TEST 3: Verifying The MAP Signal With A Multimeter

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

With this last MAP sensor test, we're gonna' either completely eliminate the MAP sensor as bad or be able to say that it has failed.

As stated at the beginning of the article, the MAP sensor creates a varying voltage signal depending on the amount of vacuum it senses.

When the MAP sensor fails... it will no longer create a varying voltage. In other words: no matter what the amount of vacuum applied to it, its output voltage will stay fixed at one single voltage value.

So, to find out if the MAP sensor is stuck, we're gonna' manually apply vacuum to it ourselves with a vacuum pump (or the good ole' lungs) and with a multimeter... verify it's voltage output.

Again, this MAP sensor bench test, we're about to perform, will let us know beyond a shadow of a doubt if the MAP sensor is bad or not.

This is what you'll need to do:

  1. 1

    Remove the MAP sensor from the top of the intake manifold.

    Reconnect the MAP sensor to its electrical connector, if you had to unplug it to remove it, since this test requires that the MAP sensor stay connected to its 3-wire electrical connector.

  2. 2

    Connect your multimeter's red test lead to the wire that connects pin number 3 of the MAP sensor.

    You'll need to back-probe the MAP sensor connector or use a wire piercing probe to connect your multimeter to this circuit (to see what a wire piercing probe looks like, click here: Wire Piercing Probe).

  3. 3

    Ground the black multimeter test lead.

    I suggest grounding the black multimeter test lead directly on the battery negative terminal.

  4. 4

    Turn the key on (but don't start the engine) and take a look at what voltage your multimeter is registering.

    It should register around 2.9 volts (± 1 V). If your multimeter is registering something different at this point... don't panic because right now you're just setting up the test.

  5. 4

    Attach a vacuum pump to the MAP sensor's vacuum inlet port using the appropriate size vacuum hose.

    If you don't have a vacuum pump... you can use the ‘good ole' lungs’ and provide the necessary with your mouth.

  6. 5

    The voltage, your multimeter is registering, should decrease as you apply vacuum with your vacuum pump (or with your mouth).

  7. 6

    The MAP sensor's voltage, as registered on your multimeter, should increase back to the original voltage value you saw in step 4 as you release the vacuum you applied with the vacuum pump (or your mouth).

NOTE: Applying vacuum, via the vacuum pump or your mouth, should make the MAP signal voltage get smaller on your multimeter. Releasing vacuum should make the MAP signal voltage get bigger (back to the original voltage you saw at the beginning of this test).

Let's take a look at what your test results mean:

CASE 1: The MAP sensor's voltage decreased and increased as you applied and released vacuum: This confirms that the MAP sensor is good and operating normally.

This test result completely eliminates the MAP sensor on your Honda as bad. If you're still having the MAP sensor trouble code come back, take a look at: MAP Sensor Is Good But Code Doesn't Go Away!.

CASE 2: The MAP sensor's voltage DID NOT decrease or increase as you applied and released vacuum: This confirms that the MAP sensor is bad and needs to be replaced.

If you need to buy the MAP sensor and would like to save some bucks, take a look at the section: Where To Buy The MAP Sensor.

MAP Sensor Is Good But Code Doesn't Go Away!

There's a good chance that, after bench-testing the MAP sensor, you've reach the conclusion that the MAP sensor is OK and not the cause of the P1128 OBD II trouble code.

So, if the sensor is good... why does the PCM keep setting the MAP sensor trouble code? Well, because several things can fool the PCM into thinking that the MAP sensor is bad... when the MAP sensor is only reporting what is actually happening.

So, to further assist you, in this section I'll offer a few testing tips and/or suggestions:


  1. Check for vacuum leaks.
    1. An intake manifold gasket leak or a ruptured vacuum hose (especially a big one) that's leaking vacuum will have a direct impact on the MAP sensor readings.
  2. check engine mechanical health.
    1. This means doing a compression test.
    2. What you're looking for is uneven wear and tear between the cylinders. This means that the compression each cylinder is producing varies more than 15%.
    3. This tutorial will help: How To Test Engine Compression (Honda 2.2L, 2.3L).
  3. Check timing belt synchronization.
    1. If you've just replaced the timing belt (or had to remove it for any reason), double check your timing marks.
  4. Make sure that the MAP sensor is getting power and ground.
    1. Take a look at the following section: P1128 Basics You Need To Know.
  5. A short in the wiring between the MAP sensor and the PCM.
    1. You'll need to check the continuity of the 3 wires between the MAP sensor and the PCM using the appropriate wiring diagram.
  6. A bad PCM.
    1. This is very rare, but it does happen.
  7. A fuel system malfunction (that is also setting other trouble codes).
    1. If there are any other trouble codes... you'll need to diagnose and repair these first (since they may be indirectly causing the P1128 code).
  8. You've replaced the throttle body gasket and used silicone sealer on it and this sealer has plugged the MAP sensor's vacuum passage and/or orifice (I've seen this happen a lot!).