This is going to be a long and technical post, but if you want to learn about car engines, this should be a good read even for the non-technician. A Honda Odyssey came into the shop with the check engine light on. I scanned the codes and found two seemingly unrelated codes. When you have multiple codes you have to decide if you can diagnose both at once or if you have to fix one before the other. The two codes that I had were P0171 Bank 1 Fuel System Too Lean and P3400 Valve Pause System Stuck Off Bank 1. Besides both being on Bank 1 of the engine, I wasn’t sure how they could be caused by the same problem.
First let’s look at the lean code, P0171 Bank 1 Fuel System Too Lean. This code gets set when the computer determines that it has been needing to add more fuel than it should to continue proper engine operation. This is called fuel trim. The base value in the scan tool is 1.00, representing 100% of normal fuel injector duration. Then the computer determines that the air/fuel ratio in the cylinders needs to be different based on various sensor readings, it will either increase or decrease how long the injectors are spraying fuel. If the mixture needs to have less fuel, needs to be leaner, it will decrease, lowering the fuel trim number. If there needs to be more fuel, it will increase. There are both short and long term fuel trim numbers. Short term changes based on current conditions, while long term changes over time to bring the short term back to around 1.00. On this particular engine, when the long term fuel trim reaches 1.45, it sets the P0171 code, because it’s been having to add a lot more fuel for a long time to compensate for the mixture being lean for a long time. This could be caused by several different things, including skewed oxygen sensor readings, or low fuel pressure.
Now let’s look at P3400 Valve Pause System Stuck Off Bank 1. Honda V6 engines have a Variable Cylinder Management (VCM) system, which enables them to run in 3 or 4 cylinder modes during low load conditions to conserve fuel. This is accomplished solenoids controlling oil flow to special rocker arms. Running a 6 cylinder engine on fewer cylinder causes the engine to vibrate, but it is equipped with active engine mounts that soak up the extra vibrations so it’s not as noticeable to the driver. There are also oil pressure switches to monitor whether the system is working properly, and if the switches are inditicating different from what is expected, it sets a code, in this case P3400.
The symptom that the car was experiencing along with the check engine light was a severe engine vibration. When I brought it into the shop, I noticed that it vibrated when idling in gear, but not when idling in park, with the engine cold. When I found that there were two different codes I had to figure out if I could diagnose both at once or only one. The troubleshooting information for the P0171 Fuel System Too Lean code listed several other codes to diagnose first before starting that code’s specific tests, but P3400 was not listed. When I searched Honda information for the P3400 code I found a service bulletin that said that if the engine oil level is good, it needs an oil pressure switch because they can fail intermittently.
I looked at the engine data list and saw that while the VCM control solenoids were all off, as expected while idling, the bank 1 oil pressure switch was reading “on”, when it should be “off”. The bank 1 oil pressure switch should only be “on” when the engine is in 3 or 4 cylinder mode. Because of how badly the engine was running, and how in depth diagnosing the lean code could get, I decided to just replace the oil pressure switch first and see if the lean code would come back. I replaced the switch, cleared the codes, and reset the PCM to make sure the fuel trim was reset. While going on an 18 mile test drive I kept an eye on the fuel trim. If there was still a problem, the number would’ve gone up towards the 1.45 threshold, but all I saw was it went down a little bit, to around 0.96.
I honestly don’t know what was causing the vibration or the computer to think that the mixture needed to be richer, but I went with my gut and it worked out, only needing the oil pressure switch. Looking back I wish I’d taken note of more data, like what the fuel trim was at when it came in, but I hope this post had enough information to make sense and be interesting. If you made it this far and liked this type of post, let me know, it was fun to take my time writing this.
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