Fuel Trim Explained: STFT vs LTFT for Humans
Key Takeaway
Fuel trims are the most useful live data your car produces. Here's how to read them like a pro.
What Are Fuel Trims?
Fuel trims show how much the ECM is adjusting the fuel injection to maintain the correct air/fuel ratio. They're expressed as a percentage.
STFT vs LTFT
| Type | Full Name | What It Shows | Response Speed |
|---|---|---|---|
| STFT | Short Term Fuel Trim | Current, real-time adjustment | Instant |
| LTFT | Long Term Fuel Trim | Learned, averaged adjustment | Slow (over time) |
How to Read the Numbers
| Value | Meaning | Health |
|---|---|---|
| 0% | No correction needed | Perfect |
| +1% to +10% | Adding fuel (slightly lean) | Normal |
| -1% to -10% | Removing fuel (slightly rich) | Normal |
| +10% to +25% | Significant lean correction | Investigate |
| > +25% | Severe lean condition | P0171 likely |
| -10% to -25% | Significant rich correction | Investigate |
| < -25% | Severe rich condition | P0172 likely |
What Fuel Trims Tell You
| Trim Pattern | Likely Cause |
|---|---|
| High positive LTFT + normal STFT | Vacuum leak (constant air ingress) |
| High positive STFT + LTFT catching up | Developing fuel delivery issue |
| High negative trims | Running rich — injector leak or MAF issue |
| One bank positive, other normal | Bank-specific vacuum leak |
ProHow Steer Helps
Steer displays fuel trim data in real-time and flags abnormal trends before they trigger a code.
Fix Any Engine Issue with Steer
Get instant plain-English explanations for any engine code on your iPhone. Steer stays connected 24/7 to monitor your car's health.
