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Check engine light still on

Why is my check engine light on after replacing parts

Seeing the check engine light on after replacing parts is a common complaint on DIY auto repair forums. The home mechanics wonder how that can be if they fixed the problem but their check engine light is still on. Well grasshopper, your shortcut bit you in the butt.

Of all the home mechanic mistakes, this one is the most common: thinking that if a sensor is listed in the trouble code that automatically means the sensor is bad. Let’s take a look at a common trouble code

P0131 02 Sensor Circuit Low Voltage (Bank I Sensor I) check engine light still on after replacing oxygen sensor

I can’t tell you how many oxygen sensors get needlessly replaced because of this code. Here’s what the code actually says versus what you THINK it says.

An oxygen sensor measures the amount of oxygen in the exhaust stream. If there’s very little oxygen, the sensor reports a low voltage; too much oxygen and it reports a high voltage. Because the engine computer is constantly trying to get the proper mixture, it swings back and forth between a rich and lean mixture. It happens fast, as often as 2 to 3 times per second. So the engine computer expects to see a switch from low voltage to high voltage and back that often. When the computer sees an oxygen sensor stuck on either high or low voltage without swings, it sets a code.

Why you should never rely on the advice of an auto parts clerk

Chances are you had the auto parts store guy read your trouble code and the clerk recommended replacing whatever parts was listed in the code. Guess what? They’re in the business of selling parts. They’re not in the business of diagnosing problems. If they were, those clerks would be working in a shop making 3X what they’re getting punching buttons behind the counter.  So you took their advice to buy a part without actually diagnosing all the possible reasons WHY the computer could set that code. In effect, you took a calculated gamble by replacing the part first. Oxygen sensors do fail, but not as often as you think.

The #1 cause for a P0131 code is a vacuum leak that allows unmetered air into the system. Too much air leans out the air/fuel mixture resulting in consistently low voltage at the oxygen sensor. You think it’s a bad sensor and replace it. But most likely you didn’t fix the broken vacuum line. Now you wonder; Why is my check engine light on after replacing parts? Well dude, you didn’t fix the problem.

There are other things that can cause a P0131 trouble code like a bad fuel pressure regulator or a clogged fuel injector that doesn’t provide as much fuel as the computer commands, so you wind up with a lean mixture and low voltage at the oxygen sensor and write in to an auto forum saying; I changed my o2 sensor and the check engine light is still on.

How to diagnose a trouble code

This is what separates the men from the boys. Top notch auto technicians NEVER replace a part without actually diagnosing the failure. They conduct tests to eliminate the problems that could cause a sensor to truthfully report a problem. Because, if the sensor is telling the truth, then the sensor isn’t at fault.

In the case of a P0131 trouble code, a technician would check fuel pressure and use a scan tool to check to see if the computer is trying to compensate for a lean mixture by adding more fuel. I understand you probably don’t have a scan tool, but the truth is you can buy a decent one for about $200 that’ll give you that kind of information. You just blew around $90 replacing a good oxygen sensor and you’ve still got a check engine light. If you’re like most DIYers, you’ll replace a few more parts hoping that you guessed right. When you’re done, you’ll most likely spend more than a scan tool would have cost.

THAT’s why your check engine light still on; because you didn’t actually diagnose and fix the right problem.

©, 2015 Rick Muscoplat



Posted on by Rick Muscoplat

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