Three Commonly Misleading OBDII Trouble Codes

Having worked in the automotive service industry for over 14 years, I can tell you that it’s not always easy to diagnose your OBDII trouble code and there’s a lot of misleading OBDII Trouble Codes. Many of these trouble codes can be very misleading or at the very least confusing for even techs, let alone the average owner and driver. Complicating matters is the fact that many times, owners and customers believe that simply retrieving the OBDII Trouble Codes from their ECU is 100% accurate.

I can tell you from firsthand and often painful experience, this isn’t always the case. Armed with the code and parts from the local auto parts store, these customers simply want the part in question replaced, rather than having a tech truly diagnose their vehicle. Customers get into the habit of taking the OBDII Trouble Codes as gospel, which can often lead to more trouble or problems.


Test a MAP Sensor – 1996 Honda Civic


We’ve had many customers simply walk in and ask for a quote to replace the part in question, rather than ask about their code and what could be causing it. We usually try to dissuade them from simply just replacing the part, and instead troubleshooting the issue, but sometimes people can be stubborn.

You can imagine their surprise when the part is replaced, but their code doesn’t go away. This is because the OBDII diagnostic system is meant to help guide you or direct you to the issue in your car.

Remember that the OBDII Trouble Codes stored in your ECU are not always 100% accurate, and you would be best served by doing just a little bit more work to find out what the problem with your car is. Read on to enjoy our DIY article on learning what codes are misleading OBDII Trouble Codes, and possibly avoiding making any expensive mistakes.

OBDII Misfire Trouble Codes



P03XX codes can be extremely difficult to diagnose and track down, especially if they are intermittent.


Misfire is one of the most misleading OBDII check engine codes you could ever have. Because so many engine inputs and values are required to determine spark and fuel events, misfire can come from any number of problems. From your mass air flow system, to leaks in your intake, spark plug gap, fuel quality, damaged wiring harnesses and more, the causes of OBDII misfire can come from anywhere.

When your ECU sees a serious variation in RPM by referencing the crankshaft position sensor speed, or camshaft position sensor, the misfire code can be triggered. This will light your Malfunction Indicator Lamp (MIL), notifying you that you’ve got to take your car in for service.

Most customers walk right in and request to simply replace the failed ignition coil in question. For example, P0301 signifies a cylinder misfire in the #1 cylinder of your engine. Some clever customers may simply purchase another ignition coil, ask to have it replaced and then wonder why the P0301 code doesn’t go away. These misfire codes are stored in your OBDII ECU as P03XX, with P0300 being a general misfire code.

OBDII Drive By Wire Trouble Codes


Relatively new to the OBDII DTC game, Drive by Wire is probably the worst invention to ever make it into a production vehicle. Throttle response is horrible in a DBW system, not to mention the handful of computers, motors, servos and valves that replace what once was a simple cable. Misleading OBDII Trouble Codes can lead to wasted time and trouble, so make sure you have the proper DBW tools to test your accelerator pedal.


As you can tell, we are not fans of Drive by Wire


Many times customers can confuse Idle Control or Accelerator Pedal OBDII trouble codes, and think that they must replace the throttle body in their car. We’ve had several customers come in and request for this job, especially in later model sporty models that get modified.

It’s a good idea to remember that when you own a vehicle with Drive by Wire, there’s a lot of things that you (or a tech) won’t be able to address. Some of these things include accelerator pedal relearning, idle relearning, throttle position relearning and reprogramming.

OBDII Exhaust Gas Recirculation (EGR) Trouble Codes


From snake oils to magic gas caps, all it takes is a walk down an aisle at your nearby “Auto Store X” to see what kind of products and demand there is for EGR codes. Most commonly misdiagnosed because of heat related issues that can also become intermittent, EGR OBDII trouble codes are definitely one of the most commonly misleading codes. I’ve seen many customers complain that a replacement EGR valve or gas cap didn’t fix the problem, when all they did was change the part. Because there can be a myriad of causes to the EGR code, from blocked intake passages, or clogged EGR control solenoids, or even leaks in the vacuum hoses.

Think we missed any misleading trouble codes? Have an experience or question about your misleading OBDII check engine light code? Let me know below.

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