Lack Of HO2S21 Switch - Sensor Indicates Rich
The oxygen sensor behind the catalytic converter (bank 2, sensor 1) isn't switching between rich and lean states properly—it's stuck signaling a rich condition. Think of it like a light switch that's jammed in the 'on' position instead of flickering.
The ECU monitors the HO2S21 sensor's voltage oscillation to detect proper combustion feedback. The sensor should rapidly switch between high voltage (rich) and low voltage (lean). If the sensor voltage remains consistently high without switching, the ECU cannot verify the catalytic converter is functioning properly.
| Parameter | Normal Range | Fault Condition |
|---|---|---|
| Sensor Voltage Switch Rate | 0.2-1.0 Hz (oscillations per second) | <0.2 Hz or no switching detected |
| Voltage Level | 0.4-0.6V (oscillating) | >0.7V constant (stuck rich) |
Code P1157 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, P1157 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.