GM Service Manual Online
For 1990-2009 cars only

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:

  1. 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.

  2. 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.

  3. This step checks the integrity of the MAP sensor signal circuit.

  4. After replacing the PCM, a new minimum throttle position and idle speed must also be established.

Step

Action

Values

Yes

No

Schematic Reference: Engine Controls Schematics

Connector End View Reference: Powertrain Control Module Connector End Views or Engine Controls Connector End Views

1

Did you perform the Diagnostic System Check - Vehicle?

--

Go to Step 2

Go to Diagnostic System Check - Vehicle

2

Is there a DTC P0641 also set?

--

Go to DTC P0641

Go to Step 3

3

  1. Turn ON the ignition, with the engine OFF.
  2. Select the manifold absolute pressure (MAP) sensor parameter on the scan tool.

Is the MAP sensor display at or below the specified value?

0.23 V

Go to Step 5

Go to Step 4

4

  1. Turn OFF the ignition.
  2. Start the engine.
  3. Operate the vehicle within the Freeze Frame conditions as noted.
  4. Observe the MAP sensor voltage on the scan tool.

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

5

  1. Turn OFF the ignition.
  2. Disconnect the MAP sensor electrical connector.
  3. Turn ON the ignition, with the engine OFF.
  4. Measure the voltage from the 5-volt reference circuit to ground, harness side with a DMM.

Is the voltage near the specified value?

5.0 V

Go to Step 6

Go to Step 8

6

  1. Turn OFF the ignition.
  2. Jumper the 5-volt reference circuit of the MAP sensor to the MAP sensor signal circuit.
  3. Turn ON the ignition, with the engine OFF.
  4. Observe the MAP sensor voltage on the scan tool.

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

  1. Check for an open in the 5-volt reference circuit between the powertrain control module (PCM) and the MAP sensor.
  2. Repair as necessary. Refer to Wiring Repairs .

Did you find and correct the condition?

--

Go to Step 12

Go to Step 11

9

  1. Check for an open in the signal circuit between the PCM and the MAP sensor.
  2. Repair as necessary. Refer to Wiring Repairs .

Did you find and correct the condition?

--

Go to Step 12

Go to Step 10

10

  1. Inspect the PCM and the MAP sensor electrical connectors for faulty terminal contact. Refer to Testing for Intermittent Conditions and Poor Connections .
  2. Repair as necessary.

Did you find and correct the condition?

--

Go to Step 12

Go to Step 11

11

  1. Replace the PCM. Refer to Control Module References for replacement, setup, and programming.
  2. Perform the idle learn procedure. Refer to Idle Learn .

Did you complete the replacement?

--

Go to Step 12

--

12

  1. Use the scan tool in order to clear the DTCs.
  2. Turn the ignition OFF for 30 seconds.
  3. Start the engine.
  4. Operate the vehicle within the Conditions for Running the DTC or until the DTC P1128 diagnostic has passed.

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?

--

Go to Diagnostic Trouble Code (DTC) List - Vehicle

System OK