Fuel trim diagnosis, step by step

The diagnostic order of operations, in ten steps

1. Is the data worth reading? Fully warm, fuel system in closed loop in the idle, 2,500 and cruise cells. Trims taken during warm up or in open loop are not a fingerprint of anything.
2. Codes and the freeze frame. Note what is stored and where it set. A code tells you which monitor failed, not which part. The freeze frame tells you the conditions, which is often the more useful half.
3. Misfire counters before trims. A misfire sends unburned oxygen past the upstream sensor and makes the trims go positive. If one cylinder has counts, deal with that first: the trims are reacting to it, not causing it.
4. Is the coolant reading believable? A warm engine near its regulating temperature, moving a little with load. One frozen cold number means the sensor; a cool reading that drops at road speed means the thermostat. Either one changes the fuel the module meters.
5. Add STFT and LTFT, per bank, per cell. Look at the shape across idle, 2,500 and cruise: high at idle only, rising with airflow, or flat everywhere. The chart in every case plots exactly this.
6. Check the MAF against the engine. Divide the MAF reading by the speed density estimate. Short at idle only means air is getting in around the MAF; short and growing with flow means the MAF reads low; above the estimate means it reads high; short at WOT only with quiet trims means the engine cannot breathe out.
7. Read the downstream sensor. Steady around 0.6 to 0.7 V means the engine is at stoichiometry after the converter. Rich while the trims add fuel means the upstream sensor is being fooled, by a bias or by a misfire.
8. Look at the WOT snapshot. Open loop, commanded rich. If the upstream sensor reads lean at WOT while the MAF agrees with the estimate, the fuel supply is not keeping up.
9. Compare the banks. On a V engine, one bank moving and the other not points to something that belongs to one bank: an injector, an exhaust leak ahead of one sensor, a sensor, a cylinder.
10. Only then decide, and confirm with a physical test. The data narrows the field. A smoke test, a fuel pressure and volume test, a sensor comparison or a misfire swap is what proves it on a real car.

Guides to the questions students ask most

• Positive fuel trim at idle: the causes, and how to tell them apart. Fuel trim high at idle but near zero at cruise usually means a vacuum leak or an exhaust leak. How to tell them apart with the MAF check and the banks.
• Negative fuel trim: the causes, and how to check each one. Negative fuel trim means the computer is pulling fuel. The usual causes, from a leaking injector to high fuel pressure, and the checks that tell them apart.
• Long term vs short term fuel trim, explained. Short term fuel trim is the instant correction; long term fuel trim is the learned one. How each behaves, why you add them, and how to read them per bank.
• MAF grams per second per litre: the rule of thumb, and a better check. A healthy engine at idle reads roughly 1 g/s of MAF per litre. Why that is only a first look, and how to check airflow properly with rpm, MAP and IAT.
• Bank 1 vs bank 2 fuel trim: what a split between the banks means. When one bank's fuel trim moves and the other stays near zero, the cause belongs to that bank. What bank 1 means, and how to read a split on a V engine.
• How to practise fuel trim diagnosis without a car. Practise fuel trim diagnosis from scan data cases: read every load cell, name the fault and the cues, and get graded on the reasoning. Free, in your browser.