Circuit Description
The mass air flow (MAF) sensor measures the amount of air ingested by the engine. The direct measurement of the air entering the engine is more accurate than calculating the airflow from the manifold absolute pressure (MAP), the intake air temperature (IAT) and the engine speed. The MAF sensor has an ignition positive voltage, ground, and a signal circuit. The IAT sensor is also part of the MAF sensor.
The MAF sensor used on this engine is a hot wire type. This engine uses the MAF sensor to measure air flow rate. The MAF output frequency is a function of the power required to keep the air flow sensing elements such as hot wires at a fixed temperature above the ambient temperature. Air flowing through the sensor cools the sensing elements. The amount of cooling is proportional to the amount of air flow. The MAF sensor requires a greater amount of current in order to maintain the hot wires at a constant temperature as the air flow increases. The MAF sensor converts the changes in current draw to a frequency signal read by the powertrain control module (PCM). The PCM calculates the air flow in grams per second or hertz, based on this signal.
The PCM monitors the MAF sensor frequency. The PCM can determine if the sensor is stuck low, stuck high, not providing the airflow value expected for a given operating condition, or that the signal appears to be stuck based on a lack of signal variation expected during the normal operation. This diagnostic checks the range and performance of the MAF sensor. The MAF system performance or rationality diagnostic uses the MAP, the IAT, and the engine speed to calculate an expected airflow rate. The PCM then compares the rate to the actual measured airflow from the MAF sensor. The PCM only compares the actual MAF value and the calculated value during conditions where the values are likely to match. This diagnostic trouble code (DTC) sets if the actual MAF reading is not within a predetermined range of the calculated reading.
Conditions for Running the DTC
| • | No active MAF DTCs |
| • | No active MAP DTCs |
| • | No active throttle position (TP) DTCs |
| • | The engine is running. |
| • | The TP sensor angle is less than 50 percent and the engine vacuum (BARO-MAP) is more than 65 kPa. |
| • | The system voltage is between 10 volts and 18 volts. |
| • | The change in TP sensor is less than 3 percent. |
| • | All above conditions are stable for 2 seconds. |
Conditions for Setting the DTC
| • | The actual MAF reading is not within a predetermined range of the calculated reading. |
| • | All conditions are met for 5 seconds. |
Action Taken When the DTC Sets
| • | The PCM illuminates the malfunction indicator lamp (MIL) on the second consecutive ignition cycle that the diagnostic runs and fails. |
| • | The PCM records the operating conditions at the time the diagnostic fails. The first time the diagnostic fails, the PCM stores this information in the Failure Records. If the diagnostic reports a failure on the second consecutive ignition cycle, the PCM records the operating conditions at the time of the failure. The PCM writes the conditions to the Freeze Frame and updates the Failure Records. |
| • | The PCM utilizes speed density such as RPM, MAP, and IAT for fuel management. |
| • | The transmission line pressure increases. |
| • | Firmer transmission shifts |
Conditions for Clearing the MIL/DTC
| • | The PCM turns OFF the malfunction indicator lamp (MIL) after 3 consecutive ignition cycles that the diagnostic runs and does not fail. |
| • | A last test failed, or current DTC, clears when the diagnostic runs and does not fail. |
| • | A history DTC clears after 40 consecutive warm-up cycles, if no failures are reported by this or any other emission related diagnostic. |
| • | Use a scan tool in order to clear the MIL and the DTC. |
Diagnostic Aids
Important: Remove any debris from the PCM connector surfaces before servicing the PCM. Inspect the PCM connector gaskets when diagnosing or replacing the PCM. Ensure that the gaskets are installed correctly. The gaskets prevent water intrusion into the PCM.
| • | The following may cause an intermittent: |
| - | Mis-routed harness |
| - | Rubbed through wire insulation |
| - | Broken wire inside the insulation |
| • | For an intermittent, refer to Symptoms . |
| • | Any unmetered air may cause this DTC to set. Check for the following: |
| - | An engine vacuum leak |
| - | The positive crankcase ventilation (PCV) system for vacuum leaks |
| - | An incorrect PCV valve |
| - | The engine oil dip stick not fully seated |
Test Description
The numbers below refer to the step numbers on the diagnostic table.
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The MAF system performance or rationality diagnostic uses the MAP sensor signal along with other inputs in order to calculate an expected airflow rate. The PCM then compares the expected flow rate to the actual measured airflow from the MAF sensor. The first few steps of this table verify that the MAP sensor is working properly. Correct any MAP sensor DTCs first.
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Release the retaining tabs in order to remove the MAP sensor.
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Using the Freeze Frame and Failure Records data may aid in locating an intermittent condition. If you cannot duplicate the DTC, the information included in the Freeze Frame and Failure Records data can help determine how many miles since the DTC set. The Fail Counter and Pass Counter can also help determine how many ignition cycles the diagnostic reported a pass or a fail. Operate the vehicle within the same Freeze Frame conditions such as RPM, load, vehicle speed, temperature, etc. that you observed. This will isolate when the DTC failed.
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Any unmetered air causes this DTC to set. Check the PCV system for vacuum leaks. Also inspect the dip stick for being pulled out. Check the oil fill cap for being loose.
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This step verifies the signal circuit from the MAF sensor electrical connector to the PCM.
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This step verifies whether a ground and B+ circuit is available.
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This step checks the signal circuit for an open.
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This step checks the signal circuit for a short to B+.
Step | Action | Value(s) | Yes | No | ||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|
1 | Did you perform the Powertrain On-Board Diagnostic (OBD) System Check? | -- | ||||||||||
If any MAP sensor DTCs are set, refer to the applicable DTC before proceeding.
Is the MAP sensor voltage outside of the specified range? | 0.8-4 V | |||||||||||
Did the MAP sensor value change smoothly through the entire range of the test without any erratic readings? | -- | |||||||||||
4 | Apply 20 inches Hg vacuum to the MAP sensor. Is the MAP sensor reading on the scan tool the same or less than the specified value? | 34 kPa | ||||||||||
5 | Disconnect the vacuum source from the MAP sensor. Does the MAP sensor reading return to the original value that was observed in step 3? | -- | ||||||||||
Does the scan tool indicate that this diagnostic failed this ignition? | -- | Go to Diagnostic Aids | ||||||||||
Did you find and correct the condition? | -- | |||||||||||
Is the voltage near the specified value? | 5 V | |||||||||||
Connect the J 35616-200 test lamp between the MAF sensor ignition positive voltage and the ground circuit at the MAF sensor harness connector. Is the test lamp illuminated? | -- | |||||||||||
Is the voltage less than the specified value? | 4.5 V | |||||||||||
Does the voltage measure near the specified value? | 0 V | |||||||||||
12 | Connect the J 35616-200 test lamp between the MAF sensor ignition positive voltage circuit and the battery ground. Is the test lamp illuminated? | -- | ||||||||||
13 |
Did you find a poor connection? | -- | ||||||||||
14 |
Did you find the MAF sensor signal circuit open or shorted? | -- | ||||||||||
15 | Locate and repair the open in the ground circuit to the MAF sensor. Refer to Wiring Repairs in Wiring Systems. Is the action complete? | -- | -- | |||||||||
16 | Locate and repair the open in the ignition positive voltage circuit to the MAF sensor. Refer to Wiring Repairs in Wiring Systems. Is the action complete? | -- | -- | |||||||||
17 | Locate and repair the short to voltage in the MAF sensor signal circuit. Refer to Wiring Repairs in Wiring Systems. Is the action complete? | -- | -- | |||||||||
18 | Replace the MAF sensor. Refer to Mass Airflow Sensor/Intake Air Temperature Sensor Replacement . Is the action complete? | -- | -- | |||||||||
19 | Replace the MAP sensor. Refer to Manifold Absolute Pressure Sensor Replacement . Is the action complete? | -- | -- | |||||||||
20 |
Important:: The replacement PCM must be programmed . Replace the PCM. Refer to Powertrain Control Module Replacement . Is the action complete? | -- | -- | |||||||||
21 |
Does the scan tool indicate that this test ran and passed? | -- | ||||||||||
22 | Select the Capture Info option and the Review Info option using the scan tool. Does the scan tool display any DTCs that you have not diagnosed? | -- | Go to the applicable DTC table | System OK |
