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Why following procedures and performing tests is needed

There are certain common trends that we all can see or predict in everything we do.  The interesting thing about automotive repair is that there are so many moving parts and because of that we can’t always assume these common trends or problems always manifest themselves in the same way.

We had a 2002 Subaru Forester L with 103027 miles come in with a Check engine light.  After pulling the codes we saw a P0130 bank one oxygen sensor incorrect information and a P0171 system too lean. When I took a step back and look at those two codes I could say the the bank 1 oxygen sensor (P0130) might be going lean and is causing the lean code (P0171).  Yet I have been doing this for way to long and realize we have to look at the data first. 

When a code is set it is accompanied by Freeze Frame Data. This Freeze Frame Data will tell the technician the environment conditions of the code. For example; speed (MPH), temperature, revolutions per minute (RPM), position of the throttle, air flow volume into the engine, fuel trims (how much fuel the intake system is using) and so on.  When I looked at the Freeze frame data I saw the the P0130 code set after the car was warmed up and had been driving at freeway speeds. I could also see the the car was adding a lot of extra fuel in the fuel trims. We first checked all my basics (vacuum leaks, oxygen sensor connector, map sensor connector, corroded grounds, check for control module updates and general engine condition). Everything check out ok, yet there was a Engine Control Modules (ECM) update for the P0130 code.  I knew I have to perform the ECM update, but since the freeze frame conditions were perfect for setting both the P0130 and the P0171 I decided to clear the codes first and test drive the car. 

I cleared the codes and moved the car in front of the shop. I was getting ready to take it for the test drive when I noticed the idle was really rough.  Then before I could get it out of the parking lot the check engine light was on again. I then pulled the car in and retrieved codes. The only code that was in the car was P0130 bank 1 oxygen sensor. At that point I know I have to do several things but do then in the correct order. Almost all the 2.5 Subaru motors have a problem with carbon build up in the throttle body.  This carbon build up can cause a rough idle. I then called the customer and told him I want to clean the throttle body and then perform the ECM update for the P0130 first before doing anything else. 

The throttle body was very dirty and the cleaning helped the idle a lot.  I then test drove the car after the ECM update and the P0130 code came back within a mile. I now know that the ECM is updated and the wiring and connector at the bank 1 oxygen sensor is ok.  I then called the customer and sold the bank 1 oxygen sensor. After replacing it I took the car on two (20 miles each) extensive test drive to set all the monitors.  I was feeling almost ready to let the car go when on the 19 mile of the second test drive the check engine light came on again. When I pulled the code it was the P0171 lean condition. 

Since we had already tested the car extensively and knew there were no vacuum leaks it was almost automatic the the Manifold Pressure Sensor (MAP sensor) had to be replaced.  In the beginning of the article I stated the we can find trends in repair procedures.  The reason I said that is because it would not be unusual to clean a throttle body, change a oxygen sensor or reprogram a ECM on a Subaru 2.5L, but it would be unusual to replace a map sensor.  In the past 28 years I have maybe only changed 2 MAP sensors on this model engine.  The MAP sensor tests the air pressure inside the intake manifold so to give data to the ECM to set the correct fuel trim.  Once we pulled the MAP sensor out we found that the measuring element was very dirty and had some damage. We replace the MAP sensor and then drove the car on several test drive for a total of about 60 mile. All monitor s passed and the car was done. 

In conclusion this car had a dirty throttle body, a bad or worn oxygen sensor, a bad or worn MAP sensor  and 1 ECM update.  You all listen to me drone on about following procedures and performing tests and this job was no exception.  One of the most important procedure in solving this car was the post test drives. If I would not have spent so much time on the final test drives I may not have found the MAP sensor on the first visit.  Thanks for reading. 


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