Technical Description of OBD-II Trouble Code
HO2S Heater Resistance (Bank 1 Sensor 3)
What does that signify?
This diagnostic trouble code (DTC) is a generic powertrain code that applies to all vehicles manufactured from 1996 onwards (Chevrolet, Ford, GMC, Mazda, Pontiac, Isuzu, etc.). While it is a generic code, the specific repair steps may vary depending on the make and model of the vehicle.
In my personal experience, a stored code P0055 indicates that the powertrain control module (PCM) has detected a malfunction in the heater circuit of the downstream (or pre-catalytic converter) oxygen (O2) sensor for engine bank one. Bank 1 refers to the engine bank that contains the number one cylinder. Sensor 3 indicates that the issue is related to the downstream sensor.
A typical O2 sensor consists of a zirconium dioxide sensing element, protected by a vented steel housing. The sensing element is connected to wire leads in the O2 sensor wiring harness with platinum electrodes. The O2 sensor transmits data to the PCM via the controller area network (CAN), providing information about the percentage of oxygen particles in the engine exhaust compared to the oxygen content of ambient air. This data is used by the PCM to calculate fuel delivery and ignition timing. During cold start conditions, the PCM uses battery voltage to preheat the O2 sensor. The O2 sensor signal circuits are accompanied by a dedicated circuit for preheating the sensor, which typically consists of a battery voltage wire and a system ground wire. The PCM applies battery voltage to the O2 sensor heater when the engine coolant temperature is low, usually until the PCM enters closed loop operation. The voltage is routed through the PCM, sometimes with the assistance of a relay and/or fuses. The circuit is activated when the ignition switch is turned on during cold start conditions. Once the engine reaches normal operating temperature, the PCM de-energizes the O2 heater circuit.
When the PCM detects a level of resistance in the O2 sensor heater circuit that exceeds the programmed limitations, it stores a code P0055 and may illuminate the malfunction indicator lamp (MIL). In certain vehicles, the MIL may only illuminate after multiple ignition cycles with a failure. If this applies to your vehicle, you will need to use the OBD-II readiness mode to ensure the success of your repairs. After performing the repairs, drive the vehicle until the PCM enters readiness mode or the code is reset.
Severity & Symptoms
A stored code P0055 should be considered severe because it indicates that the upstream O2 sensor heater is not functioning. Symptoms of this engine code may include:
Causes
Potential causes of a P0055 trouble code may include:
Possible Solutions
A good starting point is to check for technical service bulletins (TSB) specific to your vehicle. There may be a known issue with a known fix provided by the manufacturer, which can save you time and money during diagnosis.
When attempting to diagnose a code P0055, you will need a diagnostic scanner, a digital volt ohmmeter (DVOM), and a reliable source of vehicle information, such as All Data DIY.
Begin by visually inspecting the system wiring harnesses and connectors, paying close attention to harnesses near hot exhaust pipes and manifolds, as well as those near sharp edges like those found on exhaust shields.
Next, use the DVOM to test all system fuses and fusible links. Skilled technicians test these components under load, as fuses that appear operational when not loaded may fail when loaded. You can load this circuit by activating the O2 sensor heater(s).
Retrieve all stored trouble codes and freeze frame data by connecting the scanner to the vehicle diagnostic port. Write down this information, as it may be helpful if the P0055 code proves to be intermittent. Clear the codes and test drive the vehicle to see if the P0055 is immediately reset.
When the engine is cool enough to activate the O2 sensor heater and the code is reset, observe the O2 sensor heater input data using the scanner data stream. Narrow the data stream display to include only relevant data for faster response. If the engine is within the correct temperature range, the O2 sensor heater voltage should be approximately the same as the battery voltage. If a resistance issue causes the O2 sensor heater voltage to deviate from the battery voltage, a P0055 code will be stored.
You can monitor live data from the O2 sensor heater circuit by connecting the DVOM test leads to the sensor ground and battery voltage signal wires. Test the O2 sensor resistance using the DVOM, ensuring that all related controllers are disconnected before testing the system circuit resistance.
Additional diagnostic tips & notes:
