The MAF sensor: signs of wear and a live-data check

A mass air flow sensor lies gradually. The car still drives, just worse — and the numbers show it before you feel it.

The mass air flow sensor (MAF) measures how much air entered the engine. From that number the unit works out the fuel delivery. If the sensor is wrong, the whole mixture is wrong.

How it shows up

A dead sensor is obvious: the engine will not start or it stalls. Far more common is the other case — the sensor reads low:

• A flat spot when you press the throttle hard, then the car picks up.
• Hunting idle speed.
• Fuel consumption up for no obvious reason.
• Not enough pull under acceleration, with no lamp on.

All of that feels vague. The numbers in live data are clearer.

What to look at

The main parameter is the air flow itself, in grams per second. Its magnitude depends on engine size, so memorising someone else's figure is pointless. A rule is more useful:

At idle on a warm engine the flow in grams per second is roughly the engine size in litres times 2–3. For a 1.6 that is about 3–5 g/s, for a 2.0 about 4–6 g/s.

And a second, more reliable one: at 3000 rpm with no load the flow should rise about five- or sixfold compared with idle. If it only doubled, the sensor is reading low.

Alongside the flow, watch the fuel trims. A sensor reading low produces a steady positive trim: the unit sees the mixture is short and adds beyond the calculation. The same sign appears with an air leak.

The codes

• P0100 — sensor circuit.
• P0101 — reading outside the expected range. The most common code with a sensor that reads low.
• P0102 / P0103 — signal too low or too high.

What to check before replacing

1. The air filter. A clogged filter changes the flow picture. Pennies against the price of a sensor.
2. An air leak downstream of the sensor. Air that came in past the MAF does not exist as far as the unit is concerned. Look for cracked boots, dislodged hoses, a flattened throttle body gasket.
3. A dirty sensing wire. The sensor is cleaned with a dedicated cleaner — only that, not brake cleaner and not a cloth: the wire is thinner than a hair. It does not always work, but it costs almost nothing.
4. The connector. A corroded contact gives exactly the symptoms of a tired sensor.

The disconnect test

An old trick: unplug the MAF with the engine running. The unit falls back to calculating from throttle position. If the car then drives better, the sensor is lying, and almost certainly reading low.