HO2S Heater Control Circuit (Bank 2 Sensor 3)


An oxygen sensor for a one particular engine bank has delivered a voltage signal that is not within the accepted parameters, causing the PCM to store a trouble code and possibly illuminate the service engine soon light. Bank 2 typically denotes the engine bank of the engine block that does not contain the number 1 cylinder. S3 tells you that the fault is found in sensor 3, or the downstream sensor (aft of the catalytic converter/s).

Code Set Parameters

A universal resistance level of 8 ohms is normal for this particular circuit. Variations of greater than 10-percent in either direction will result in a stored code and an illuminated malfunction indicator lamp. Heater circuit voltage should duplicate battery voltage, with a variation of 10-percent being sufficient to initiate a stored trouble code in the PCM. Engine running (closed loop) oxygen sensor readings that remain constant for an extended period of time (typically more than 8-seconds) can cause a code too be stored and a malfunction indicator to be illuminated.


These could include a lean or rich condition in the engine, decreased fuel efficiency, black smoke from the tailpipe, and an illuminated service engine soon lamp.

Common Causes

Possible causes include a faulty oxygen sensor, faulty electrical connector at the oxygen sensor, faulty or shorted wiring in the oxygen sensor circuit, a blown fuse for the oxygen sensor (if applicable), low or excessive fuel pressure, a vacuum leak on the engine, excessive exhaust leaks, or a faulty PCM. PCM failure is rare.

Common Misdiagnosis

Technicians report replacement of oxygen sensors, when lean or rich engine conditions are the cause of incorrect readings, is fairly common. Replacement of the incorrect oxygen sensor is also a possibility.


  • In order for the engine to function properly, an air fuel mixture of 14:7 (air:fuel) must be provided
  • The PCM is used to control fuel delivery, ignition timing, and fuel injector pulse
  • The PCM gathers input signals from various engine drivability sensors to calculate the fuel delivery strategy
  • One of the most important of these sensors is the Heated Oxygen Sensor or the Air/Fuel Sensor (Japanese cars, such as Toyota)
  • It is used to detect oxygen levels in the exhaust system
  • This data informs the PCM of how rich/lean the engine is running
  • The “heater” portion of the oxygen sensor is an electrical circuit that is dedicated exclusively to warming up the oxygen sensor/s
  • Most oxygen sensors in OBD-II equipped vehicles are of the heated variety
  • Each heated oxygen sensor has its own integrated heater
  • By heating the oxygen sensor using electrical voltage, the PCM is permitting it to become operational sooner than if it was heated using only exhaust gases
  • This decreases emissions and the time needed for the PCM to enter closed loop mode. Most heated oxygen sensors use a battery voltage (approximately 12.6 to 13.8-volts)
  • On some models the voltage is delivered via a fused circuit
  • The heated oxygen sensor circuit is completed with a ground supplied by the PCM (with the KOEO or KOER)
  • Other manufacturers use a computer controlled supply of battery voltage and a continuous ground (no fuse in needed in this design)
  • If the PCM detects a fault in heater system circuit voltage (or resistance) a code will be set and a service engine soon lamp may be illuminated
  • Several tools will be instrumental in successfully diagnosing this code
  • A suitable OBD-II scanner (or code reader) and a digital volt/ohmmeter will be most helpful in performing a successful diagnosis
  • A manufacturer’s service manual (or the equivalent) will also be necessary
  • Begin with a visual inspection of all wiring and connectors
  • Repair or replace damaged, disconnected, shorted, or corroded wiring, connectors, and components as necessary
  • Always retest the system after repairs are completed to ensure success. If all system wiring, connectors, and components (Including fuses) appear to be in normal working order, connect the scanner (or code reader) to the diagnostic connector and record all stored codes and freeze frame data
  • This information can be extremely helpful in diagnosing intermittent conditions that may have contributed to this code being stored
  • Continue by clearing the code and operating the vehicle to see if it returns
  • This will help to determine whether or not the malfunction is intermittent
  • After the codes are cleared, test drive the vehicle to see if the code returns
  • If the code fails to immediately return, you may have an intermittent condition
  • Intermittent conditions can prove to be quite a challenge to diagnose and in extreme cases may have to be allowed to worsen before a correct diagnosis can be made. Look for wiring that has been damaged by road debris or vehicle collisions
  • Wiring harnesses that are burnt by exhaust pipes are also fairly common
  • Removal of the oxygen sensor can be helpful, assuming that it is relatively accessible
  • Examine the sensor for signs of unwanted debris or discoloration
  • Silicon compounds that are burnt by the engine and carried away in the exhaust can foul oxygen sensors permanently
  • Repair faulty wiring or replace oxygen sensors as necessary
  • Clear codes and test drive the vehicle afterward to ensure that the repair was successful. If no obvious problem is found, then use a scanner to view data from the scanner in question
  • Narrowing the scanner’s data stream to include only this sensor will increase the accuracy and response time of the data stream
  • Using a digital volt/ohmmeter to obtain live data is also an acceptable method for monitoring sensor function and will provide you with more accurate readings
  • Upstream (pre catalytic converter) heated oxygen sensor readings should constantly fluctuate between approximately 100-millivolts and 900-millivolts, after the engine reaches normal operating temperature and the PCM enters closed loop operation
  • Downstream oxygen sensors should find a point near the center of a lean and rich condition and remain within 100 to 200 millivolts of that point until a significant engine RPM change is realized If your live data testing (or the scanner data display) reveals a similar reading, then unplug the oxygen sensor connector in question and inspect the pins for signs of damage or corrosion
  • If the connector is faulty, repair or replace it as required and clear the codes
  • Test drive the vehicle to make sure that the repair was successful. If the connector pins appear to be in good shape, then prepare to perform resistance and continuity testing on the oxygen sensor by disconnecting the electrical connector
  • If live data suggests that the oxygen sensor is not performing properly, disconnect the connector and test the sensor itself
  • If the battery voltage feed circuit for the oxygen sensor heater indicates “no resistance”, suspect a defective oxygen sensor
  • Test the oxygen sensor as suggested by the manufacturer and compare your findings with manufacturer’s specifications
  • Replace the sensor as needed and clear the codes afterward
  • Test drive the vehicle afterwards to ensure that a successful repair was performed. If the oxygen sensor conforms to the manufacturer’s specifications, prepare to test system circuits for resistance and continuity by disconnecting the electrical connector/s from all related control modules (especially the PCM)
  • Consult the manufacturer’s service manual and perform continuity and resistance testing on all system circuits
  • Compare your actual findings with manufacturer’s specifications and repair or replace circuits, components, or connectors as required
  • Once the repairs are made, clear the codes and test drive the vehicle to ensure that the repair was successful. If oxygen sensor and system circuits are comparable to the manufacturer’s specs, suspect a defective PCM (or other related controller)
  • Controller failure is rare and replacement of such will require reprogramming. Test voltage to the individual sensor heater circuits (power and ground) using a voltage drop method
  • Test voltages should be near that of the fully charged battery