Fuel Trim Malfunction (Bank 2)

Fuel trim is the ECM adjustment to the air-fuel mixture. P0173 means the Bank 2 fuel trim correction has exceeded the ECM threshold, indicating the engine cannot properly maintain the correct air-fuel ratio on that bank.

P0173 is a general fuel trim malfunction code for Bank 2 that indicates the fuel trim is outside normal operating limits. P0174 specifically indicates a lean condition on Bank 2. P0173 can be triggered by either rich or lean fuel trim deviations.

A clogged catalytic converter can cause back-pressure that affects fuel trims, but it is not a common cause. A failed converter is more likely to set catalyst efficiency codes like P0430.

The OBD II regulation requires the system to flag the fuel delivery system when it can no longer hold emissions at or below 1.5 times the applicable FTP standards, and — on vehicles with adaptive feedback control — when that control has used up all the adjustment the manufacturer allows (Title 13 CCR 1968.2, section (e)(6.2)). The second case is the useful one to understand: the code means the computer ran out of correction, which is why the underlying fault is often something it was compensating for silently for a long time. This is the threshold the monitor is designed around, not a description of your specific fault — the regulation says what the system must detect, not how to find it on your vehicle.