Technical Description of OBD-II Trouble Code
HO2S Heater Control Circuit (Bank 1 Sensor 3)
What does that signify?
This diagnostic trouble code (DTC) is a standard powertrain code that is applicable to OBD-II equipped vehicles, including VW Volkswagen, Audi, Mazda, Ford, Chevy, and others. Although it is a generic code, the specific repair steps may vary depending on the make and model of the vehicle.
Fuel-injected vehicles utilize heated oxygen sensors in the exhaust system before and after the catalytic converters to determine the oxygen content. This feedback is used to adjust the fuel system in order to maintain the proper air/fuel ratio of 14.7:1.
The oxygen sensors use a heated circuit to warm up the sensor for faster feedback operation. Depending on the vehicle, the oxygen sensor may have three or four wires, with two wires typically used for sensor feedback to the Powertrain Control Module (PCM) / Engine Control Module (ECM), and the other wires used to power the heated circuit. Three-wire sensors are usually grounded through the exhaust system, while four-wire sensors have a separate ground wire.
The P0042 code pertains to the third sensor downstream from the engine on bank 1, which is located on the side of the engine with the #1 cylinder. The heater circuit may receive power or ground from the PCM/ECM or another source that can be controlled by the PCM/ECM.
Note: Exercise caution when working around an exhaust system that has recently been running, as it can become very hot.
Symptoms
Symptoms of a P0042 DTC include the illumination of the Malfunction Indicator Lamp (MIL). You will likely not notice any other symptoms associated with the failure of the heated circuit, as it only operates momentarily when the vehicle is first started. This sensor is also located after the catalytic converter, so it will not affect the air/fuel ratio input to the PCM/ECM; its primary purpose is to verify the efficiency of the catalytic converter.
Causes
Potential causes of a P0042 trouble code may include:
Possible Solutions
Visually inspect the oxygen sensor wiring for any damaged or loose wiring leading to the sensor.
Unplug the oxygen sensor and use a digital volt ohm meter (DVOM) set to the ohms scale to test the resistance of the heater circuit, using a wiring diagram for reference. The heater circuit inside the sensor should exhibit some resistance, while excessive resistance or an over limit reading would indicate an open in the heated portion of the circuit, necessitating the replacement of the oxygen sensor.
Back-probe the ground wire at the connector and check for resistance between a known good ground and the connector to the oxygen sensor.
Back-probe the power supply wire at the connector with the DVOM set to DC volts, placing the positive lead on the power supply wire and the negative lead on a known good ground, to check for power supply at the oxygen sensor. If no power is present at the connector during initial car startup (cold start), there may be an issue with the power supply circuit to the oxygen sensor or with the PCM itself.
