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Technical Description of OBD-II Trouble Code
Correlation between MAP/MAF and Throttle Position
What does that signify?
This generic powertrain code (DTC) is applicable to all vehicle makes and models (1996-newer). However, the specific repair steps may vary slightly depending on the model.
The generic trouble code P0068 indicates an engine management issue. There is a discrepancy among the computer’s sensors regarding the amount of air entering the intake manifold.
The PCM relies on three sensors – the mass airflow sensor, throttle position sensor, and the Manifold Absolute Pressure (MAP) sensor – to determine air flow volume and calculate fuel and timing strategies. Although there are many sensors on the engine, these three are associated with this code.
The mass airflow sensor is positioned between the air cleaner and the throttle body. Its purpose is to measure the amount of air passing through the throttle body. It contains a thin piece of resistance wire that stretches across the sensor’s inlet.
The computer supplies voltage to heat the wire to a specific temperature. As air volume increases, more voltage is required to maintain the temperature. Conversely, less voltage is needed as air volume decreases. The computer interprets this voltage as an indication of air volume.
The throttle position sensor is located on the opposite side of the throttle plate in the throttle body. When the throttle plate is closed, it blocks airflow into the engine. However, air necessary for idle bypasses the throttle valve through an idle air control motor.
Most newer vehicles have a throttle position sensor on the floorboards at the top of the gas pedal. As the pedal is pressed, the sensor sends a voltage to an electric motor that controls the opening of the throttle plate.
The throttle position sensor functions as a rheostat. At idle, with the throttle plate closed, the sensor registers a voltage close to 0.5 volts. When the throttle is opened during acceleration, the voltage rises to approximately 5 volts. The transition from 0.5 to 5 volts should be smooth. The engine’s computer interprets this voltage increase as a signal indicating the amount of airflow and the rate of opening.
The Manifold Absolute Pressure (MAP) sensor plays a dual role in this scenario. It senses the manifold pressure, which is adjusted for air density based on temperature, humidity, and altitude. It is also connected to the intake manifold through a hose. When the throttle suddenly opens, the manifold pressure drops and then rises again as the air flow increases.
The engine management computer requires all three of these sensors to accurately determine the duration for opening the injectors and the amount of spark advance needed to maintain a fuel ratio of 14.5/1. If one of these sensors or a mechanical issue causes a discrepancy among the rest, the computer cannot make the necessary adjustments and triggers the P0068 trouble code.
Symptoms
The symptoms experienced with a P0068 code depend on the cause of the overboost condition:
Causes
Possible causes for this DTC include:
Diagnostic Steps & Possible Solutions
As an auto technician, it is advisable to start with the most common problems. You will need a volt/ohmmeter, a wire-piercing probe for the meter, a can of carburetor cleaner, and a can of intake cleaner. Repair any identified issues and start the vehicle to determine if the problem is resolved. If not, continue with the following procedures.
With the engine off, open the hood and inspect the air cleaner element.
Check for any loose clamps or leaks in the piping from the mass airflow sensor to the throttle body.
Examine all the vacuum lines on the intake manifold for obstructions, cracks, or looseness that could cause vacuum loss.
Disconnect each of the sensors and check the connector for corrosion and any pushed out or bent pins.
Start the engine and use the carburetor cleaner to detect any leaks in the intake manifold. A short spray of carburetor cleaner over a leak will noticeably change the engine rpm. Hold the can at arm’s length to avoid contact with your eyes. Spray all connection points on the manifold to check for leaks.
Loosen the clamp on the pipe connecting the mass airflow sensor to the throttle body. Look inside the throttle body to see if it is coated with coke, a black greasy substance. If it is, pinch the tube from the intake cleaner bottle between the pipe and the throttle body. Push the pipe onto the throttle body and start the engine. Begin spraying until the can is empty. Remove the pipe and reconnect it to the throttle body.
Test the mass airflow sensor. Disconnect the connector from the sensor. Turn the ignition on with the engine off. There are three wires – 12-volt power, sensor ground, and signal (usually yellow). Use the red voltmeter lead to probe the connector for 12 volts, while keeping the black lead on ground. If there is no voltage, there may be an ignition or wiring problem. Reinstall the connector and check the sensor ground, which should be under 100 mv. If the sensor has 12 volts and is out of range on the ground, replace the sensor. This is a basic test. If it passes but the problem persists, the mass airflow sensor may still be faulty. Have it checked on a graphing computer, such as the Tech II.
Check the operation of the throttle position sensor. Ensure it is properly seated and the bolts are tight. It has a three-wire connector – dark blue for the signal, grey for the 5-volt reference voltage, and black or orange for the PCM negative wire.
– Connect the red voltmeter lead to the blue signal wire and the black voltmeter lead to a ground. Turn the key on with the engine off. If the sensor is functioning properly, it will have less than 1 volt with the throttle closed. As the throttle is advanced, the voltage will smoothly rise to approximately 4 volts without any dropouts or glitches.
Check the MAP sensor. Turn the key on and backprobe the power reference wire with the red voltmeter lead, while connecting the black lead to ground. With the key on and the engine off, the voltage should be between 4.5 and 5 volts. Start the engine, and the voltage should be between 0.5 and 1.5 volts, depending on altitude and temperature. Increase the engine rpm, and the voltage should react to the throttle opening by dropping and rising again. If not, replace the MAP sensor.
Professional Service
While we always recommend resolving this issue through mechanical means, if that is not possible, we offer the option to permanently delete this specific DTC. All other codes will continue to function. If you are interested in this option, please upload your ECU file to our portal.
