Safety Warning
DIY auto repair can cause serious injury, fire, or vehicle damage. These guides are for informational purposes only. Always follow OEM torque specs, wear PPE, and consult a certified mechanic if you are unsure. You are solely responsible for your safety.
Fuel Trim System Monitor Diagnostic Operation
In order to provide the best possible combination of driveability, fuel economy, and emission control, the Powertrain Control Module (PCM) uses a closed loop air/fuel metering system. The PCM monitors the heated oxygen sensor (HO2S) signal voltage and when in closed loop adjusts fuel delivery based on the HO2S signal voltage. Changes in fuel delivery will be indicated by the long term and the short term fuel trim values that are displayed on the scan tool. The ideal fuel trim value is around zero percent. The PCM will add fuel when the heated oxygen sensor signal is indicating a lean condition. Additional fuel is indicated by fuel trim values that are above zero percent. The PCM will reduce the amount of fuel delivered when a rich condition is indicated by the HO2S. Fuel trim values below zero percent indicate a reduction in fuel. A fuel trim DTC can be set when exhaust emissions reach excessive levels because of a lean or rich condition.
In order to meet OBD-II requirements, the control module uses weighted fuel trim cells in order to determine the need to set a fuel trim DTC. A fuel trim DTC can only be set if the fuel trim counts in the weighted fuel trim cells exceed the specifications. A vehicle that has a fuel trim problem that is causing a concern under certain conditions but operates fine under other conditions may not set a fuel trim DTC. For example an engine that is idling high due to a small vacuum leak or an engine that is running rough due to a large vacuum leak may set an idle speed DTC or an HO2S DTC but not a fuel trim DTC. A fuel trim DTC may be triggered by many different vehicle faults. Use all of the diagnostic information available when diagnosing a fuel trim fault. See FUEL TRIM OPERATION table.
| HO2S | Short Term FT Action | Long Term FT Action | Short Term FT Response |
|---|---|---|---|
| Lean Condition | Quick Increase Of 1% or More | Slow Increase Of 1% or More | Return To Zero Per Cent |
| Desired Condition | Stays Near Zero Per Cent | Stays At Learned +/- Value | Stays Near Zero Per Cent |
| Rich Condition | Quick Reference Of 1% or Less | Slow Decrease Of 1% or Less | Return To Zero Percent |
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When to See a Mechanic
Stop DIY work and contact a certified mechanic immediately if any of the following apply:
- • You smell fuel, burning insulation, or see smoke.
- • Brakes feel soft, pull hard to one side, or make grinding noises.
- • The engine overheats, stalls repeatedly, or misfires under load.
- • You are missing required tools, torque specs, or safe lifting equipment.
- • You are not confident in the next step or safety outcome.