How To Diagnose Trouble Code P1128 MAP Sensor Signal Lower Than Expected

There are quite a few things that can fool the PCM into thinking the MAP sensor is bad when it really isn't.

So, the absolutely best way to find out if the MAP sensor has failed (and thus causing the P1128 DTC) is to bench test it with a multimeter.

This is a fast and very easy test and that involves: connecting your multimeter to the MAP signal wire, manually applying vacuum via a vacuum pump (or the ‘good ole' lungs’) and seeing the actual voltage output of the MAP sensor.

If the MAP sensor if fried... it won't be able to react to the vacuum you're applying to it. Don't worry, in the following pages I've explained how to do this test in a step-by-step way.

This is a brief summary of the 3 tests in this tutorial:

  1. Verifying The MAP sensor is getting power.
    1. We'll use a multimeter to make sure the MAP sensor is getting 5 Volts DC from the PCM.
    2. TEST 1: Verifying The MAP Sensor Has Power.
  2. Verifying that the MAP sensor has a good path to ground.
    1. We'll be using a multimeter to verify this ground path.
    2. TEST 2: Verifying The MAP Sensor Has Ground.
  3. Manually apply and release vacuum to see if the MAP sensor responds to these changes in pressure.
    1. You can either use a vacuum pump or the ‘good ole’ lungs’ for this simple test.
    2. TEST 3: Verifying The MAP Signal With A Multimeter.

I recommend that you start with TEST 1: Verifying The MAP Sensor Has Power, but you can modify the following tests to fit your particular diagnostic needs.

TEST 1: Verifying The MAP Sensor Has Power

How To Test A P1128 Diagnostic Trouble Code (Honda 2.2L, 2.3L)

The very first thing we'll do, to find out if the MAP sensor is bad and causing a P1128 DTC, is to make sure it's getting power.

The PCM supplies this power in the form of 5 Volts DC and we can easily test for these 5 Volts with a multimeter.

NOTE: This test can be done with the MAP sensor connected or disconnected to its electrical connector.

Alright, this is what you'll need to do:

  1. 1

    Place your multimeter in Volts DC mode.

    If you don't have a multimeter or need to upgrade yours, check out my recommendation here: Abe's multimeter Recommendation (at: easyautodiagnostics.com).

  2. 2

    Probe the wire that connects to the pin labeled with the number 1, in the image viewer, with the red multimeter test lead.

    NOTE: Do not probe the front of the connector with the multimeter's test lead or you'll damage the connector's metal terminal. You should back-probe the connector or use a wire-piercing probe on the wire.

  3. 3

    Ground the multimeter's black test lead on a clean and unpainted metal surface on the engine...or better yet, on the battery's negative post.

  4. 4

    Turn the Key On (but engine off).

    Your multimeter should show you 5 Volts DC.

OK, now that the testing part is done... let's take a look at what your results mean:

CASE 1: The multimeter registered 5 Volts: So far so good, since this is the correct and expected test result. The next step is to check that the MAP sensor is getting ground. For the ground test, go to TEST 2: Verifying The MAP Sensor Has Ground.

CASE 2: The multimeter DID NOT register 5 Volts: Double check your connections and retest. If you still don't see the 5 Volts... then this test result lets you know that the MAP sensor is not BAD, since without these 5 Volts DC, the MAP sensor can not function.

Although it's beyond the scope of this article to troubleshoot the cause of these missing 5 Volts, you have now eliminated the MAP sensor as BAD. Resolving the issue that is keeping these 5 Volts from being supplied will solve the MAP sensor issue on your Honda vehicle.

TEST 2: Verifying The MAP Sensor Has Ground

How To Test A P1128 Diagnostic Trouble Code (Honda 2.2L, 2.3L)

So far, you've confirmed code P1128 (with your scan tool) and that the MAP sensor has power (5 Volts DC) in TEST 1...

...In this test step, you're gonna' verify that the MAP sensor is getting ground using your multimeter once again.

IMPORTANT: Remember, this ground is provided directly by the PCM, so be very careful and don't short this wire to battery power (12 Volts), or you WILL FRY the PCM.

Alright, this is what you'll need to do:

  1. 1

    With your multimeter still in Volts DC mode from the previous test and the Key On (but engine off).

  2. 2

    Probe the middle wire of the MAP sensor connector with the black multimeter test lead.

    This is the wire that connects to the pin labeled with the number 2 of the MAP sensor, (see photo in the image viewer).

    You test the ground circuit with the connector connected to the MAP sensor or not.

  3. 3

    Connect the multimeter's red test lead to the battery's positive (+) post.

  4. 4

    Your multimeter should show 12 Volts DC.

OK, let's take a look at what your results mean:


CASE 1: The multimeter registered 12 Volts. This is the normal and correct test result and lets you know that the PCM is providing a good path to ground for the MAP sensor.

The next step, and the last one, is see if the MAP sensor can react to the changes in air pressure while you apply (and release) vacuum to it. For this test go to: TEST 3: Verifying The MAP Signal With A Multimeter.

CASE 2: The multimeter DID NOT register 12 Volts. Double check your multimeter connections and repeat the test. If your multimeter still doesn't show 12 Volts, then the MAP is not fried and not the cause of the MAP Diagnostic Trouble Code (DTC) issue.

Here's why: Without a good path to ground, that the PCM provides internally, the MAP sensor will not work. With this test result, you have eliminated the MAP sensor as BAD.