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

OBD II trouble code P1129 MAP Sensor Signal Higher Than Expected is usually caused by a bad MAP sensor, but not always.

In a nutshell, this trouble code is telling us that the PCM (Powertrain Control Module = Fuel Injection Computer) knows the MAP sensor is not reporting the correct air pressure value for the current engine operating conditions.

But, this is as far as the computer can go since it can't know for a certainty that the MAP sensor is fried or if something else is causing the problem (it's up to us to find out).

In this tutorial I'll go over the basics you need about trouble code P1129 and how to test the MAP sensor in a step-by-step manner.

P1129 Basics You Need To Know

To better understand why the PCM sets a P1129: MAP Sensor Signal Higher Than Expected  trouble code, we need to know that the fuel injection system on your Honda (or Acura) is a speed density type.

Which simply put (and I want to emphasize the words: ‘simply put’) means that the fuel injection computer uses engine speed, intake air temperature, and engine load to calculate the amount of air the engine is breathing.

Once the fuel injection computer calculates this amount of air, then it now calculates the correct amount of fuel to inject into the cylinders.

The MAP sensor is the main component that helps the fuel injection computer calculate engine load. How? By measuring the amount of vacuum the engine is creating. As you're already aware, engine vacuum varies greatly depending on how much you're loading your Honda's engine (or, in other words, how much your mashing on the gas pedal).

The PCM is pre-programmed with a table of MAP values for any given engine operating condition (such as: idle, hard acceleration, deceleration, etc.) and when calculating the correct air/fuel mixture values, it takes into consideration throttle angle (from the throttle position sensor), engine speed (from the crank position sensor), and a few others to further fine tune the air/fuel mixture the engine needs to run optimally under any operating load condition.

It's when the MAP sensor reports a vacuum pressure that's not within the range of the actual operating condition of the engine, that the PCM sets a P1129.

To be a bit more specific, it's when the MAP sensor doesn't report a low vacuum input, when it should, that the PCM sets P1129.

How The MAP Sensor Works

As stated above, the MAP sensor is tasked with measuring the amount of vacuum the engine is creating.

The amount of vacuum pressure varies depending on engine load. Here are some more specifics:

1. At idle, manifold vacuum is higher.
   • When manifold vacuum is higher, the MAP sensor sends the PCM a lower voltage.
   • The PCM, using the inputs from the MAP sensor and other sensors, injects less fuel into all its cylinders.
2. Under load (let's say accelerating the engine to pass someone on the highway), manifold vacuum is lower.
   • When manifold vacuum is lower, the MAP sensor sends the PCM a bigger voltage.
   • The PCM, using the inputs from the MAP sensor and other sensors, injects more fuel into all its cylinders.

Here's a brief description of the MAP sensor's 3 circuits:

1. One wire supplies 5 Volts DC.
   • This is the MAP sensor's power source.
   • 5 Volts are fed only with the Key On Engine Off (KOEO) or Key On Engine Running (KOER).
   • The fuel injection computer (PCM) feeds these 5 Volts to the MAP sensor.
   • MAP sensor pin labeled with the number 1, in the image above, gets this power.
2. One wire feeds Ground.
   • In tech circles, this circuit is know as the low reference circuit.
   • The PCM provides this Ground internally.
   • Ground provided by the middle wire of the MAP sensor connector. MAP sensor pin labeled with the number 2, in the image above, gets this Ground.
3. One wire is the MAP sensor signal circuit.
   • This wire sends the signal the MAP sensor creates to the computer (PCM).
   • MAP sensor pin labeled with the number 3, in the image above, outputs this MAP signal to the fuel injection computer.

Symptoms Of A P1129 Diagnostic Trouble Code

Since the MAP sensor's vacuum pressure input is used to calculate engine load and thus fuel injection and timing advance, when the MAP sensor fails, the PCM won't be able to keep your Honda (or Acura) running smoothly.

You'll see one or more of the following symptoms:

1. Check engine light (CEL) is shining nice and bright on the instrument cluster.
2. DTC P1129 is present.
3. DTC P0108 MAP Sensor Circuit High Voltage.
4. Rough idle.
5. ‘Rotten egg’ smell coming from the exhaust.
6. Won't pass the state mandated emissions test.
7. Bad gas mileage.
8. Lack of power, rough idle, or hesitation.
9. Engine cranks a long time before starting.

Where To Buy The MAP Sensor

If the MAP sensor failed the bench test and you need to buy one, check out the following recommendations. I think they'll help you comparison shop for the MAP sensor (of known automotive name brands -no knock-off sensors):

Not sure if the MAP sensor will fit your particular Honda? Don't worry. Once you get to the site, they'll ask you for the specifics of your particular Honda (or Acura) vehicle to make sure it fits. If it doesn't, they'll show you several that will. It's that easy to find the right part!