What does it mean?
OBD II fault code P0038 is defined as “ Oxygen Sensor Heater Control Circuit High (Bank 1 Sensor 2), and is set when the PCM (Powertrain Control Module) detects a signal voltage from oxygen sensor #2 that does not fall within the predefined range for that sensor. Note that “Bank 1” refers to the bank of cylinders that contains cylinder #1, and that “Sensor 2” refers to the oxygen sensor that is located after (downstream of) the catalytic converter(s).
Regardless of the number of oxygen sensors present in a vehicle, each sensor is provided with a dedicated circuit to control the heating of the sensor element. Heating the element electrically brings the sensors into operation much sooner than would have been the case if the sensors were heated by the exhaust stream; by bringing the sensors into operation sooner, emissions are reduced significantly.
On most applications, the electrical resistance of an oxygen sensor will be 8 Ohms (or very close to it), and deviations of 10% of this value will set code P0038 and trigger the CHECK ENGINE light. Input voltages in all oxygen sensor heater control circuits should be equal to battery voltage; deviations of 10% will also set code P0038 and trigger the CHECK ENGINE light.
Note that the electrical resistance may differ on some applications- consult the manual on the exact values for the application being worked on. During normal (closed loop) engine operation, the signal voltage from the #2 oxygen sensor(s) will vary constantly, and in cases where the PCM detects no changes in the sensor signal voltage(s) for a period of about 8 continuous seconds, code P0038 can be set and the CHECK ENGINE light triggered.
Oxygen sensor #2 monitors the functioning of the catalytic converter, whereas sensor #1 monitors fuel/air metering. In practical terms, this means that sensor #1 measures the amount of oxygen in the exhaust stream (as a function of the combustion process) before it enters the catalytic converter, while sensor #2 measures how well the converter(s) copes with cleaning up the exhaust stream. NOTE: Some Japanese cars, such as Toyota, do not use conventional oxygen sensors.
Instead, these applications use AIR/FUEL RATIO sensors that are NOT interchangeable with conventional oxygen sensors. Consult the relevant repair manual for these applications for the correct testing/diagnostic/repair procedures. The image below shows the differences between conventional oxygen sensors and air/fuel sensors.
Note: Circuit High Input codes are mostly caused by defective alternators that develop excessive voltages, although there may be other causes as well, such as short circuits that “leak” current into a particular system from other, unrelated systems that work on higher voltages. Note that in the case of short circuits that leak power into a control circuit, there is likely to be other, seemingly unrelated trouble codes present along with the code(s) being investigated.
Diagnosing a “high input” code will always involve a thorough testing of the charging system as a first step, followed by measures to isolate the system from all other possible sources of power during resistance, continuity, and reference voltage tests.