The car has started jerking under acceleration, the idle is uneven, and the check engine light is on. Diagnostics throws up a code from the P0105 to P0109 series and the mechanic mentions the MAP sensor. In this article we explain what that sensor does, how to tell when it has failed, and how you can test it yourself before going for a replacement.
What the MAP sensor is and what exactly it measures
The MAP sensor (short for Manifold Absolute Pressure) measures the air pressure behind the throttle body, in that part of the engine where air mixes with fuel before entering the cylinders. The engine's computer, the ECU (Engine Control Unit), uses that signal to calculate how much fuel to inject and when to fire the spark plug.
At idle there's a vacuum in the intake manifold, so the MAP reads a low pressure, typically around 30-40 kPa. When you put your foot to the floor, the throttle opens wide, the vacuum disappears and the pressure climbs close to atmospheric, around 100 kPa. On turbocharged engines with an intercooler (TDI - turbo diesel with direct injection, TSI and TFSI - turbo petrols with direct injection), the MAP sensor is usually placed downstream of the intercooler and there it also reads turbo boost pressure, meaning over 200 kPa under full throttle.
The difference between MAP and MAF sensors
Drivers often confuse MAP and MAF (Mass Air Flow) sensors. The difference is simple. The MAF sits in the pipe between the air filter and the throttle body and measures the mass of air entering the engine. The MAP is screwed into the intake manifold or pushed through a rubber adapter behind the throttle and measures pressure.
Many older petrol engines run with a MAP sensor only (the so-called speed-density system, where the ECU calculates air mass from pressure, temperature and engine rpm). Most modern turbo engines have both: a MAF at the air inlet and a MAP downstream of the intercooler. So a faulty MAP sensor doesn't mean the MAF is fine, or the other way round, and both should be checked separately on diagnostics.
Symptoms of a faulty MAP sensor
The most common signs that the MAP sensor is lying or has failed:
- jerking and bucking under acceleration, especially in the mid rpm range
- loss of power, the engine doesn't pull like it used to
- uneven or high idle, occasionally stalling at the lights
- higher fuel consumption
- black smoke from the exhaust on petrol engines, a rich mixture
- a diesel that bogs down when you press the throttle
- the check engine light and codes from the P0105, P0106, P0107, P0108 or P0109 series on OBD (On-Board Diagnostics, the standard diagnostic protocol)
A failure is often not a complete loss of signal but a drift. The sensor starts reading a value 10-20 kPa off the actual one, the ECU trusts that number and meters fuel incorrectly. In half the cases the car still drives, just worse, and the driver thinks it must be the spark plugs or the filters.
How to test the MAP sensor
Before swapping the part, always confirm the sensor is actually lying. Three steps worth going through:
- Visual. Look at the vacuum hoses around the sensor. Cracked, soft or loose rubber hoses often produce the same symptoms as a faulty sensor and cost a couple of marks. If the sensor sits in a rubber adapter, that adapter is known to crack with age and heat.
- 5V supply. With a multimeter, check that the sensor is getting supply voltage from the ECU. Typically there are three or four pins: 5V supply, ground, signal, and on some sensors the intake air temperature. Without supply the sensor obviously won't work, and there the problem is in the wiring, not the sensor.
- Diagnostics and live data. On modern digital sensors the most reliable check is diagnostics that reads the signal in real time. Start the engine, watch the MAP value at idle (it should be around 30-40 kPa), then briefly open the throttle and watch it climb. If at idle it shows a value close to atmospheric (around 100 kPa) instead of 30-40 kPa, either the sensor or a hose is lying.
For analogue sensors a multimeter is enough, you measure the signal voltage and watch how it changes as you open the throttle. For digital sensors on the CAN bus a multimeter doesn't make much sense, you need a diagnostic tool there.
Cleaning, replacement, and what to watch for on LPG cars
Before you order a new sensor, try cleaning it. Take it off the intake manifold, spray it with CRC contact cleaner (an electronic contact cleaning spray, not MAF cleaner and definitely not petrol) and let it dry. You'd be surprised how often deposits of soot and oil vapour from the crankcase breather bring the sensor back to life. At the same time, replace the vacuum hose if it's cracked.
If the diaphragm or chip inside the sensor is damaged, cleaning won't help and a new part is the only option. Pay attention to the original part number, because two sensors that look identical can have different calibrations and the ECU won't run properly with the wrong one.
LPG cars deserve special attention. Many sequential LPG systems (STAG, Lovato, BRC and others) follow the petrol MAP sensor signal in order to correct gas dosing. When the MAP lies, the LPG computer gets a wrong picture of engine load and the gas mixture goes in the wrong direction. The driver then thinks the problem is on the LPG side - the reducer, filters, injectors - when it's actually the petrol sensor pulling the whole system by the nose. Auto Gas Gaga handles all work on LPG systems, so in cases like these we always check the MAP first before touching the gas side.
When to head to a workshop
If you've passed the visual check, replaced a suspect hose, and the car still doesn't behave normally, it's time for the workshop. In the workshop we first look at live data on diagnostics and compare the MAP signal with the MAF, rpm and intake temperature. Only when we see the sensor really is lying do we change the part.
The cost of replacement depends on the car model, whether the part is original, and availability in BiH, so it falls into a wide range. The specific price for your engine is best confirmed right before the job. The price depends on the condition of the car and the workshop - get in touch for an estimate.
If you're not sure what exactly is lying under the hood, book an appointment and we'll run diagnostics on the spot. Better to check now than to drive for months with the wrong mixture and ruin the catalytic converter or oxygen sensor.