HO2S Heater Control Circuit (Bank 1, Sensor 3)
The oxygen sensor heater for Bank 1, Sensor 3 (located after the catalytic converter) isn't warming up properly, like a broken heating element in an oven that won't reach temperature. The ECU detected the heater circuit isn't drawing the expected electrical current or isn't functioning.
The ECM monitors the oxygen sensor heater circuit's current draw and response time during cold start and operation. It expects the heater to draw a specific amperage range within milliseconds of engine start. If current is absent, too low, or the sensor doesn't reach operating temperature, the fault is triggered.
| Parameter | Normal Range | Fault Condition |
|---|---|---|
| Heater Current Draw | 10-14 amps (typical) | 0 amps or <5 amps |
| Sensor Response Time | Reaches 600°C within 10-20 seconds | Exceeds 30+ seconds or fails to warm |
Code P0042 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, P0042 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.