[Solved] P0171 and P0174 - 2019 Chevrolet Traverse 3.6L

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Here is a list of tools I use every day at the shop. Let me know if you use them too in the comments!

Customer Complaint:

This 2019 Chevrolet Traverse with a 3.6 liter engine came into the shop with a complaint of a check engine light and stalling problem. The customer states that the vehicle will randomly shut off.

The codes in this vehicle were:

P0171 - Fuel Trim System Lean Bank 1

P0174 - Fuel Trim System Lean Bank 2

General Information:

Whenever I diagnose a vehicle, I have a certain process.

The first few steps of the process are:

1. Verify the complaint - In this step, I make sure I can duplicate the problem with the vehicle.

2. Check for codes - I use an advanced scan tool to retrieve the codes that have set the check engine light.

3. Check for TSBs - TSB stands for “Technical Service Bulletin”. They are documents sent from the manufacture, in this case General Motors, reporting known issues with a vehicle. Some TSBs include recalls, updated information, software update numbers, and other valuable information.

In my experience, the manufacture will know about a problem with a vehicle before independent repair shops experience it. This is why reading TSBs is important. There is a high chance that the problem has already been solved with information found in a TSB.

In this case study, I used a TSB to solve this issue.

Diagnostics:

When I retrieved the codes from this Chevy Traverse, I got two codes:

P0171 - Fuel Trim System Lean Bank 1

P0174 - Fuel Trim System Lean Bank 2

These codes can point to a ton of possible causes.

Anything ranging from a vacuum leak to a weak fuel pump can set these codes.

I wanted to check live data since I still had the scan tool connected to see if the vehicle was still running lean.

It was running very lean. As you can see in the picture below, total fuel trims for both banks were over 40%.

If you want to learn how to read fuel trims, I highly suggest ScannerDanner on Youtube.

As I stated before, in my diagnostic process, I research any TSBs.

For this specific vehicle, there is a TSB that applies.

The TSB number that I followed was:

#PIP5549A

I don’t want to restate all of the TSB. It basically said a customer may bring their vehicle in with these codes and the possible causes are a vacuum leak in the intake or a faulty purge valve.

With this knowledge, I used it to determine my next step.

I decided to check for a vacuum leak first.

When I check for vacuum leaks, I use a smoke machine. The one I use is in the picture below. I got it off of Amazon and it works very well.

During the smoke test, I looked for leaks and did not see any. I was convinced that it was not a vacuum leak.

The next step was to test the purge valve.

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To check the purge valve, I used a scan tool with bidirectional controls. The one I use is the Snap On Zeus, but there are plenty of great scan tools on Amazon cheaper than the Snap On Zeus.

I decided to connect a manual vacuum gauge to the purge valve. I did this by disconnecting the line that goes the other EVAP components in the rear of the vehicle and connecting a manual gauge in its place.

When the purge valve is commanded at 0%, it is not suppose to allow vacuum through.

But as you can see from the picture below, the gauge shows a vacuum even with the purge valve commanded at 0%.

Purge Valve commanded closed with the scan tool

Purge valve still allowing vacuum even when commanded closed

While the purge valve was still connected to the manual gauge, I checked the fuel trims.

The fuel trims came immediately down into an acceptable range.

I have found the problem!

The purge valve was stuck open when it was supposed to be close. This caused access air to enter the engine and cause a lean running condition. This also caused excessive vacuum inside the fuel tank. In the picture with the bad fuel trims, the pressure in the fuel tank was less than -9 inH20. I was a little surprised that it did not cause EVAP codes.

For more information on the EVAP system, check out this article.

Ford vehicles have a similar problem with their purge valves, but instead of lean codes, they have rich codes. Check out these case studies to see what I’m talking about.

2015 Mustang with p1450 code

2012 Ford F-150 with rich codes

After replacing the purge valve, the vehicle was fixed!

The purge valve is located near the rear of the engine bay. In this picture, I have the hose disconnected for testing.

Below are some pictures of the purge valve operating as intended.

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