DTC P1128 – MAP Signal Lower Than Expected – My Pro Street

DTC P1128 – MAP Signal Lower Than Expected – My Pro Street


The check engine light is never a sign you want to see from your daily driven vehicle, but with the proper steps you can easily locate and resolve the problem at hand. In the case of this OBDII DTC P1128 found in Honda or Acura vehicles, this OBDII DTC Trouble code has to do with the Manifold Absolute Pressure sensor.

The exact verbiage for this trouble code is MAP Signal lower than Expected : DTC P1128 and it’s a sign that either the wiring harness in your engine bay or your MAP sensor is having a problem. There’s quite a few issues that can cause this check engine code, but it’s certainly one Malfunction Indicator Lamp (MIL) status that any shadetree mechanic working at home can fix.


Symptoms of DTC P1128


  • Sluggish acceleration
  • Poor Fuel economy
  • Bucking or irregular driving

What could be wrong?


DTC P1128 can also be accompanied by DTC P1129 which is for the MAP sensor sending too high of a voltage signal. Because your Honda or Acura relies on an accurate reading from the pressure sensor to operate your engine’s fuel and ignition trims, it’s a vital part of how your ECU controls your engine.

The MAP sensor reads vacuum all the way to up 10 psi of pressure, and when the voltage being sent to the ECU is too low (or too high) the OBDII DTC Trouble Code P1128 is triggered, or P1129 for too high of a signal

Today we’ll be showing you how to resolve the DTC P1128 trouble code in a 2005 Acura TSX with the K24A2 engine in it. This TSX carries a VIN Code of F, although all K24 equipped TSX’s will be the same in the way you will be testing the MAP sensor and wiring.


The tools you will need to complete the process of testing your MAP sensor and fixing your DTC P1128 error, are a phillips screwdriver and a voltmeter or multimeter. If you are not familiar with a voltmeter and don’t know how to use one, check out our handy Multimeter guide here.

Where is my MAP sensor located?

Your K24A2 engine has the manifold absolute pressure sensor located in the throttle body, and near the #4 intake runner of your intake manifold plenum. To find this sensor, open your hood and look near your coolant overflow tank. This Acura MAP sensor is connected by way of a three pin weatherproof connector.


This MAP sensor carries a Honda OE part number of 37830-PGK-A01, and we’ll be showing you how to test your wiring harness to make sure it’s reading your MAP sensor correctly.

Let’s begin our tutorial on servicing your Acura MAP sensor, by unplugging the MAP sensor and examining the wiring connector.


This three pin connector is broken up into three different terminals and values. They are labeled 1-3 in the diagram above. Before we test the MAP sensor, we’ll be checking the Acura wiring harness to make sure that the car is sending power and ground to the MAP sensor.

To fix your P1128 OBDII trouble code and check your engine harness, unplug the MAP sensor and refer to the chart above for harness arrangement. Remember you are testing the harness side of the MAP sensor, not the sensor at this time. Check the PIN 2 terminal at the wiring harness, this pin is the switched power source for your MAP sensor.

Turn your multimeter to the DC range and put the black lead on the negative terminal of your battery. With the red probe, gently touch the front of the MAP sensor connector to check for power. Make sure your key is in the ignition and turned to the “ON” position. If you have power at the middle terminal, now check the PIN 1 terminal on your engine harness for low reference signal or the ground supplied by your Acura ECU.


If you have power and ground at these two pins, it’s time to plug your MAP connector back into the sensor and backprobe the PIN 3. This can be a difficult task because of where your radiator and coolant overflow tank are located.

Try bending a paper clip into a L shape, and push it into the back of the PIN 3 connector. Using this method, you should be able to touch the red lead to this paperclip to read the voltage being sent from the MAP sensor.

From this point you can do one of two things to test the sensor output to your ECU. You can either use an air compressor to gently blow air into your intake tract, or you can start the car but using extreme caution as the cooling fans are just inches away from your fingers and tools. With the key set to the ON position, you should see the voltage being sent from the MAP sensor to your ECU.

When the pressure in your intake manifold is lower or greater vacuum, the MAP sensor sends the PCM a higher voltage, and when the pressure is higher the MAP sensor signal is lower.

If your MAP sensor is “lazy” or does not respond or even register a signal here, it’s time to replace your MAP sensor. As you do not require calibration, all it takes is for you to undo the two bolts that hold the MAP sensor in place, and replace it to resolve your DTC P1128 trouble code.

Having problems still? Not sure what you are doing is right? Have more questions about the DTC P1128? Leave us a message below!


  1. I have my multimeter set to the DC voltage range and all 3 pins are about 5v. Power and ground. Also when I backhole the pin and start the car pin 3 is also around 5v so if it has power and ground my map sensor is working what is the problem then? Still getting the p1128 code and my car still isn’t driving right.

    • Hi Alex, thanks for reading.

      If you’ve got 5 volts at all three pins, there’s a definite wiring issue in your vehicle or your MAP has failed in an odd way. Can’t say we’ve ever seen the 5 volts at all three terminals..

      Is there any other sensor that’s failed? It’s just your MAP sensor? Try replacing your MAP sensor if no other sensors are having issues. Check your TPS to see if a ground signal is present at the harness. If there’s a ground there, chances are your MAP is fried.

      Thanks for commenting, let us know how it turns out!

      • Hi I have 5 volts from pin 3 and 2.9 volts from pin 2 with map plugged in key on but not running. But when car is started I have volts from pin 3 and 1.2 volts from pin 2 . I’ve changed ecu and ran a separate map sensor plug derect to the ecu to cross out the harness being a problem and changed map sensor 3 different times and code still there

        • Hi Pierre, couple of questions for you sir.

          When you turn the key to start the car, you say you have X volts from PIN 3 and 1.2 volts from PIN 2. Are you still using the stock MAP sensor location? Are you sure you have tried checking continuity between the ECU and the MAP harness sir? Let us know and we can help!