2006 Chevrolet Trailblazer P0171
Bashing the dealership at this point would be very easy, but I think we need to sit back and take a look at the entire picture. I looked up the causes for this code on this vehicle through Identifix and the two most common were intake manifold gaskets and O2 sensors.
- Scan Tool $174.95
Most customers are not willing to pay for diagnostics, especially if they are going to take more than one hour. When you have the option of trying to diagnose an intermittent problem which you know you are not going to get paid for versus repairing or replacing the most probable causes, which would you do?
This is where customer communication comes in. If the dealership called the customer and explained the options and risks, they did their job. We really don’t know if they did or not, but we have to give them the benefit of a doubt, until we know different.
Another occasional cause for the P0171 code on this vehicle is a defective MAF. I sent a new MAF to the shop to try and about two weeks later they called to say the code has again set. We again replaced the MAF back to the original. We have now exhausted all the known causes for the P0171 code on this vehicle. It is time to put our diagnostic caps on and get to work.
There are four master technicians in my department at Wells. We all had a part in this diagnosis but the leader was Jim.
The following are the steps we took to find the cause of this issue and our thought processes along the way.
Remember, when the code was cleared it can take two weeks or more for it to reset. In addition when this code would set it was always a short time after a cold start This means we may need the vehicle for a couples weeks just get the required data at the right time. There are plug-in tools for recording PID information, but in this case we felt we needed more than just a general view.
We did look at the freeze frame data and it gave us a few pieces of valuable information. Freeze frames are stored in the computer’s memory when a trouble code sets. They do not give you all the parameters, but they can give you a brief look at what was going on at the time the code was recorded. Freeze frames are only one frame long, you cannot see what happened before or after the code was set.
Looking at the two freeze frames side by side we could see many of the parameters are very similar. The short and long term fuel trims and coolant temperatures were very close the two previous times the code set. The most interesting piece of information was the run time.
When an engine starts an internal clock in the computer also begins. The two run times the code was set were nearly identical, right around eight minutes after the engine was started. This information was nowhere near enough to come to a conclusion, but it did tell us we need to record a movie when the problem occurs.
A movie will record PID information before and after the code is recorded (check engine light illuminates). The plan is to drive the vehicle after it had been sitting overnight and when the check engine light comes “on” push the record button on the scan tool. Because the code does not set after every cold soak, we may need it for a while to get the correct information. In the video we mention it took several test drives to get the information. In reality it took about ten. Ten test drives, can you feel the diagnostic charges increasing?
After recording the movie Jim put it onto a computer to trace all the parameters. Tracing means you can look at it as a line graph instead of just number values going up and down. The four parameters that popped out immediately were short and long term fuel trims and front and rear O2 sensors. Another advantage of a movie recording is that you can see the point where the record button was pushed and move the graphs forward and backward from there. You can also observe up to 16 graphs side by side if you like.
After noticing the strange swings in the fuel trim and O2 sensor graphs our first thought was the O2 sensor was going to sleep or losing connection to the PCM. Notice how quickly the O2 sensor drops off and the immediate reaction to richen the mixture by the short term fuel trim. When you see a drastic change in a sensor reading like the front O2 was producing here, either there is a connection issue, sensor failure or or there is unwarranted amounts of oxygen suddenly in the exhaust system.
To confirm the front O2 had a good connection and was functioning correctly, we looked at how the rear O2 was reacting in the same time frames. The O2 sensor is there to inform the computer of fuel trim changes and the rear O2’s main purpose is to report catalytic converter efficiency. The job of the catalytic converter is to convert harmful pollutants into less harmful emissions before they ever leave the car’s exhaust system. For example, carbon monoxide will be converted to carbon dioxide by adding a molecule of oxygen. This is one of the reasons a converter will store oxygen. But when its storage capacity has reached the limit it then just passes the oxygen through the system and to the atmosphere. Also, for this reason the rear O2 sensor will normally read a little bias rich.
In our trace the rear O2 was going extremely lean at nearly the same time the front O2 was reporting a lean condition. By the rear O2 reacting the same as the front O2 this told us both sensors are likely to just be doing what they are supposed to do. They were reporting at that moment in the drive trace the system was running extremely lean, which explains why the computer was storing a P0171 lean mixture code.
Could an engine running condition cause this large amount of oxygen? When an engine misfires, the oxygen in the combustion chamber does not burn or when a vacuum leak of this size occurs the idle will be unstable. There were no other drivability issues with this vehicle, which told us the engine components and sensors were most likely functioning properly.
At this point of the diagnosis we were stumped. If the excess amount of oxygen was not being produced by the engine where else could it be coming from? Our first thought was a possible crack in the exhaust system before the oxygen sensor. If this was the case and it was drawing this amount of oxygen we felt the driver would hear the leak. Also how could the leak not be there when the engine was cold, then appear for a short period of time during warm up and then seal itself off again for the duration of the drive? This did not make sense.
It was then I saw the lights come “on” in Jim’s head. He said “it is somewhere in the AIR system”. The Secondary Air Injection Reaction system (AIR) pumps air into the exhaust system during warm up to help the catalytic convertor get to operating temperature quicker. He was right it was the only thing that made sense. If the AIR was coming or staying “on” when it was commanded “off” the computer would think the system was running too lean!
Our next step was to find out just how the AIR system worked on this vehicle. On this vehicle the PCM controls the activation of the AIR system by simultaneously supplying grounds to the AIR pump and valve relays. When the system is shut down, the AIR pump will remain “on” for a short period of time. This is where the description of operation left us. But isn’t it interesting, the pump will remain “on” for a short period of time after it is commanded “off”? Why would you program the computer to hold the pump “on” after the system is shut down and why didn’t it tell us how long it was going to be left “on”?
The following is a theoretical synopsis, we did not find this information documented. The AIR solenoid directs the flow of the air pressure from the pump. We found the AIR solenoid incorporates a pressure sensor. When the computer deactivates the solenoid and continues to run the pump we concluded it is done to check the integrity of the AIR pump and hoses. By running the AIR pump with the solenoid closed a pressure should build confirming the pump is pumping and the system has no leaks. What would happen if the computer commands the solenoid to close and it intermittently sticks open for a short time? The computer would set a P0171 lean mixture code assuming the solenoid closed.
As you know by watching the video, replacing the AIR solenoid repaired the issue. Our question then was, “why didn’t it set a code for the AIR pump or for a leak in the AIR system? The best we could conclude was this type of monitor check was in its beginning stages and there was no programing for it as of yet in the computer. Had the programming been incorporated the diagnosis would have been much easier.
This vehicle was tied up for about two weeks and our actual diagnostic time was roughly 9 hours in total. We do not charge for anything, but a shop or dealership would have charged $900 or more just for the diagnosis. A pretty big pill for the customer to swallow! This is where customer communication comes in. After looking at the entire results, it would have cost less to do the job the right way the first time.
Link to video
Link to WellsTech Channel
Link to Wells website