[vwong]

I posted this in the Mechanical Maintenance section (http://www.e90post.com/forums/showthread.php?t=905610), but got no response at all. I'm hoping to get more responses here in this section.


Originally posted in the Mechanical Maintenance section:

I did a search on P0171, and found threads mostly related to cars with tune and/or DP. My car is bone stock, therefore I decided to start a new post.

For the past couple months, I have seen the "Service Engine Soon" light popped up twice. Both instances were almost identical, and both didn't trigger the half check engine light (only the Service Engine Soon light came on). I was stuck in traffic going about 25mph in 2nd gear with very light throttle. Suddenly, the "Service Engine Soon" light popped up on the dash. The car would still drive normal. When I got home, pulled the code and it read P0171 Fuel Trim Too Lean (Bank 1). I just cleared the code without doing anything. The car drives normally, everything seems to be fine. Should I be worried? Is this a symptom of something about to break?

My car is out of warranty, therefore I tried to diagnose/fix the problem myself instead of getting raped by the stealer (already got raped by the stealer recently for a problem I knew before taking the car to them). So any help would be appreciated.


Latest as of today, 11/6:

First of all, in my original post, I said I was stuck in traffic going about 25mph in 2nd gear. My estimate was off after seeing the same thing yesterday. It was more like just under 20mph in 2nd gear.

The same code popped up more often recently. I've had this 3 times in the last week, most recent was this morning on my way to work. There were 3 different scenarios, I'll describe them below.

Scenario #1 (happened 3 times already): crawling in traffic at just under 20mph in 2nd gear. Engine was probably turning around 1k rpm. The Service Engine Soon light just popped up.

Scenario #2 (happened once): idling in a parking lot, and the SES light just popped up. The idle appeared to be normal when it happened.

Scenario #3 (happened once): again crawling in traffic at less than 10mph in 1st gear. Engine speed was again probably about 1k rpm. The SES light popped up.

All five times, the car appeared to operate normal, no weird vibration, no weird noises. And all five times, the car seemed to drive normal with the SES light on. This problem is causing me to get worried. What should I check first?

Thanks in advance.

[enrita]

O2 sensors maybe. Take it to a shop with the code not erased.

[vwong]

The code has been the same in all five instances, P0171 Fuel Trim Too Lean (Bank 1). There was no other code though. So in case it is the O2 sensor, will it be the pre-cat or post-cat O2 sensor?

[messar]

I'm seeing the same thing, although it's bank 2 (so P0174).

I've looked at the O2 sensors using a scanner, and they seem to be switching correctly (strangely, I can't seem to find voltage for the S1 sensors, only S2, but I can see current for S1, and it is switching as expected).

Next plan is to pull the cover, and see if I can find a leak somewhere - any ideas from anyone what would leak that only affects 3 cylinders??

Thanks,

matt

[ZBMW]

That could mean you are in need of new injectors if they haven't been replaced with the latest (and most expensive) ones. Any stumbles when the engine is cold?