Lack Of HO2S21 Switch - Adaptive Fuel At Limit
The oxygen sensor after the catalytic converter (bank 2) isn't switching fast enough, so the engine computer can't fine-tune fuel mixture and has maxed out its correction attempts. Think of it like a thermostat that can't read room temperature, so the furnace keeps guessing and gives up.
The ECM monitors HO2S21 (heated oxygen sensor, bank 2, sensor 1 position) switching frequency to detect rich/lean conditions and adjust fuel trim. When the sensor response is too slow or absent, the ECM can't make corrections and fuel trim reaches its maximum adaptive limit, triggering the fault.
| Parameter | Normal Range | Fault Condition |
|---|---|---|
| HO2S21 Switch Rate | 0.5–2 Hz at steady cruise | Below 0.5 Hz or no switching detected |
| Fuel Trim Adaptive Limit | ±10% adjustment range | Reaches ±25% or higher (max limit exceeded) |
Code P1155 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, P1155 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.