P0173: the fuel trim system on bank 2 isn't behaving correctly
Fuel trim is how much the engine control unit is adjusting the fuel mixture away from its baseline setting, based on what the oxygen sensors report. This code means bank 2's fuel trim itself isn't behaving the way the computer expects, rather than simply reporting that the mixture has gone too lean or too rich. It's a broader fault with the trim calculation than the specific lean and rich codes.
You might notice
- The engine warning light comes on
- The engine may idle unevenly, hesitate, or feel like it's surging slightly
- fuel trim
- — how much the computer is adjusting the fuel mixture away from its baseline setting
- bank 2
- — the second side of an engine built with two cylinder banks, such as a V6 or V8
What the code itself says
P0173 is a powertrain (engine, gearbox and drive system) code, and the 0 after the letter means it is a generic code with a standard meaning.
Grouped here as: Engine running, fuelling and misfires — Air metering, fuelling, ignition, misfire, idle, boost, knock, oxygen sensors and the catalyst, all working together in the running engine. That grouping is ours, to help explain the code. The code's official family is the letter at the start.
This page is generic. Your car isn't.
A code on its own only tells you what the sensor saw, not what's actually wrong — and that changes next to your other codes. Read this one together with the rest of your scan and we'll work out what's really going on.
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