Do It Yourself Auto Repair, Case Study #1

This is my first, in a series of many, case studies for the "Do It Your Self" (or DIY) vehicle repair folks out there. Additionally, this is some good information for Junior and Intermediate Level Automotive Technicians!

Some vehicle history first. We'll start off with calling the DIYer Jim. Jim was literally pulling his hair out. He needed to get his vehicle to pass the states emissions test in order to get a current Inspection Sticker. He borrowed a friends "mini scan tool" and got the code p0171, left bank lean. Meaning the left bank O2 sensor had "seen" a continuous lean condition and triggered a code to set in the PCM and turn on the Check Engine Light.

Jim is "web savvy", so he went online and did some homework. All the recommendations pointed to the left bank O2 sensor ("left" being the operative word!). He went to his local parts supplier and purchased the sensor. Jim replaced the O2 sensor, cleared the code and had his wife drive the vehicle for a few days. At approximately 75 miles later, the Check Engine Light came on and the same code was triggered.

Jim went back online and dug deeper. It was suggested that a possible cause could have been the vacuum lines had cracked and were leaking; causing this lean condition and/or the lower intake gasket had failed. These are common problems with this type of engine. He also verified this with some "mechanic friends" of his. (I'll comment on that later)

Well, Jim went all out! First he tackled the daunting task of replacing the lower intake gasket. Luck was not in his favor and in the process, two of the intake bolts hole threads stripped out.

After re-drilling and tapping the bolt holes, he got the lower intake gasket and the upper plenum gaskets replaced. Jim also replaced every vacuum line! He cleared the code and again had his wife drive the vehicle for a few days. At this point he thought he "nailed" the problem.

Once again, the nasty Check Engine Light came on at about 75 miles. Well you can imagine Jim's frustration level by now. He got my business card from his local parts supplier and gave me a call. When I arrived on site, his wife gave me Jim's written history and a list of what he wanted me to check (he's a meticulous guy!). Needless to say, I immediately connected my Scantool (I use an OTC Genysis) and noted that the O2 sensor 1/1 (left bank) was "fixed" lean. I also noted fuel trim on the left bank was "maxed" and on cold start the idle was very low.

I then shut down the engine, removed the air intake ducting and noted that the "left bank" O2 sensor was an original factory installed part. I restarted the vehicle, connected my lab scope and verified the O2 sensor's condition. (But I've got to say that I knew which side of the engine was "left" and which side was "right").

I think you see the "moral" to this story. Poor Jim had replaced the wrong O2 sensor; he'd gotten "bad advice" online AND from "mechanic friends" (who turned out to be really fellow DIYers and technicians with very little experience!). He was lead to believe that the "left" bank was in relation to the "drivers" side of the vehicle, when in fact the "left" side was really in relation to when you were standing in front of the vehicle FACING the front of the engine. (Simplified)

Lessons Learned: (I love this part, because if we all learn something new everyday ........ WE are all that much better!)

1. I'm going to say this over and over and over again ... Back to Basics! Yeah, these new cars can be complicated, when it comes to engine performance issues, start with the BASICS and then "move on" ie: verify which side of the engine is which!

2. Did I mention Back to Basics? (I'll stop now!)

3. Testing is the key to your successful diagnosis! And knowing what to test for is equally important. Lets review; Jim had a Scantool that displayed basic codes, monitor status and limited data stream. A simple test was: disconnect the O2 electrical connector and view which O2 was actually the cause. Suspected O2 was lean, disconnected O2 should read mid range, .4 -.5 vac. Or rev the engine to 2k rpm (in park!) and "view" the O2's "switching" from rich to lean.

4. Jim had the DIY manual for the vehicle AND a wiring diagram. A simple test (not definitive) is to test, with a digital multimeter,the voltage of the suspected O2 sensor(less than .40 vac is lean, greater than .40 vac is rich. This may have pointed him in the right direction!

(Though I don't recommend this unless you can find your way around a wiring diagram!)

5. Don't "lock yourself in" on the recommendations of others! Get some professional advice! Hey, this is the "New Millennium"and there are plenty of places out there to give you FREE advice.

Jim spent a good amount of money in vain and got a few grey hairs in the process. But, hopefully, he'll never forget this lesson and will learn from it. I've kept the testing procedures in the very broadest of terms, so please don't take them as the gospel truth. I recommend finding the component test information in your DIY manual or OEM manual first. When seeking advice ensure the person is a certified technician, either A.S.E. or OEM Factory trained.

Until next time,

Chris is a retired Navy Veteran, who owns Advanced Diagnostic Services. Specializing in mobile automotive diagnostics and technician training. He currently writes on a wide variety of subjects and for 2 of his local news publications.

Advanced Diagnostic Services, Mobile Automotive Diagnostics and Technician Training

www.mobilediag.net/4

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