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Oxygen Sensor Troubleshooting Tips From Delphi

Oxygen Sensor Aging

Of the many possible HO2S Diagnostic Trouble Codes (DTCs), “Slow Response” is the most frequent code seen. Age, contamination, and extreme heat can affect the oxygen sensor’s response characteristics. Degradation of the signal can be in the form of an extended response time (period duration) or a shift in the sensor voltage curve (sensor shift biased). Both conditions reduce the oxygen window, thereby reducing the catalyst’s capacity for exhaust gas conversion.

Although a change in reaction time can be registered and stored as a code in memory, it cannot be compensated for. The PCM (Powertrain Control Module) can correct a shift in a voltage curve within defined limits (adaptation) using the post-cat oxygen sensor as a second control loop.

To find out if the computer is “in control” of a vehicle’s air/fuel mixture, you need to view the oxygen sensor signal and determine the computer command.

Oxygen Sensor Contamination

Coolant contamination of oxygen sensor

Since the beginning of closed loop fuel control systems, O2 sensors have been subject to contamination. Much like spark plug fouling, you can ‘read’ the sensor’s environment. An extremely rich mixture exposes the sensor to extra unburned fuel which in turn leaves sooty black deposits.

Earlier years saw O2 sensors being contaminated with lead from leaded fuel, which is all but gone today. Lead poisoning can still occur, however, when a steel fuel tank is exposed to overly large doses of fuel system cleaners or alcohol. While rare, the result can be the lead/tin alloy referred to as ‘terne’ plating inside some older steel fuel tanks being emulsified by the exposure to the overuse of cleaners and alcohol.

Once the lead/tin from the plating is in the fuel system there will be in contact with the O2 sensor. Oil deposits can also end up contaminating an O2 sensor similar to a spark plug when the engine’s rings/cylinder walls are worn or more commonly when there is a PCV system or intake manifold causing oil to enter the combustion chamber. Probably the most common source of O2 sensor contamination is from engine coolant leaks such as a leaky intake manifold or cylinder head gasket.

Planar style O2 sensors are manufactured in both the conventional switching Zirconia as well as the wide band type. The planar element has the ability to provide a sealed air reference, providing its own electrochemical oxygen “pump” in both rich and lean exhaust mixtures, preventing the possibility of air reference contamination. This sensor also utilizes an anti-contaminate coating. This construction helps to resist sensor contamination

A Diagnostic Strategy for Technicians


A diagnostic strategy should be efficient as well as accurate. It must lead the technician through steps that will correctly identify the root cause of the malfunction. These steps must be thorough enough to ensure malfunctions aren’t overlooked, but not so cumbersome that they cause the technician to waste a lot of time.


The first step of your diagnostic process should always be to, “Verify the Concern.” The final step of your diagnostic strategy should be “Repair and Verify the Fix.”


Verify the Concern

The first part of this step is to obtain as much information as possible. Review the OBD II I/M Check results shown on the Vehicle Inspection Report (VIR). Then, ask questions like:

      Are there any drivability concerns?

      When does the condition occur?

      Where does the condition occur?

      How long does the condition last?

      How often does the condition occur?

      Are there aftermarket accessories on the vehicle?

      Has any other parts been replaced recently?


A check sheet can be a useful tool that the technician can use to record notes. The check sheet should contain questions that helps the technician obtain additional details that will assist them to diagnose and repair the vehicle. In order to verify the concern, the technician should be familiar with the normal operation of the system and refer to the owner or service manual for any information needed.


Preliminary Checks

      Conduct a thorough visual inspection

      Detect unusual sounds or odors

      Review available service history

      Check Bulletins and Other Service Information

      Perform Published Diagnostic System Checks

      Determine the proper operation of the system


Scan Tool Testing

      Check and record any stored DTCs

      Check and record freeze frame data

      Check and record the data parameters associated with the system involved

      Perform bi-directional testing on components (when available)


Based on the results of the Preliminary Checks and Scan Tool Testing, move into a diagnostic process based on the individual components involved.


Individual Component Testing

      Use the appropriate service and repair information to diagnose the component(s)

      Determine if the component is mechanically able to operate

      Determine if the component is electronically able to operate (if applicable)

      When applicable, verify the electrical control signal to the component


Repair and Verify Fix

After isolating the cause, make the repairs and validate for proper operation. Verify that the malfunction has been corrected, which may involve road testing the vehicle under specific conditions. Mode $06 in the scan tool may assist with verifying the fix.

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