P0171 Repair Story

After chasing down and solving my P0171 check engine light (CEL) I thought I'd share my story and how it was ultimately solved in hopes that it can help someone else down the road.

Background: I recently purchased a 1999 Toyota Tacoma SR5 V6. I got it for a good price partially because it had a check engine light. I was told that the mass airflow (MAF) sensor was replaced and that he believed there was a vacuum leak. Looking at the truck it was mostly all original meaning hard cracked vacuum lines and even a large crack on the air intake hose.

When I pulled the code I then searched and found a ton of posts about what causes the P0171 and some troubleshooting tips. These posts were helpful but also full of back and forth and not straight forward. I also watched some YouTube videos about it and here is what I found most helpful.

The most common issues that causes were:
- Vacuum leak (intake)
- Vacuum leak (manifold)
- Dirty/bad MAF sensor
- Clogged fuel filter
- Weak/dying fuel pump
- Bad O2 sensor
- Air filter
- Bad PCV

As this was a new to me truck with no repair history I knew I wanted to give it a tune up and was okay with buying some parts. With that in mind I started with a tune up, new:
- Spark plugs
- Spark plug wires
- Vacuum lines (all were rock hard with some cracks at the ends)
- PCV valve (the old one was bad)
- Air intake hose (because mine was cracked)

After installing all of this I erased the code but it came back so I move on to a few more maintenance items that could be the cause.
- Fuel filter (the date on mine was at least 10 years old)
- MAF sensor (the new one that was on the truck was not OEM)

Code was still not solved. At this point I decided to actually learn about the code and how to troubleshoot it. I learned about the code and about fuel trims and some steps to narrow down the issue as opposed to just replacing more parts. Reading live data showed that I still had a vacuum leak. I decided to start tearing down the manifold to replace the gaskets. As I removed the throttle body housing I saw something odd, the throttle body gasket was installed incorrectly! I could not believe it. I pulled it, cleaned it, and then reinstalled everything and checked my fuel trims. Everything was fixed!

This was a freak issue but by knowing that it was a vacuum leak and not an O2 sensor or a fuel pump issue I avoided throwing more parts at the problem that would not have solved it.

So here is what I suggest for people looking into this issue.

First: Keep it simple. Clean your MAF, check your PCV, look for cracked/disconnected vacuum lines.

Second: Get your hands on a obd2 scanner that can read live data. These can be had for less than taking your truck to a mechanic to diagnose. You want to be able to see the short term fuel trim (STFT) and long term fuel trim (LTFT) readings. This data will help you pin point where the issue is.

Third: Get an understanding of fuel trim, what is does, and how it works. YouTube is your friend.

Fourth: Watch this video and narrow down your issue.

Fifth: Dig in to what you can, take things apart and don't assume it was done right.

Hope this helps someone find and fix there issue quickly.

 

Good advice... just wait until you do a proper diagnosis and replace a bad part with a brand new bad part and then you go down THAT rabbit hole. You noticed it immediatly, me, my brain will kick into diagnosis mode while I sleep and at 3 AM I will wake up and have fixed it.