Rick's Free Auto Repair Advice

Free Check Engine Light Test at Auto Parts Stores — It’s Not a Diagnostic

Why a Free Check Engine Light Test At An Auto Parts Store Isn’t a Real Diagnostic

Most auto parts stores offer a free check engine light test in order to sell you whatever part is listed in the code. But never confuse the ability to read a trouble code with a diagnostic. They’re not the same thing. 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.

A free check engine light test will always result in a recommendation to replace a part

But 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.

Recommending a part replacement without a real diagnostic is insane

Here’s a real wold example of how crazy that is: You go to a doctor and they take your blood pressure reading. It’s low so they automatically recommend a heart transplant. No actual diagnosis of what’s causing the low blood pressure; just an assumption that if the pressure is low, it’s a bad pump. Automatic heart replacement. That’s what you get from an auto parts store.

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, as opposed to making close to minimum wage at an auto parts store. So when they perform a free check engine light test, all they’re doing is pulling the code an automatically recommending the replacement of whatever part is listed in the code.

Here’s how the free check engine light test costs you money

You take your car to an auto parts store and they pull the trouble code. It’s a P0131, which is a very common trouble code. The definition is: O2 Sensor Circuit Low Voltage Bank 1 Sensor 1. I guarantee you that the store will recommend replacing the oxygen sensor simply because the oxygen sensor is mentioned in the code.

Image of an air/fuel sensor that is often unnecessarily replaced when a DIYer gets a free check engine light test

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 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.

©, 2022 Rick Muscoplat




Posted on by Rick Muscoplat

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