD II fault code P0238 is a generic code that is defined as “Manifold absolute pressure (MAP) sensor “A” – TC system high input”, and is set when the PCM (Powertrain Control Module) detects an abnormally high reading or value in the turbocharger’s boost control circuit. Note that “high input” in this definition could be a higher than expected signal voltage from a defective MAP/boost pressure sensor, or, “high input” could pertain to a higher than expected signal voltage caused by an actual boost pressure that is higher than would be expected given the operating conditions (such as the current engine speed and throttle opening) that obtain at any given moment.
Also, note the following to avoid confusion-
- The term MAP (Manifold Absolute Pressure) sensor is sometimes used interchangeably with “Boost Control Sensor”, but be aware that some applications use a dedicated turbo -, or supercharger boost control sensor to monitor boost pressure.
- In the definition above, “TC”, is an abbreviation for “turbocharger”
- On many applications, the various sensors in a particular system are labelled “A”, “B”, “C”, and so forth, but be aware that there is no convention that governs how manufacturers label sensors in the same system(s). For instance, one manufacturer may label say, the MAP sensor as sensor “A”, while another may label the same sensor as sensor “B”. Refer to the manual for the application being worked on to determine how sensors in any given system are labelled.
While the sole function of a turbocharger is to compress the air that enters an engine to increase engine power, the volume and pressure of the compressed air needs to be controlled to within very narrow margins to both ensure that maximum engine power is available over as wide a range of the engine’s power band as possible, and to protect the engine against over boost conditions that can damage, or even destroy the engine.
To accomplish both tasks, engine management systems use pressure sensors in the intake manifold to monitor the total pressure of the intake air. On most applications, this sensor is known as the Manifold Absolute Pressure sensor, and it works by changing its internal resistance in response to changes in the pressure of the intake air. These sensors are supplied with a 5-volt reference voltage by the PCM, and as the pressure in the manifold rises, the resistance in the sensor increases by a predictable amount, which causes less current to flow back to the PCM, and vice versa, as the pressure decreases.
NOTE: On some applications, MAP, or boost pressure sensors are electrically opposite; refer to the manual for the application for detailed information on this very important point.
This return current is known as the signal voltage, which the PCM interprets as pressure. The PCM uses this information to control a dumping mechanism, known as a “waste gate” in the turbocharger that controls, or limits the volume of exhaust gas that is used to drive the turbochargers’ turbine wheel. Thus, if the MAP sensor signals the PCM that the maximum allowable boost pressure had been reached, the PCM delivers a signal to the waste gate actuator to open the mechanism to allow some of the exhaust gas, also referred to as “drive pressure”, to bypass the turbine wheel by flowing directly into the exhaust system. Conversely, in order to maintain the boost pressure at optimum levels, the PCM commands the waste gate closed when the pressure in the manifold drops.
In a fully functional system, the PCM keeps the boost pressure within the permissible range by continually controlling the movement of the waste gate, with effective control relying on a continuous signal from the MAP sensor, which signal will continuously increase and decrease in direct response to the continually changing engine speed that is a feature of normal driving.
However, due to harsh operating conditions, the waste gate mechanism is prone to failures such as binding, sticking, or even fracturing. When this happens, and depending on the exact nature of the problem, the drive pressure cannot always be effectively controlled, which can cause potentially dangerous over boost conditions to develop. Thus, to prevent damage to the engine, most applications will enter a fail-safe, or limp mode when an over boost condition exists. On most applications, the PCM will also set code P0238, and illuminate a warning light on the first failure cycle.
The image below shows details of a waste gate mechanism (circled in red) on a typical turbocharger as might be found on almost any stock application.
What are the common causes of code P0238 ?
Some common causes of P0238 could include the following-
- Damaged, burnt, shorted, disconnected, or corroded wiring and/or connectors
- Defective MAP or boost control sensor
- Damaged, split, dislodged, hardened, or perished vacuum lines if the boost control system is vacuum operated
- Defective vacuum actuator or check valve(s) if the boost control system is vacuum operated
- Mechanical failure of the waste gate mechanism
- Failed or failing PCM. Note that this is a rare event, and the fault must be sought elsewhere before any controller is replaced.
- In some cases, clogged or even partially clogged catalytic converters and/or mufflers can prevent or inhibit the free flow of exhaust gas, which can cause over boost conditions to develop at high engine speeds
NOTE: As stated elsewhere, issues such as leaking exhaust manifold gaskets or damage to the turbine or compressor wheel (or both) are more likely to produce under boost conditions than over boost conditions.
What are the symptoms of code P0238 ?
Some common symptoms of P0238 could include the following-
- Stored trouble code and an illuminated warning light
- Most applications will experience a serious loss of engine power
- The application may enter a fail-safe, or limp mode that will endure until the problem is resolved
- Non-metal parts of the inlet ducting may burst, rupture, or become dislodged under excessive boost pressure conditions
- Depending on the nature of the problem, the turbocharger may emit whistling, knocking, grinding, or hissing sounds that may or may not vary with changing engine speeds
- Engine may misfire, or detonation may be present on all cylinders. Note however that these conditions will be indicated knock sensor or misfire codes that are the result, rather than the cause of P0238
- Note that if the cause of P0238 is a defective MAP or boost pressure sensor, or involves wiring, the boost pressure gauge (if fitted) may not indicate an excessive boost pressure condition
- Depending on the nature of the mechanical failure, spark plug fouling may occur, and particularly when excessive smoke from the tail pipe is present
How do you troubleshoot code P0238 ?
NOTE #1: Take note that mechanical failures such as excessive free play in the shaft linking the turbine and turbine wheels, worn shaft bearings, worn shaft oil seals, or damage to the turbine/compressor wheels are more likely to produce an under boost -, than an over boost condition. While these are serious defects that must be corrected without delay, or before putting the vehicle back into service, the first step in diagnosing this code (after eliminating wiring/electrical issues) should involve inspecting the condition, and operation of the waste gate mechanism and its actuator.
NOTE #2: Apart from a repair manual and a digital multimeter, a hand-held vacuum pump fitted with a graduated gauge, as well as a turbocharger boost pressure gauge will be most helpful in diagnosing code P0238 if the boost control system is vacuum operated.
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: Scan the entire engine management system for other codes (both active and pending) that relate to the boost control system, and in particular, for any codes that indicate short circuits or other issues in any part of the engine management system. On most applications, several engine sensors share one of two reference voltage circuits, which means that a failure or defect in one circuit can affect the operation of several engine sensors simultaneously. Thus, if any codes are present that relate to “Reference Voltage” problems without referring to a specific sensor, circuit, component, or system, investigate and resolve these codes first, before attempting a diagnosis and repair of code P0238.
If no other codes are present, refer to the manual to locate, and identify all relevant components, as well as the function, routing, color-coding, and location of all associated wiring, and if applicable, all associated vacuum lines and vacuum operated solenoids and/or check valves.
Perform a thorough visual inspection of all associated wiring /connectors; look for damaged, burnt, shorted, disconnected, or corroded wiring and/or connectors. Make repairs as required.
If the waste gate mechanism is vacuum controlled, look for damaged, split. Dislodged, perforated, or perished vacuum lines, and replace all lines that re in a less than perfect condition. Also, test the operation of all vacuum check valves by sucking air through them in the direction indicated on the valve by an arrow or other device. Replace all vacuum check valves that allow airflow in both directions, or that do not allow airflow in the indicated direction.
NOTE: If the application is not fitted with a boost pressure gauge on the dashboard, now would be a good time to check the turbochargers’ actual boost pressure with the test gauge to verify if an actual over boost condition exists or not. Most manufacturers supply a dedicated test point on the inlet ducting for just this purpose, so refer to the manual for the application to locate this point. Be sure to follow the instructions provided in the manual exactly during this step to be sure of obtaining accurate test results.
If no visible damage to wiring is found, and there is no actual over boost condition present, prepare to perform reference voltage, resistance, continuity, and ground connectivity checks on all associated wiring, but be sure to disconnect all wiring from the PCM during this step to prevent damage to the controller.
Pay particular attention to the reference voltage circuit, which should carry a current of between 4.5 volts, and around 4.8, to 4.9 volts when the circuit is activated. If the reading is lower, perform a resistance check on this circuit, and make repairs, or replace wiring as required to ensure that the correct current reaches the MAP of boost control sensor.
NOTE: If neither reference voltage, nor ground is present, refer to the manual for details on how to test the relevant terminals on the PCM directly. If there is no current on the PCM connector, replace the PCM. Also, bear in mind that the PCM supplies the ground for this circuit on most applications, so refer to the manual for details (KOER/KOEO) on how to establish the ground connection.
Also, be sure to check the internal resistance of the MAP sensor as per the instructions provided in the manual, since this value is a reasonably good indicator of the overall condition of the sensor. Compare this reading with the value stated in the manual, and replace he sensor if its resistance does not fall within the range specified by the manufacturer.
If the code persists, but all wiring is free of defects and known to be good, use the scanner to command the waste gate fully open and closed multiple times. On some scanners, this will be indicated as a percentage of opening, while on others it may be indicated as degrees of rotation.
Regardless of the unit displayed, compare this reading with the values stated in the manual for both the fully closed and fully open position. This value will be expressed as a voltage: thus, if the fully open position yields a voltage value that exceeds the maximum permissible voltage, suspect a wiring issue, a defective position sensor on the waste gate, or a maladjusted actuator control rod.
Similarly, if the fully closed position yields a voltage value that is higher than the minimum allowable value, suspect a maladjusted control rod, a defective position switch, or some sort of problem inside the turbocharger that prevents the waste gate from closing fully. If this turns out to be case, the wiser option is to refer the vehicle for professional diagnosis and repair.
Investigate the possible causes listed in Step 4 one by one. For instance, consult the manual for details on how to determine the correct adjustment of the actuator control rod, and follow the instructions EXACTLY to confirm that maladjustment is the cause of the code, or can be ruled out as a possible code of the code.
Similarly, disconnect the control rod if the actuator is vacuum operated, and see if the waste gate mechanism moves freely throughout its permissible range by moving it manually. Also, test the vacuum actuator to verify that it reacts to the prescribed vacuum, but more importantly, that it holds a vacuum. Replace the vacuum actuator if it does not perform as expected, or if a manually drawn vacuum decays- however slowly.
Clear all codes after all repairs are complete, and operate the vehicle for at least one drive cycle before scanning the system again to see if the code returns.
If Steps 1 through 5 did not resolve the problem, suspect a defective PCM, but since this is an exceedingly rare event, it is far more likely that a mechanical failure of the waste gate mechanism inside the turbocharger is the cause of the problem.
It is sometimes more cost effective to replace the turbocharger than to attempt repairs, but in all cases where turbochargers are removed and replaced, the process has to follow certain prescribed steps to ensure proper lubrication of the moving parts. Consult the manual on the correct procedure(s) to follow to prepare and install the replacement turbocharger, and be sure to follow the instructions provided EXACTLY to prevent premature failure of the replacement turbocharger.
Alternatively, refer the vehicle to the dealer or other competent repair shop for professional diagnosis and repair.
Codes Related to P0238
- 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”