HO2S Heater Control Circuit (Bank 1 Sensor 3)
Diagnostic trouble code P0042 indicates that the powertrain control module (PCM) has detected a malfunction in the heater control circuit for the heated oxygen sensor located in Bank 1, Sensor 3. The PCM sets this code when the voltage or resistance in the heater circuit falls outside of the expected manufacturer specifications, preventing the sensor from reaching operating temperature quickly.
While this code will not typically strand you or cause immediate drivability issues, it should be addressed soon. A malfunctioning O2 sensor heater delays the sensor from reaching operating temperature, which can increase emissions and slightly reduce fuel economy.
Common symptoms
- Illuminated Check Engine Light (MIL)
- Slight decrease in fuel efficiency
- Failure to pass an emissions test
- Engine may run slightly rich or lean during warmup
- No noticeable drivability issues in most cases
Likely causes
- Faulty Bank 1 Sensor 3 Heated Oxygen Sensor (HO2S)
- Blown fuse for the oxygen sensor heater circuit
- Damaged, burned, or shorted wiring in the HO2S heater circuit
- Corroded or loose electrical connector at the oxygen sensor
- Poor ground connection for the heater circuit
- Faulty Powertrain Control Module (PCM) (rare)
What to check before replacing parts
Scan for other codes and clear them
Look for: Check if P0042 returns immediately or after a drive cycle to confirm it is a hard fault.
Inspect the oxygen sensor heater circuit fuse
Look for: A blown fuse indicates a short in the heater circuit or the sensor itself, which must be resolved before replacing the fuse.
Visually inspect the wiring and connector for Bank 1 Sensor 3
Look for: Look for melted wires from exhaust heat, chafing, corrosion in the connector pins, or loose connections.
Test the heater circuit resistance on the oxygen sensor
Look for: Using a multimeter, measure the resistance across the heater pins on the sensor. If the resistance is infinite (open) or out of specification, the sensor is faulty.
Check for voltage and ground at the sensor connector
Look for: With the key on, engine off, verify that the harness side of the connector has battery voltage and a good ground on the heater circuit pins. Lack of voltage or ground indicates a wiring or PCM issue.
This page explains the generic P0042 definition and is general reference information, not a diagnosis for your specific vehicle. Actual causes, severity, and repairs vary by make, model, engine, and mileage — always confirm with a scan tool and a qualified technician before replacing parts.
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