The amount of fuel trim required is determined by the ECM's monitoring of the residual oxygen in the exhaust stream before it gets to the catalytic converter, using what is commonly referred to as an oxygen (O₂) sensor. A more accurate name is the lambda sensor, and its job is to tell the ECM whether the residual oxygen in the exhaust stream is a result of lambda being greater than, less than or equal to 1.0. Remember, when lambda equals 1.0, the air/fuel ratio is stoichiometric — and that's where we want to try and keep it.

The first O₂ sensor was the zirconium dioxide (ZrO₂) sensor. ZrO₂ is an insulator when cold, but becomes a conductor when heated, particularly for oxygen ions. It works like a little battery cell, with voltage being generated as more oxygen ions migrate from one electrode to the other. The inside of the thimble-shaped sensor is exposed to ambient air, and the outside is exposed to the exhaust gas stream.

This basic capture shows how STFT varies constantly to maintain lambda, while LTFT is more stable because it's a learned correction.

Checking freeze frame data is an important diagnostic step. This P0171 (System lean-bank 1) code was recorded at idle.

The TiO₂ sensor functions more like a semiconductor in that its resistance increases with the presence of residual oxygen. When O₂ levels are high, resistance is high, and when O₂ levels are low, resistance is low. By applying a reference voltage across this resistance, the resulting voltage drop provides the input the ECM needs to know the oxygen concentration. Like the ZrO₂ sensor, it, too, is designed to switch almost instantly at lambda equals 1.0.