What P0173 means
P means powertrain (engine and transmission). The 0 marks a generic code defined by SAE, so the same number means the same thing across makes. The third character, 1, places it in the fuel and air metering subsystem; the last two digits identify the specific fault.
Fuel trim is how much the engine computer is correcting its base fuel calculation, using the upstream O2 sensors as feedback. Positive trim means it is adding fuel to correct a lean mixture; negative means it is removing fuel to correct a rich one. These codes set when the correction reaches its limit.
Bank 2 is the side opposite cylinder 1. Inline engines have only bank 1, so a bank 2 code on an inline engine usually indicates a manufacturer-specific use.
What “Fuel Trim” means here
A general fuel trim fault: the module's fuel correction for that bank reached or stayed at a limit without the more specific lean or rich code being set. Read both short and long term trims to see which way it is correcting.
What you may notice
Symptoms depend on the vehicle and on how the module responds to the fault. Any of these can accompany P0173:
- Hesitation or rough idle
- Higher fuel use
- Misfire codes on some engines
- Fuel smell or black smoke when rich
Where to start diagnosing P0173
- Read short and long term fuel trims at idle and at 2,500 rpm. Trims that are high at idle but improve with rpm point to a vacuum leak; trims that are high everywhere point to fuel delivery or air metering.
- Check for vacuum and intake leaks, including PCV hoses.
- Check MAF readings and fuel pressure.
- If both banks are affected, look for a shared cause; if only one, look at that bank.
Clear the code only after recording freeze frame data: it shows the engine speed, load and temperature at the moment the fault set, which is often the best clue to reproducing it.
Live data to watch
These standard OBD2 values relate to P0173. Support varies by vehicle; a value your car does not report will simply be missing.
| Value | What to look for |
|---|---|
| Short and long term fuel trim (PIDs 0x06–0x09) | Combined trims well beyond about ±10% deserve investigation. |
| MAF air flow rate (PID 0x10) | Under-reporting causes lean trims. |
| O2 sensor voltage (PIDs 0x14–0x1B) | Confirms what the trims are reacting to. |
Related codes
Codes from the same fuel trim system family. If several are stored together, the shared part — wiring, a common reference voltage, or the component itself — is the place to start.
Further reading
Frequently Asked Questions
What does P0173 mean?
P0173 is the generic SAE J2012 code for "Fuel Trim (Bank 2)". In plain terms, it means the engine computer's fuel trim system monitor has reported "Fuel Trim (Bank 2)".
Can I keep driving with P0173?
Usually drivable in the short term. A persistently lean engine can misfire and run hot, so do not ignore it.
What should I check first for P0173?
Read short and long term fuel trims at idle and at 2,500 rpm. Trims that are high at idle but improve with rpm point to a vacuum leak; trims that are high everywhere point to fuel delivery or air metering.
Definition source: SAE J2012 / ISO 15031-6 generic DTC definitions. This is the generic meaning that applies across makes; a manufacturer can define a code more narrowly or add its own sub-codes, so confirm against the service information for your vehicle before replacing parts. Written by the OBDAssistant engineering team — see our editorial policy.