Understand the downside of a free OBDII code read

Why a Free OBDII Code Read At An Auto Parts Store Isn’t a Real Diagnostic

Most auto parts stores offer a free OBDII code read for a check engine light in order to sell you whatever part is listed in the code. That’s the business they’re in. They’re not in the diagnostic business. So, never confuse the ability to read a trouble code with an actual diagnostic. They’re not the same thing. This article discusses the difference between a real diagnostic versus a free OBDII code read from an auto parts store.

A free OBDII code read will always result in a recommendation to replace a part

However, 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 the sensor could be telling the truth. If it is, and you replace it, you’ll be wasting your money. Let’s take a look at some common trouble codes and the misdiagnosis that often happens due to a free OBDII code read from an auto parts store.

Recommending a part replacement without a real diagnostic is insane

Here’s a real wold example of how crazy it is to replace a part without doing a diagnostic. Suppose you go to a doctor, and they take your blood pressure reading, and the blood pressure monitor device shows a low blood pressure reading. The doctor walks in, sees the reading, and says, “Yeah, let’s replace your heart.” Would you call that a diagnostic? Nope. It’s the same thing that happens at auto parts stores. They see the code that shows an out-of-spec reading, and they automatically recommend replacing whatever part is listed in the code.

Auto parts store clerks are not auto technicians, and they don’t have diagnostic training

If they were actual auto technicians, they’d be making at least double the hourly wage by working in a real shop instead of making close to minimum wage at an auto parts store. So when they perform a free check OBDII code read, all they’re doing is pulling the code and automatically recommending the replacement of whatever part is listed in the code.

Here’s how the free OBDII code read costs you money

Suppose the auto parts store pulls a P0131 code, which is a very common trouble code. The definition is: O2 Sensor Circuit Low Voltage Bank 1 Sensor 1. I guarantee you the store will recommend replacing the oxygen sensor simply because the oxygen sensor is mentioned in the code.

A free check engine light test for a P0131 will most likely result ina recommendation for a new oxygen sensor and that rarely fixes the problem.

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 a P0131 code?

• A leak in the exhaust that’s drawing in oxygen from outside air.
• A vacuum leak. Again, the 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 leftover 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 above?

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.

©, 2022 Rick Muscoplat