Fuel Trim too Rich (Bank 1)
Your engine is running too rich on Bank 1, meaning it's burning more fuel than air—like adding too much syrup to pancake batter. The ECU can't trim the fuel mixture back enough to reach the proper balance.
The ECU monitors oxygen sensors (O2) on Bank 1 to calculate fuel trim corrections. It compares actual air-fuel ratio against the target 14.7:1 ratio and adjusts injector pulse width accordingly. When long-term fuel trim exceeds maximum positive correction limit, P0173 triggers.
| Parameter | Normal Range | Fault Condition |
|---|---|---|
| Long-Term Fuel Trim (LTFT) | -10% to +10% | >+25% (cannot enrich further) |
| O2 Sensor Response | 0.1–0.9V cycling | Stuck lean (>0.5V sustained) |
Code P0173 is a moderate fault. You can generally drive to a workshop, but avoid long trips or high-load driving (motorway, uphill towing) until it is diagnosed. If the code keeps returning after clearing, or if you notice the symptoms listed above worsening, do not delay professional diagnosis. Many moderate codes have multiple possible root causes — a mechanic with live OBD data can identify the exact fault more efficiently than part-by-part trial and error.
Once the fault is repaired, P0173 can be cleared using any OBD-II scanner. Connect the scanner, navigate to "Clear Codes" or "Erase DTCs," and confirm. The check engine light turns off immediately.
The code will return if the root cause was not actually fixed. The ECM re-detects the fault within 1–3 drive cycles and sets the code again.