Fuel trims explained: short term, long term and what's normal

What fuel trim is

The engine computer calculates how much fuel to inject based on airflow, engine speed and other inputs. Then it checks the upstream oxygen sensor to see if the result is lean or rich. If the sensor says lean, it adds fuel. If it says rich, it takes fuel away. That correction is fuel trim, shown as a percentage.

Fuel trim only works in closed loop, after the engine and oxygen sensors are warm. In open loop, such as right after a cold start or at wide open throttle, the computer ignores the sensor and trims stay at or near fixed values.

Short term vs long term fuel trim

Short term fuel trim (STFT) reacts in fractions of a second. With a narrowband upstream sensor it swings back and forth around zero as the sensor switches between lean and rich. This is normal.

Long term fuel trim (LTFT) is the learned average. If STFT keeps leaning one way, the computer moves LTFT in that direction so STFT can return toward zero. LTFT is stored in memory and is erased when you clear codes or, on many cars, disconnect the battery.

Add them together for the total fuel trim. That total is what tells you how far off the base mixture is. If LTFT is plus 12 and STFT is plus 3, the computer is adding about 15 percent fuel.

Note.Exact limits for setting codes vary by manufacturer. Treat the numbers above as general rules of thumb, not fixed thresholds.

Use idle vs cruise to narrow it down

One of the most useful tests is comparing trims at idle with trims at about 2,500 rpm or a steady cruise. Do it with a warm engine and the scanner showing STFT and LTFT for each bank.

• Lean at idle, close to normal at higher rpm: classic vacuum leak. A small leak is a big share of total airflow at idle but a tiny share at higher rpm. Check intake hoses, PCV hoses, brake booster line and intake gaskets.
• Lean everywhere or worse under load: fuel delivery. Think weak fuel pump, clogged filter, failing pressure regulator or restricted injectors.
• Lean everywhere, roughly the same: the MAF sensor may be reading low. A dirty MAF is a common cause.
• Rich at idle: look at a purge valve stuck open, a leaking injector or high fuel pressure.
• Only one bank off on a V engine: the problem is on that side, such as a vacuum leak at that bank's intake gasket or an exhaust leak ahead of that sensor.

Fuel trims and common codes

When total trim stays near the correction limit, the computer sets a fuel system code. P0171 and P0174 mean system too lean on bank 1 and bank 2. P0172 means system too rich on bank 1. Misfires such as P0300 can also appear with a lean condition. Unburned oxygen from a misfiring cylinder makes the oxygen sensor read lean, so the computer adds fuel even though the real problem is ignition.

Freeze frame data stores fuel trims at the moment the code set. Comparing those numbers with live data during a test drive is a fast way to confirm a repair. See freeze frame data explained.

How to read fuel trims with a scanner

Any scanner that shows live data can read fuel trims. A tool that can graph several values at once makes the swings much easier to see. Our guide to the best OBD2 scanners for DIY mechanics covers tools with graphing, and the wireless Bluetooth scan tool guide covers phone and tablet options.

1. 1

   Warm the engine fully so it enters closed loop.

2. 2

   Open live data and select STFT and LTFT for bank 1 and bank 2, plus rpm and coolant temperature.

3. 3

   Record the numbers at idle.

4. 4

   Hold about 2,500 rpm in park or neutral, or cruise steadily, and record again.

5. 5

   Enter the numbers into the fuel trim reader and compare the pattern with the list above.

Check your numbers in the fuel trim reader →

Frequently asked questions

What is a normal long term fuel trim? Close to zero is ideal. Values within about plus or minus 10 percent are generally considered normal on a healthy engine.

Is positive fuel trim lean or rich? Positive means the computer is adding fuel because the oxygen sensor reads lean. Negative means it is removing fuel because it reads rich.

Why does short term fuel trim jump around? It reacts to every switch of the upstream oxygen sensor. Fast swings around zero are normal. What matters is where the average sits.

Does clearing codes reset fuel trims? On most vehicles, yes. Long term fuel trim returns to zero and relearns over the next drives.