Fuel Trims on the 3.3T Explained: STFT, LTFT, and What They Mean

What Are Fuel Trims?

Fuel trims are how your ECU keeps things on target. They're shown as percentages, and they represent how much the ECU is correcting fuel delivery in real-time.

There are two main types:

• STFT (Short-Term Fuel Trim): Live, moment-to-moment corrections based on the oxygen sensors.
• LTFT (Long-Term Fuel Trim): Learned corrections that smooth things out over time. The ECU uses this to "remember" consistent trends.

Here’s what the numbers mean:

• +10% trim = ECU is adding 10% more fuel than it expected
• –10% trim = ECU is pulling 10% because it's richer than expected

A higher fuel trim does not mean your engine is running lean—it means it would be lean if the ECU weren't correcting for it. As long as the ECU has enough range to compensate, you're still running safely at the correct AFR.

Logging using a JB4? Trims are displayed a bit differently..

JB4 is a popular piggyback and logger on this platform—but its fuel trim display is offset.

On JB4 logs, a value of 25.00 = 0% STFT.

So if you see 27.00, that means a +2% correction.

And if you see 23.00, that means a –2% correction.

Also, as of writing, JB4 does not show LTFT values—only STFT with the offset applied.

Just something to be aware of if you’re comparing to a scanner or tuner log.

What’s Normal?

What counts as “normal” for fuel trims really depends on how your car is set up:

• Using a piggyback (JB4, ProTuner, RaceChip, etc.)? → Check out the “What About Piggybacks?” section
• Running auxiliary fueling (CPI or WMI)? → See “How Auxiliary Fueling Affects Trims”
• Using both? → You’ll want to read both sections and consider how the effects stack.

But if you’re just running an ECU tune—or using your piggyback strictly for logging—then there’s a pretty well-established baseline:

• –5% to +10% is perfectly normal
• +20% or more often means you’re outside the ECU’s expected range (possibly due to ethanol, airflow mods, or a fueling issue)
• Once the ECU hits its correction limit (usually ±25%), it will trigger a fault

How Auxiliary Fueling Affects Trims

When you’re running CPI (Charge Pipe Injection) or WMI (Water-Methanol Injection), you're injecting fuel outside the ECU’s control. Since this fuel doesn’t go through the factory DI system, the ECU has no idea it’s there—so it doesn’t reduce direct injector duty in response. (at-least, not until the fuel trim system starts doing its job)

The result? The ECU delivers what it thinks is the right amount of fuel—but the O2 sensors detect a rich condition, and the ECU pulls fuel to compensate. In your trims, this shows up as:

• Negative STFTs
• Sometimes unusually low LTFTs if the system adapts long-term

This is totally normal on aux setups without a matching tune. With a proper tune, we can calibrate the base fueling so trims stay closer to where they should be—but because the ECU still can’t “see” the aux fuel directly, some trim correction is always expected.

What About Piggybacks? (JB4, ProTuner, RaceChip, etc.)

Piggybacks like JB4 increase boost by intercepting and altering sensor signals—usually from the MAP or TMAP sensors—so the ECU thinks it's making less boost than it really is.

That means:

• More air is entering the engine than the ECU is aware of
• The O2 sensors detect extra oxygen
• The ECU responds by raising fuel trims to correct the air/fuel ratio

This is why piggybacked setups with boost increases often show +10–20% STFT, even on stock fuel. It’s the ECU doing its best with incomplete information.

P0171, P0172, P0174, P0175: What the Codes Mean

These common fuel trim codes show up when the ECU detects that the average trim stays too rich or too lean during steady-state cruising.

• P0171 / P0174: System too lean (Bank 1 / Bank 2)
• P0172 / P0175: System too rich (Bank 1 / Bank 2)

These codes don’t mean your AFR is actually wrong in the moment—it means the ECU has had to correct too much for too long.

Typical causes include:

• Running ethanol or gasoline without a matching tune (e.g., E50+ on a stock map, or 93 on a E85 map)
• Vacuum leaks, which lean out the mixture
• Bad sensors, especially MAF or O2

These codes are often resolved by adjusting the tune to match your actual fuel type and setup—especially when switching between gasoline and ethanol blends.