OBD II trouble code P1128 MAP Sensor Signal Lower 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 P1128 and how to test the MAP sensor in a step-by-step manner.
Contents of this tutorial:
- P1128 Basics You Need To Know.
- How The MAP Sensor Works.
- Symptoms Of A P1128 Diagnostic Trouble Code.
- How To Diagnose Trouble Code P1128: MAP Sensor Signal Lower Than Expected.
- TEST 1: Verifying The MAP Sensor Has Power.
- TEST 2: Verifying The MAP Sensor Has Ground.
- TEST 3: Verifying The MAP Signal With A Multimeter.
- MAP Sensor Is Good But Code Doesn't Go Away!
- Where To Buy Your Honda MAP Sensor Cheaper.
- More Honda Test Tutorials.
P1128 Basics You Need To Know
To better understand why the PCM sets a P1128: MAP Sensor Signal Lower Than Expected trouble code, we need to know that for any given engine operating condition (such as: idle, hard acceleration, deceleration, etc.) the PCM is pre-programmed to know how much intake manifold vacuum there should be.
This info is based on several inputs like: engine speed (crank position sensor), throttle angle (throttle position sensor), engine coolant temp (ECT sensor), etc.
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 P1128 (remember, it's the MAP sensor's job to measure intake manifold pressure).
To be a bit more specific, it's when the MAP sensor doesn't report a high vacuum input, when it should, that a P1128 trouble code sets on your Honda (and lights up the check engine light).
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:
- 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.
- 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 higher voltage.
- The PCM, using the inputs from the MAP sensor and other sensors, injects more fuel into all its cylinders.
As you already know, the MAP sensor has 3 wires in its connector. Each one has a specific job. Here's a brief description of the function of each:
- 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.
- 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.
- 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 computer.
Symptoms Of A P1128 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:
- Check engine light (CEL) is shining nice and bright on the instrument cluster.
- DTC P1128 is present.
- DTC P0107 MAP Sensor Circuit Low Voltage.
- Rough idle.
- ‘Rotten egg’ smell coming from the exhaust.
- Won't pass the state mandated emissions test.
- Bad gas mileage.
- Lack of power, rough idle, or hesitation.
- Engine cranks a long time before starting.
Let's go to the next subheading and let's get testing!