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Auto parts store check engine light diagnostics

Why you should never trust an auto parts store check engine light diagnostic

Why auto parts stores offer free check engine light diagnostic

Most auto parts stores offer free check engine light diagnostic in order to sell you a replacement part. But never confuse the ability to read a trouble code with a diagnostic. You may find an Autozone free code reading service, but reading the code and recommending the replacement of whatever part is mentioned in the code is NOT a diagnostic.

In the case of a P0131 code, the vast

Flange mount oxygen sensor

Flange mount oxygen sensor

majority of auto parts stores will recommend replacing the oxygen sensor based on the assumption that the sensor has failed. That can happen, but it’s not the most likely cause of the P0131 trouble code.

An OBD II code never tells you to replace the part

A trouble code only tells you that the sensor reading is out of range. You have to find out WHY the reading is out of range. Sure, it could be a bad sensor, but it could also be telling the truth. Let’s take a look at some common trouble codes and the misdiagnosis that often happens at auto parts stores.

Cast study on P0131

Let’s say you take your car to Advance Auto for a free OBD II check and they find P0131 02 Sensor Circuit Low Voltage (Bank I Sensor I). Since the words “oxygen sensor” is mentioned in the code, it’s likely that the store clerk will recommend replacing the oxygen sensor. But that’s not a diagnostic based recommendation. In fact, it’s just the opposite. It’s a lazy person’s guess that can result in wasting money on parts.

What a P0131 code really means

An oxygen sensor simply reports the level of oxygen left over in the exhaust after combustion.

Since the air/fuel mixture is constantly changing, so is the oxygen level in the exhaust. A normal oxygen sensor will change rapidly from lean to rich and vice versa.

What does a low voltage (lean) O2 sensor reading actually mean?

It means the computer has commanded an air/fuel mixture that has too little fuel. So the combustion burns up all the fuel, leaving excess oxygen in the exhaust. High levels of oxygen in the exhaust will cause the oxygen sensor to report low voltage.

The computer will see this report from the oxygen sensor and adjust the air/fuel mixture to compensate. In most cases, it overshoots and adds too much fuel, so you the oxygen sensor reports a high voltage

What does high voltage (rich) O2 sensor reading mean?

A high voltage (rich) exhaust reading means there was too much fuel in the combustion chamber that used up all the oxygen and the combustion event stopped, pushing excess fuel but no oxygen into the exhaust.

Potential causes of a P0131

The P0131 code means that the 02 sensor is reporting a constantly low voltage reading (not switching back and forth between high and low). What could cause that?

• Leak in the exhaust that’s drawing in oxygen from outside air.
• Engine vacuum leak. Again, engine is drawing in too much oxygen
• Clogged fuel injectors/low fuel pressure that adds too little fuel to the air/fuel mix resulting in too much left over oxygen in the exhaust
• A dead oxygen sensor.

What’s the lesson here for following auto parts store diagnostics?

Did you notice that a dead oxygen sensor is at the bottom of the list?

Whether you got the trouble code read at Autozone, Advance Auto, or O’Reilley’s, changing the oxygen sensor for a P0131 code, without doing any other diagnostic work on the other common causes, means you are likely replacing a perfectly good part.

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Posted on by Rick Muscoplat

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