Code reader: fault vs. pending?
 

Based on my autolocking malfunction as described in this thread I bought a code reader.

As a number of people noted whatever is causing this problem did not show up. The code reader did not list any "current" fault codes but rather a "pending" code of P0174, system too lean, which probably means I need to clean or replace my MAF sensor.

However, what is the difference between a fault and a pending fault? Is there a way to know when this code was generated? i.e. could it be old and was just never cleared? I'm not noticing any driveability or mpg issues. Don't want to throw time/money at some mysterious code that may mean nothing.

Thx.

according to actron:

What are Pending Codes?
"OBD II Pending Codes are also referred to as "continuous monitor" and "maturing codes". An intermittent fault will cause the computer to store a pending code in memory. If the fault does not recur within 40 warm-up cycles, the code will be cleared from memory. If the fault recurs a specific number of times, the code will then mature into a diagnostic trouble code (DTC) and the MIL or "check engine" light will turn on. "

i would advise you to clear the code, and see if it comes back. that way you're not changing anything without reason

Will do, thanks. My Autel manual does not have this definition...but what do you want for $20?:)

Nothing wrong with cleaning or replacing the MAF but it's more likely that you have a vacuum leak. Start here:
- How to diagnose a BMW E39 engine misfire (1)

In addition to what was stated already, three pending codes turns into 1 fault code.

The concept of the FTP plays a role in each of those pending faults, so, for more details, I typed /ftp F3 into the bestlinks for you to find this:
- How to identify all BMW computer-specific OBDII DTC diagnostic fault codes (1) & how to better understand the key EPA federal test procedure (FTP) concept of the BMW SES "drive cycle" (1) & how to diagnose a typical BMW E39 engine misfire (1)

Good question! Maybe. But I don't know of any way.

Most of us just clear the codes, and then look for pending codes after driving the FTP (about 11 miles - but it could be as much as about 50 miles if you don't know how to drive the FTP).

OBD codes are for emissions problems, many of which are unnoticeable to the driver. A code will be thrown for a misfire of 1 in 200, for example, which you can't feel until/unless the ECU shuts down the fuel injection to that cylinder (ask me how I know).

So it looks like this code actually had a reason for being: today the car stalled out a few times, while moving at low speeds. It started right up again, and then ran fine, then stalled again. I am thinking that perhaps this code is indicative of a failing O2 sensor. The codes were cleared last week when I had the car inspected but I will re-check what's in there tomorrow.

Could P0174 indicate a bad oxygen sensor? I'm now wondering if this has also been causing my poor MPG issue as described here.

Thoughts?

P0174 (and it's brother P0171) indicate a lean condition which is indicative of a vacuum leak. It is doubtful this is caused by a faulty O2 sensor. Vacuum leaks usually cause poor engine performance (e.g. misfire). Reduced gas mileage may be a by-product of poor engine performance.

The most common cause of the P0174 code is a vacuum leak.

That said, it's entirely possible that you need to replace your oxygen sensors if they have about 100,000 miles on them.

Does your scanner show the oxygen sensor voltages and switching times?

BTW, is the CEL lit?

I was thinking/hoping that such sporadic loss of power (car dies, then runs fine, then dies, runs fine on highway, gets 30MPG on highway etc) would be more likely to be a sensor issue that a vacuum leak. My O2 sensors have at least 50k miles on them. I'm also still thinking MAF, either dirty or fouled sensor. My scanner is just basic OBDII. I will re-scan in the morning and see if it's still P0174 or something else.

No CEL, and no CEL after the engine died, either, but I'm sure there are pending codes.

What you need to do is some diagnostic work and stop guessing.
Look in the threads I pointed you to for pictures of the most common hoses that cause a P0174 lean condition vacuum leak.

I had a P0174 (among others) for example, and my symptoms were exactly the same as yours.
- Does the order of the misfire OBDII DTCs diagnostic trouble codes actually matter (1)

No codes or pending codes in the computer. Visually checked hoses for cracks and found none. Can't think what would cause stalling and not throw any code at all. Not equipped for a pressure test so might have to bring to my mechanic for diagnosis.

The bolded would make me believe you have either water accumulation in the fuel filter or it's clogged up with debris. If you have changed the fuel filter not long ago, disregard my comment.

That is a good thought but my fuel filter is new as of July 2011. However the cut outs did occur less than a mile after filling up a near empty tank with gas. I suppose it's possible that there was some condensation in the tank or some debris. However I drove a mile or two on the highway with no issue and then it conked out again when I parked. So far today no problem. Lots if speculation but so far no obvious answers. Having it smoke tested and also going to check battery and alternator voltage.

Just spoke to mechanic. Smoke test showed no vacuum leaks. Alternator OK (thank you Jesus) and battery OK.

However GT1 shows a bunch of faults in fuel mixture, running both a bit rich and a bit lean, so something is going on. Car is being evaluated for a few days.

Once the dust is settled, it would be nice if you can update with the findings. The both "rich & lean" is very intriguing.

I absolutely will. My money is on MAF. He is going to monitor the computer while he drives it to see what is happening with the fuel to air ratio. He said faulty O2 sensors would throw codes that are not appearing.

yep. I replaced my oxygen sensor at about 110,000 mi because I felt guilty.

As far as the MAF, it seems to me that monitoring the mass of air at idle would show you if it's defective. I have a $27 ELM327 OBDII device that communicates with my snmartphone running the Torque app. It displays the mass air flow.

Anyway, I'm sure the mechanic will solve the problem. Please update us with the solution.

tagged cause im curious too

Here is an update:

They found nothing wrong (except the codes, below). He drove the car at least 40 miles, city and highway. There are codes indicating 8-10% below proper fuel trim levels...too low to set off the CEL (or not often enough to meet the drive cycle criteria) but enough to show up as pending. As noted, alternator is fine. No vacuum leaks...and they also pressure tested it while I was there and it was fine. As an experiment they took out my MAF box and swapped in a new one. I am going to drive the car for a week and then bring it back and they will read the codes again. Gotta love VAC Motorsports.

Quite the mystery, but it is driving fine now so who knows. I will report back in a week, or if the car stalls out again.

EDIT: one thing I forgot, they took off that little plastic nipple on the rear of the engine...it's part of the vacuum system, not sure what it's called. It showed some wear and slight cracking at the edges. Replaced preventively but I doubt that was the problem.

Does that mean your short term fuel trimis -8% to -10%?

Was that a vacuum cap that they replaced?

I believe the piece is #9 here but am not certain. Tech pulled it out by hand, no tool.

http://www.realoem.com/bmw/diagrams/v/u/73.png

As far as the trim, all I can tell you is what he told me, which is that the codes indicated 8-10% below the normal operating fuel to air ratios. I didn't see the readout. I have been noticing poorer in town MPG, though 8-10% would be barely noticeable, just a few MPG.

Yes, that is a vacuum cap.

Since the P0174 code indicates a lean condition, so I suspect that 8-10% is the short term fuel trim. That would indicate that the ECU is adding more fuel because the oxygen sensor indicates the mixture is lean.

Did the mechanic look at the performance of the oxygen sensors?

That little "endcap" is actually covered in gory detail in these threads:
- How to locate all problematic (between 1/8" & 9/32" ID) 3.5x1.8mm, 3.3x1.8mm & (between 17/64" & 9/32" ID) 7mm ID vacuum tubing (single material), vacuum hoses (multiple material), 3.3mm OD curved vacuum pipes (rigid tubes), 3.5mm & 7mm ID vacuum endcaps (closed end) & 7x3mm manifold o-rings (1) & 7.52X3.52mm and 9.2X2.8mm fuel injection o-rings (1) & gaskets (1) on the M54 engine & where in the USA to get new vacuum tubing & vacuum caps (1) & what SAE sizes to get for all the metric M54 engine vacuum tubes, hoses, pipes, and caps (1) & correcting the F-connector errors in the realoem diagrams (1) & finding the ends of hard-to-locate vacuum tubes (1) & sorely needed clarification on how the M54 CCV vacuum port works on the M52 CCV valve connection to the fuel pressure regulator connection (1) & how to make, borrow, or buy lean-condition misfire test tools to test for vacuum leaks & lean conditions (1) (2)

Note: Ignore the caption of this picture below of 'my' endcap - the answer is in the threads above.

http://www.bimmerfest.com/forums/att...1&d=1319779118

We decided not to throw (more) money at the problem at this point. He said the ECU has a bunch of monitors for the O2 sensors and that they would be throwing specific codes if they were bad...those codes were not present. My guess is, in a week if the trim level codes are still there, we will have eliminated the MAF as the problem and then try to swap out the O2 sensors. Trying to eliminate each piece of the puzzle rather than swap a bunch of things at once.

My original suspicion was the O2 sensors.

That's a reasonable approach. I understand not throwing parts and money at the problem. These parts are not cheap. But what I was suggesting is that the mechanic should look at the signal from the MAF and the oxygen sensors before replacing those sensors.

Anyway, keep working at it, you'll get it fixed.

Latest update:

Drove the car with the new MAF installed, for two weeks. The car has not stalled again and is driving fine. We hooked it up to the computer yesterday and there were no codes. However, we then looked at the adaptation ratios and they are still off, by about the same amount. So, the MAF did not solve the problem. We re-set the adaptations to zero (this was not done when the MAF was put in), so we could start fresh to see what the car is doing. I am going to drive it for another few weeks and then we will look again. He noted that there is a software update available for the ECU, which costs $150. He said I could do it if I wanted, that it would not hurt the car, and might help.

Thoughts on this?

I also noticed that the Aux blower is coming on (in winter) which is not normal. We then checked the rad fan and--guess what?--the fan clutch has too much play. So, that will be this spring's project.

It never ends.