3 Misleading Types of OBD-II Trouble Code Descriptions

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parts girl
Counter Personnel

I utilize several major auto parts franchise chains. I welcome the diversity which comes with each one; from the “mom & pop” store, which has been in town for longer than I have been alive, to the ultra-modern and brightly lit super stores, which do business on a global level. Having said that, I must say that I have noticed a frustrating trend concerning the latter:

Several major auto parts stores offer free diagnostic trouble code retrieval. Typically, inexperienced counter personnel use a scanner, or code reader, to pull an OBD-II trouble code from a customer’s car; off-handed repair advice is then communicated in an effort to facilitate a sale. If the recommended component replacement rectifies customer complaint, then all is well in the world. If it does not, the frustrating part begins.


There are several OBD-II diagnostic trouble codes which can be misleading, in their description. A code, alone, does not constitute an accurate diagnosis. Customers arrive at my shop believing that they have had a “free-diagnosis” performed at Brand-X Auto Parts; Brand-X parts in hand, they exit the vehicle and inquire regarding the installation price. When I attempt to politely inform the clever consumer that the purchased component is unlikely to effectively repair his vehicle, what I get is more attitude. Not desiring to offend the customer, I agree to install his/her part, against my best advice and with a specific disclaimer. After the component is installed and the service engine soon light returns (as expected), an accurate diagnosis is recommended and guess what — more attitude. In the long run, this customer has spent more money than if a proper diagnosis had been performed, initially.

Exhaust Gas Recirculation (EGR) Codes

Exhaust Gas Recirculation Valve (EGR)

In my neck of the woods, this is the current reigning king of the “snap-judgment” codes. Any thing from a P0401 Insufficient EGR Flow to a P0402 Excessive EGR Flow will likely result in a shiny, new EGR valve from Brand-X; the problem is that it will not rectify the condition for setting the code. A thorough and accurate diagnosis must be performed prior to condemning a particular component. Concerning the EGR system, the EGR valve is the least likely component to fail. Granted, there are exceptions to any rule and there are times when a carline or manufacturer will have an issue with EGR valves, but clogged intake passages, faulty delta process flow EGR sensors, EGR control solenoids, and vacuum and exhaust hose leaks occur with a much greater degree of regularity than faulty EGR valves, themselves.

Lean Exhaust Gas Codes

oxygen sensor
Oxygen Sensor

Specifically, trouble codes P0171 Lean Exhaust Bank 1 and P0174 Lean Exhaust Bank 2, are frequently misunderstood. Several times during the last three years, I have had customers show up with brand-new, expensive oxygen sensors from Brand-X Auto Parts. “Free Diagnostic Certificate” in hand, they are confident that these new sensors will ensure that a Federal emissions certificate will be obtainable. Again, I am forced to explain what only time and experience has taught my crew, and I. A lean exhaust condition, not faulty oxygen sensors, is at the root of the problem. On one particular occasion, I could hear the sound of a loud vacuum leak emitted from under the hood of a Ford Explorer, even before the customer informed me that her new oxygen sensors were on the front seat. Lean exhaust is caused by one of two things: An excess of air to the engine or insufficient fuel. Before you purchase hundreds of dollars worth of oxygen sensors, consider whether or not you have a vacuum leak at the intake; then check your records for a fuel filter change. If you haven’t changed your fuel filter in 100,000 miles, then you may want to start there. Chances are that it will fix your problem.

Misfire Codes

ign coil
Ignition Coil

The OBD-II Diagnostic System’s ability to pin-point a misfiring cylinder is extraordinary. The word “misfire”, however, can be misleading. One of the major parameters for a P03_ _ misfire code being set occurs when the powertrain control module (PCM) detects a variation in RPMs, between the crankshaft and camshaft sensors, which exceeds the maximum allowable amount in degrees. This variation is caused by a weak or failing cylinder, but can be contributed to a variety of factors. “Misfire” seems to denote a problem with the ignition system but a misfire code can be caused by a lack of fuel, insufficient ignition spark, or lack of compression to the affected cylinder. In addition, excessive vacuum deposits in any cylinder will spoil the air/fuel mixture and render the cylinder impotent. Before you perform an expensive maintenance tune-up, in order to correct a misfire code, perform a complete and accurate diagnosis. The money that you save might be your own.

S.M. Darby

S.M. Darby

I am a freelance author with over 25 years of experience as a professional, ASE certified automotive technician and shop owner, muscle car enthusiast, avid street racer, and classic car restoration specialist.