Catalyst Temperature Sensor (Bank 1 Sensor 1)
Your catalytic converter's temperature sensor isn't sending the right signal to the engine computer. Think of it like a thermometer that's broken or disconnected—the engine can't tell how hot the converter is getting.
The ECU monitors catalyst inlet temperature to verify converter efficiency and prevent overheating. It expects a voltage signal that rises as exhaust temperature increases. If the signal is out of range, missing, or doesn't respond properly to load changes, a fault is triggered.
| Parameter | Normal Range | Fault Condition |
|---|---|---|
| Sensor Voltage | 0.2–4.8V (varies with temp) | Below 0.1V or above 4.9V, or no response to load |
| Catalyst Inlet Temp | 300–900°F under load | Stays below 200°F or erratic readings |
Code P0425 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, P0425 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.