7 Symptoms of a Broken MAP Sensor

car engine with MAP sensor highlighted
A Failing MAP Sensor Could Stop this Engine in its Tracks!.

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In modern engines, the engine control module (ECM) measures or calculates air flow via either a mass air flow (MAF) or manifold absolute pressure (MAP) sensor. Turbocharged engines may use both, but naturally-aspirated engines typically use one or the other. If the MAP sensor is failing or broken, the ECM – and thus the engine – can't function properly. By maintaining and repairing your MAP sensor, you'll keep your engine running smoothly.

How a MAP Sensor Works

2002 Subaru Outback 2.5 MAP sensor highlighted
This MAP Sensor is Mounted Directly to the Intake Manifold, but Others Might Be Connected by a Hose. https://www.flickr.com/photos/104031407@N06/28146737598

The ECM uses MAP sensor data to run crucial calculations such as engine load, fuel injector pulse, and spark advance. When at rest, the MAP sensor reads atmospheric pressure at sea level (29.93 in. Hg). Because atmospheric pressure varies with weather and altitude, the ECM calculates this “zero” point just before the engine starts, fine-tuning spark and fuel injection mapping from that point.

When idling, intake pressure usually ranges from 16-22 in. Hg. Because this is lower than atmospheric pressure, air rushes into the intake. When the driver uses the engine to brake, pressure can go as low as 10 in. Hg. Upon accelerating, however, the open throttle body allows air to rush in faster, increasing pressure in the intake. At wide-open throttle, intake and atmospheric pressure are nearly equal.

Signs of a Broken MAP Sensor

check engine light in a late-model car
MAP Sensor Problems Could Trigger a DTC and Check Engine Light.  https://www.gettyimages.com/license/689808938

MAP sensors fail by getting clogged, contaminated, or damaged. Sometimes, engine heat “overcooks” the MAP sensor's electronics or cracks vacuum lines. If the MAP sensor goes bad, the ECM can’t accurately calculate engine load, which means the air-fuel ratio will become either too rich (more fuel) or too lean (less fuel).

So, how will you know that your MAP sensor is going bad? Here are the key problems to look out for:

  1. Poor Fuel Economy. If the ECM is reading low or no vacuum, it assumes the engine is at high load, so it dumps in more fuel and advances spark timing. This leads to excessive fuel consumption, poor fuel economy, and possibly detonation.
  2. Lack of Power. If the ECM is reading high vacuum, it assumes the engine load is low, so it cuts fuel injection and retards spark timing. On the one hand, fuel consumption will go down, which seems like a good thing. However, if too little fuel is consumed, the engine may lack power for acceleration and passing.
  3. Failed Emissions Inspection. Because fuel injection doesn’t correspond to engine load, a broken MAP sensor can lead to an increase in harmful emissions. Excessive fuel generates higher hydrocarbon (HC) and carbon monoxide (CO) emissions, while insufficient fuel may lead to higher nitrogen oxide (NOx) emissions.
  4. Rough Idle. Insufficient fuel injection starves the engine for fuel, leading to rough idling and perhaps even random cylinder misfiring.
  5. Hard Starting. Similarly, an excessively rich or lean mix makes the engine hard to start. If you can only start the engine when your foot is on the accelerator, you probably have a MAP sensor problem. 
  6. Hesitation or Stalling. When starting from a stop or trying a passing maneuver, stepping on the gas might not give you any joy, especially if the ECM is giving you a lean mixture based on faulty MAP sensor readings.
  7. Check Engine LightDepending on the age of your vehicle, MAP sensor diagnostic trouble codes (DTC) may range from simple circuit or sensor faults to correlation or range faults. A dead MAP sensor won’t read anything, while a failing MAP sensor might give the ECM data that makes no sense, such as low engine vacuum when the throttle position sensor (TPS) and crankshaft position sensor (CKP) show the engine at idle. 

What You Can Do About MAP Sensor Problems

Bluetooth OBD2 scan tool
A Bluetooth OBD2 Scan Tool is an Inexpensive but Powerful Tool for Diagnosing All Kinds of Engine Problems, Such as a Failing MAP Sensor. https://www.publicdomainpictures.net/en/view-image.php?image=252067

A functional MAP sensor is a critical part of the maintenance of your vehicle. If you suspect that you may have a problem with your MAP sensor, check the following elements first before racing to replace the sensor. 

  1. Electrical. Start by checking the connector and wiring. The connector should be securely connected, and the pins should be clean and straight. Corrosion or bent pins can cause MAP sensor signal problems. Similarly, the wiring between the ECM and MAP sensor should be intact. Chafing could cause short circuits, and breaks could cause open circuits.
  2. Hose. Some MAP sensors are connected to the intake manifold by a hose. Check that the MAP sensor hose is connected and intact. Also, check that the port is free of carbon deposits or other debris, which could block the hose and lead to poor MAP sensor readings.
  3. Sensor. If the sensor is connected properly, both electrically and to the intake manifold, use a scan tool or voltage meter and vacuum gun to check MAP sensor output. You’ll have to look up a chart to measure voltage against no vacuum and full vacuum. If MAP sensor output doesn’t match the chart, it’s safe to say the sensor should be replaced.