Circuit Description
The DTC P1128 Manifold Absolute Pressure (MAP) Sensor Low Pressure diagnostic monitors the MAP sensor response to pressure changes in the intake manifold. The pressure changes occur based on engine load and the throttle valve opening. The MAP sensor has the following circuits:
| • | A 5-volt reference circuit |
| • | A low reference circuit |
| • | A signal circuit |
The powertrain control module (PCM) supplies 5 volts to the MAP sensor on the 5-volt reference circuit. The PCM also provides a ground on the low reference circuit. The PCM receives a low voltage on the signal circuit when manifold vacuum is high at idle or deceleration. The PCM receives a high voltage when manifold vacuum is low at wide open throttle (WOT) or when the ignition is ON and the engine is OFF. If the MAP sensor output is less than the expected value, a DTC P1128 can set.
The following table illustrates the difference between MAP kPa, signal voltage and engine vacuum.
Engine Condition | MAP kPa | MAP Signal Voltage | Manifold Vacuum |
|---|---|---|---|
Idle | Low | Low | High |
Deceleration | Low | Low | High |
Ignition ON, Engine OFF | High | High | Zero |
Wide-Open Throttle | High | High | Low |
DTC Descriptor
This diagnostic procedure supports the following DTC:
DTC P1128 Manifold Absolute Pressure (MAP) Sensor Low Pressure
Conditions for Running the DTC
| • | DTC P0107, P0108, P0117, P0118, P0122, P0123, P0171, P0172, P0174, P0175, P0201-P0206, P0222, P0223, P0300-P0306, P0335, P0401, P0403, P0404, P0443, P0496, P0501, P0506, P0507, P1129, P2100, P2101, P2108, P2122, P2123, P2127, P2128, P2135, P2138, P2176, P2227, P2228, P2229, P2413, P2646, P2647, P2648, P2649, and U0107 are not set. |
| • | The engine speed is between 1,100-6,300 RPM. |
| • | The engine coolant temperature is at least 69°C (156°F). |
| • | The vehicle speed is 24 km/h (15 mph) or more. |
| • | The throttle position (TP) sensor angle is 20 percent when the engine speed is 1,000 RPM. |
| • | The TP sensor angle is at least 33 percent when the engine speed is 3,000 RPM. |
| • | DTC P1128 runs once per drive cycle when the above conditions are met. |
Conditions for Setting the DTC
Condition 1
| • | The MAP sensor output is 54 kPa or less when the barometric pressure is 104 kPa. |
| • | The above condition is present for at least 2 seconds. |
Condition 2
| • | The MAP sensor output is 36 kPa or less when the barometric pressure is 61 kPa. |
| • | The above condition is present for at least 2 seconds. |
Action Taken When the DTC Sets
| • | The PCM illuminates the malfunction indicator lamp (MIL) after 2 consecutive ignition cycles in which the diagnostic runs with the fault active. |
| • | The PCM records the operating conditions at the time the diagnostic fails. This information is stored in the Freeze Frame buffer. |
| • | The PCM enters the Fail Safe Function and uses default MAP values based on the throttle position. The engine operates with the fuel control system in Open Loop and the fuel cutoff is lowered to 3,000 RPM. |
Conditions for Clearing the MIL/DTC
| • | The MIL turns OFF after 3 consecutively passing trips without a fault present. |
| • | A history DTC clears after 40 consecutive warm-up cycles without a fault. |
| • | Perform the scan tool Clear DTC Information function. |
Diagnostic Aids
Inspect for a restriction or blockage in the vacuum passage to the MAP sensor orifice.
An intermittent malfunction may be caused by a fault in the MAP sensor circuit. Inspect the wiring harness and the components for an intermittent condition. Refer to Intermittent Conditions .
If the DTC P1128 cannot be duplicated, the information included in the Freeze Frame data can be useful in determining the vehicle operating conditions when the DTC was first set.
Test Description
The numbers below refer to the step numbers in the diagnostic table:
-
The Diagnostic System Check - Vehicle prompts the technician to complete some basic checks and store the Freeze Frame data on the scan tool, if applicable. This creates an electronic copy of the data taken when the fault occurred. The information is then stored in the scan tool for later reference.
-
This step checks for the correct reference circuit voltage. The scan tool display of MAP sensor voltage will indicate more than 5 volts with an open in the sensor input circuit. This is caused by the scan tool software and should be considered normal.
-
This step checks the integrity of the MAP sensor signal circuit.
-
After replacing the PCM, a new minimum throttle position and idle speed must also be established.
Step | Action | Values | Yes | No |
|---|---|---|---|---|
Connector End View Reference: Powertrain Control Module Connector End Views or Engine Controls Connector End Views | ||||
Did you perform the Diagnostic System Check - Vehicle? | -- | Go to Step 2 | ||
2 | Is there a DTC P0641 also set? | -- | Go to DTC P0641 | Go to Step 3 |
3 |
Is the MAP sensor display at or below the specified value? | 0.23 V | Go to Step 5 | Go to Step 4 |
4 |
Did a DTC P1128 set or did the MAP sensor voltage go below the specified value? | 0.23 V | Go to Step 5 | Go to Diagnostic Aids |
Is the voltage near the specified value? | 5.0 V | Go to Step 6 | Go to Step 8 | |
Is the scan tool voltage near the specified value? | 5.0 V | Go to Step 7 | Go to Step 9 | |
7 | Replace the MAP sensor. Refer to Manifold Absolute Pressure Sensor Replacement . Did you complete the replacement? | -- | Go to Step 12 | -- |
8 |
Did you find and correct the condition? | -- | Go to Step 12 | Go to Step 11 |
9 |
Did you find and correct the condition? | -- | Go to Step 12 | Go to Step 10 |
10 |
Did you find and correct the condition? | -- | Go to Step 12 | Go to Step 11 |
Did you complete the replacement? | -- | Go to Step 12 | -- | |
12 |
Does the DTC run and pass? | -- | Go to Step 13 | Go to Step 2 |
13 | With a scan tool, observe the stored information, Capture Info. Does the scan tool display any DTCs that you have not diagnosed? | -- | System OK | |
