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Recognizing a Broken MAP Sensor

A broken MAP sensor, like this one, can create engine knock and cause you to fail an emissions test.

Engines need a lot of things to run smoothly. There’s a small army of sensors that regulate vehicle operation and performance (or lack thereof), including the manifold absolute pressure, or MAP sensor. A broken MAP sensor can present itself in several ways. If left unaddressed for an extended period of time, it can cause headaches. Learn the signs, and take action if you suspect an issue.

How It Works

The proper ratio of air to fuel is a big factor in keeping your engine running, and the MAP sensor plays a key role in keeping this balance in check. For simplicity’s sake, let’s say the MAP sensor measures the volume of air flowing through an engine. The reality is more complicated. It measures the vacuum inside the manifold created by downward strokes of the piston and calculates the absolute pressure with regard to atmospheric pressure. Atmospheric pressure is normally 14.7 psi at sea level, but changes with altitude. The MAP sensor measures barometric fluctuations and gives feedback about air flow. The ECU (electronic control unit or main computer) then uses that information to determine the right amount of fuel to inject and when to send a spark.

MAP sensors use power, ground and signal wires to report to the ECU, and most commonly fail because they’ve taken on water or just gotten old. When this happens, you might notice the following issues.

Fuel Consumption Issues

If the ECU is receiving inaccurate or missing data about air flow, it will make a guess as to how much fuel to send into the chamber for combustion. Without specifics, this results in a lean (too little fuel) or rich (too much fuel) mixture. When the ratio is off, your engine can’t be as efficient as it was designed to be, and fuel consumption will rise. Furthermore, a consistently rich mixture will cause nasty buildup of gunk and heat, resulting in untimed detonation (engine knock). Prolonged detonation can lead to major engine damage.

Performance ProblemsRecognizing a Broken MAP Sensor

Without efficient combustion, you lose power. As is the case with lean mixtures, there is not enough fuel supplied for a big enough explosion in the combustion chamber. This means performance suffers noticeably. Maybe it’s a rough idle, because the system is allowing too much air in. It can also show up as hesitation when accelerating or shifting into drive. Severe cases might cause stalling on startup.

Bad Scans

Generally, your car will let you know something is up with a check engine light. A trained mechanic can plug in an OBD II device that scans the vehicle’s computer for trouble codes. If a code indicating trouble with a MAP sensor shows up, remember that it could be the sensor itself, but it could also be a vacuum issue. A professional will know how to check the lines for any leaks or clogs. On the rare occasion you’re not alerted on your dash, you might find you’ve failed an emissions test. Again, a mechanic will be able to sort out whether it’s the MAP sensor itself, another part of the air delivery system or something entirely different.

Remember, many of these symptoms could point to other problems, so don’t jump to conclusions without a scan and a little professional diagnosis.

Check out all the relays, sensors and switches available on NAPA Online or trust one of our 17,000 NAPA AutoCare locations for routine maintenance and repairs. For more information on the effect of a broken MAP sensor on your vehicle, chat with a knowledgeable expert at your local NAPA AUTO PARTS store

Photo courtesy of Flickr.


Blair Lampe View All

Blair Lampe is a New York-based professional mechanic, blogger, theater technician, and speechwriter.  In her downtime she enjoys backpacking wherever her boots will carry her, rock climbing, experimental theatre, a crisp rosé , and showering love on her 2001 Sierra truck.

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