What does a short-term fuel trim of -20 indicate?
4 Answers
Short-term fuel trim refers to a correction value stored in permanent memory, meaning the memory is retained even after the ignition is turned off. This value itself holds little significance as it is merely an adjustment parameter. Below is expanded information on short-term fuel trim: 1. Overview: Short-term fuel trim is a real-time correction control of fuel injection volume based on oxygen sensor feedback regarding the air-fuel mixture's richness or leanness during previous operating cycles. 2. Air-fuel mixture concentration: When the mixture is too rich (high concentration), the oxygen sensor's output voltage increases, causing the short-term fuel trim to decrease - manifested as reduced fuel injection pulse width, displaying values below 1 on diagnostic tools. Conversely, when the mixture is too lean, short-term fuel trim increases, showing values greater than 1 on diagnostic equipment.
I had this same trouble code pop up on my car recently, and it took me forever to figure it out. A -20% value means the car is desperately reducing fuel injection, most likely due to an overly rich air-fuel mixture. Don't rush to replace parts yet—checking real-time data with a diagnostic tool is the most reliable approach. Focus on the oxygen sensor voltage; if it's consistently stuck above 0.7V, there's an 80% chance it's an air-fuel ratio signal issue. In my case, it turned out to be a faulty mass air flow sensor—when I opened it up, the blades were clogged with willow catkins. Another hidden culprit could be a stuck-open purge valve in the charcoal canister, which would cause a strong gasoline smell from the fuel tank. Also, test the fuel pressure; anything over 300kPa definitely indicates a problem with the pressure regulator's relief valve. Remember to check both cold-start and warm-engine data, as short-term fuel trim negative values tend to fluctuate more in winter.
I've encountered this issue three times. A short-term fuel trim of -20% is the ECU's self-rescue behavior. Simply put, the oxygen sensor before the exhaust pipe detects the exhaust and says 'too rich,' prompting the engine control unit to reduce fuel injection. Focus on checking the fuel system: Spray carburetor cleaner at the intake manifold seams and listen to the engine RPM. If the RPM surges like it's on stimulants, it's definitely a vacuum leak at the intake manifold gasket. Leaking fuel injectors are also common—after turning off the engine, remove the fuel rail to check for fuel droplets. Last week, while helping a neighbor inspect their car, I found a faulty coolant temperature sensor causing the ECU to misjudge a cold start and inject extra fuel. Replacing it with a fifty-dollar sensor fixed the issue immediately.
Check out my car repair notes: Short-term fuel trim -20% ≈ deducts the equivalent of 1 injection out of every 5 sprays. Common culprits fall into three categories: First, sensing errors—front O2 sensor poisoning/intake air temperature sensor false reporting/MAF sensor fouling; Second, execution errors—excessive fuel pressure/fuel injector dripping/purge valve stuck open; Third, air leak actors—crankcase ventilation valve diaphragm rupture or brake booster hose cracking. The most extreme case was a car with aftermarket air intake causing abnormal airflow turbulence, making the factory MAF sensor readings inaccurate. Recommendation: Start by reading freeze frame data to identify engine operating conditions during data anomalies before troubleshooting.
