How To Test: P1129 OBD II Trouble Code (Honda 2.2L, 2.3L)

How To Diagnose Trouble Code P1129 MAP Sensor Signal Higher Than Expected

The thing to remember, about code P1129, is that it's not telling us the MAP sensor is bad. It's just telling us that the info the MAP sensor is reporting just doesn't correspond to what the engine is doing.

It could be that the MAP sensor is truly fried. Unfortunately, quite a few things can fool the PCM into thinking the MAP sensor is bad (when it really isn't). This is why we need to test it.

The absolutely best way to test the MAP sensor and find out if it's bad (and thus causing the P1129 DTC) is to bench test it with a multimeter.

This is one of the easiest test you'll ever do and it doesn't involve a scan tool. The test involves: connecting your multimeter to the MAP signal wire, applying vacuum via a vacuum pump (or the ‘good ole' lungs’) and checking to see if the MAP sensor's voltage decreases (as you apply vacuum).

If the MAP sensor is fried, you won't see the output voltage decrease (on your multimeter). It doesn't get any easier than that! In the following pages, you'll see the whole test explained 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.
   • We'll use a multimeter to make sure the MAP sensor is getting 5 Volts DC from the PCM.
   • TEST 1: Verifying The MAP Sensor Has Power.
2. Verifying that the MAP sensor has a good path to Ground.
   • We'll be using a multimeter to verify this Ground path.
   • 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.
   • You can either use a vacuum pump or the ‘good ole' lungs’ for this simple test.
   • 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

Before we test the MAP sensor's output voltage, we're gonna make sure the MAP sensor is getting both power and Ground.

In this test section, the very first thing we'll do is make sure the MAP sensor is 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, let's get started:

1. 1

   Disconnect the MAP sensor from its electrical connector.

2. 2

   Place your multimeter in Volts DC mode.
   
   If you don't have a multimeter or need to upgrade yours, check out my recommendation here: Buying A Digital Multimeter For Automotive Diagnostic Testing (at: easyautodiagnostics.com).

3. 3

   With the red multimeter test lead, probe the wire that connects to the pin labeled with the number 1 in the image above.
   
   NOTE: You should avoid probing the front of the connector with the multimeter's test lead or you'll damage the connector's metal terminal. Instead, use a back-probe on the connector or use a wire-piercing probe on the wire.

4. 4

   Connect the multimeter's black test lead to the battery's negative post.

5. 5

   Turn the key to the ON position (but engine OFF).

6. 6

   Your multimeter should show you 4.5 to 5 Volts DC.

OK, let's see what your test 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

The next step, after confirming that the MAP sensor is getting power, is to check it's Ground circuit.

What we'll do, using the multimeter in Volts DC mode, is make sure the MAP sensor has a good path to Ground.

IMPORTANT: Be careful when testing this circuit because this Ground is provided directly by the PCM. DO NOT short this wire (circuit) to battery power (12 Volts), or you WILL FRY the PCM.

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

1. 1

   Disconnect the MAP sensor from its electrical connector.

2. 2

   Place your multimeter in Volts DC mode.

3. 3

   Turn the key to the ON position but don't crank or start the engine.

4. 4

   With the black multimeter test lead probe the middle wire of the MAP sensor connector.
   
   This is the wire that connects to the pin labeled with the number 2 of the MAP sensor (see the image above).

5. 5

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

6. 6

   Your multimeter should show 10 to 12 Volts DC.

OK, let's interpret your test result:

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.