Fuel trims: how to read them

Two numbers that show what the unit thinks of the mixture. They find an air leak before the lamp comes on.

The control unit does not know how much fuel is needed — it calculates it and then checks the result against the oxygen sensor. If the mixture came out different from the intention, it corrects the delivery. The size of that correction is the fuel trim.

There are two numbers, and they mean different things:

• Short term — the correction right now. It swings back and forth several times a second, and that is normal.
• Long term — the accumulated one. That is what shows an old problem: the unit has learned that it always has to add.

What counts as normal

The two together around zero, give or take five per cent. Ten per cent is a conversation, fifteen is trouble, twenty-five and above means the unit is at the end of its range and the lamp is coming.

Plus and minus are two different problems

A steady plus means the unit is adding fuel because the mixture comes out lean. Causes, cheapest first:

1. An air leak — a cracked hose, a boot that slipped off, a tired gasket.
2. A mass air flow sensor reading low.
3. A weak fuel pump or a clogged filter.
4. A tired oxygen sensor.

Beyond that lies P0171.

A steady minus means it is taking fuel away because there is too much.

1. A leaking injector.
2. Excessive rail pressure.
3. A dirty air filter.
4. An oxygen sensor reporting lean where it is rich.

Beyond that lies P0172.

A trick that halves the search in a minute

Look at the trims at idle, then at 2500 rpm.

• A large plus at idle and almost nothing at speed — an air leak. At idle the manifold is under vacuum and extra air gets in easily; at higher revs its share becomes negligible.
• About the same plus everywhere — not a leak but the supply: the air meter, the pump, the filter.

That one experiment splits a dozen causes in half and costs nothing.

Where to look

Fuel trims are in the live data of any OBD-II application. V engines have two pairs, bank 1 and bank 2; a difference between the banks is itself a hint which side to search.