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It’s very common now we use some entry-level diagnostic tool to check the engine issue without going to the dealer’s. But sometimes even you Google the meaning of those codes, we are still lost. In fact, I heard that pros learn from experience as well.

Here below, I use trouble code P2277 B2S3 as example, to show that how to verify which O2 sensor goes wrong.

First we need to know some definition of particular words.

Circuit/open circuit: Most of the time, it is used when the detection of a fixed value or a specific circuit high or low is not feasible. It can also be used with circuit low and high fault codes at the same time, provided that all three circuit conditions can be detected.

Range/performance: Refers to the circuit within the normal operating range, but is unreasonable or incorrect for the current operating conditions. It can be used to indicate stuck or distorted values, poor performance of circuits, components or systems.

Low circuit: It means that the circuit voltage, frequency, or other characteristics measured at the input of the control module are below the normal operating range.

High circuit: means that the circuit voltage, frequency, or other characteristics measured at the input of the control module are higher than the normal operating range.

Bank: Refers to a combination of cylinders that share a common control sensor, the first bank always contains cylinder 1, and the second bank refers to the opposite bank; “bank” will be replaced by B in some vehicle fault code explanations. B1 refers to bank 1; B2 refers to bank 2.

The position of the sensor: Refers to the air flow relative to the engine: from the intake system to the exhaust system; or relative to the engine fuel flow: from the fuel tank to the engine, in the order of 1, 2, 3, etc. The fault code will use "S" or "column" to indicate the location, such as S1 refers to the first column, that is, the first one.

Left/Right and Front/Rear: From the driver’s angle.

A, B:Refers to components, customized by the manufacturer.

Intermittent/Unstable: It means that the signal is temporarily discontinuous, but the fault duration is not long enough to be considered as an open circuit or short circuit, or the rate of change is too high.

So we could now know that P2277, B2S3 means that the second exhaust pipe the third O2 sensor has issue.
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