P0055 code definition
Heater Circuit Resistance (Bank 1, Sensor 3)
What the P0055 code means
When this code is set it means that the Engine Control Module (ECM) has detected a problem with the amount of resistance in the heater circuit for the third oxygen sensor on bank one of the engine.
What causes the P0055 code?
The most common cause for this code is the heater inside the oxygen sensor burning out. When this happens, it will cause the circuit to either incomplete, or open, or short out. Essentially, the ECM has a preset amount of resistance that it wants to see for the sensor to heat up properly.
When the circuit is open, there will be an infinite amount of resistance because current can no longer travel along a complete path. When the sensor shorts out, the resistance is too low and the sensor will not heat up enough for the sensor to operate properly.
What are some of the symptoms of the P0055 code?
There are no perceptible symptoms for a P0055. The only symptom of P0055 is that the sensor will not begin operating correctly until the exhaust has heated it up sufficiently, as opposed to the built-in heater heating it up. This could cause an increase in emissions, which is why the Check Engine Light is illuminated.
How does a mechanic diagnose the P0055 code?
The diagnosis starts with a visual inspection of the sensor and its associated wiring. Besides a failed sensor heater, the most common issue that causes this code is damaged wiring or connectors. The damage could be caused by rubbing on body components, or from road hazards.
Once the visual inspection is completed, the next step is to measure the resistance of the heater in the oxygen sensor. This is done by unplugging the connector for the oxygen sensor and using a multimeter to measure the resistance, measured in ohms, across the heater terminals. The service manual will have a specification for a known good sensor. If the reading is out of specification, then the sensor will need to be replaced.
If the sensor is good, the next step is to measure the resistance of the wiring from the ECM to the oxygen sensor plug. Usually, you never have to go this far because the sensor is almost always at fault. Generally, you'd like to see the resistance of the wiring to be less than half an ohm.
If that checks out, the most likely culprit lies within the Engine Control Module itself. The ECM usually activates and grounds the heater circuit. There can be problems in the circuit; the contacts in the ECM may get burned and cause a higher than normal resistance reading. This would definitely cause the P0055 code to set.
Common mistakes when diagnosing the P0055 code
- Not properly performing tests and diagnostics
As stated earlier, the sensor itself is often the problem. Often times a mechanic may just replace the sensor without any diagnostics, which isn't always the best path to repair.
How serious is the P0055 code?
Generally this code is not very serious. However, there is a very slight chance that a problem in the heater circuit could burn the contacts in the Engine Control Module. Replacing an ECM can be quite costly.
What repairs can fix the P0055 code?
- Replacing the Bank 1, Sensor 3 oxygen sensor
- Repairing a chaffed or broken wire in the heater circuit
- Replacing the Engine Control Module (in rare cases)
Need help with a P0055 code?
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This article originally appeared on YourMechanic.com as P0055 OBD-II Trouble Code: Heater Circuit Resistance (Bank 1, Sensor 3) and was authored by Evan Clay.