Service Engine light and Oxygen Sensors

[Riggodeaux]

My 'service engine soon' light is on and I've been reading up on possible causes. I will run the codes this weekend. I anticipate cleaning the MAF sensor, regardless, but since the CCV components were replaced 14 months/24k miles ago, just before I bought the vehicle, I'm assuming [perhaps erroneously] I won't have to do the CCV job. One possible source I've seen id'd is an oxygen sensor - and I see reports that replacing them [x4, parts cost maybe $280] as a 100k mile maintenance item. I'm overdue for that, if so, and need to do spark plugs, as well, due on the same schedule. Have any of you done the sensors as a 100k miles preventive maintenance item? I have viewed a couple of good videos on this DIY. Any tips on which codes point to the CCV/vacuum leaks, as opposed to the MAF or O2 sensors? Thanks in advance for any guidance.

[white46]

Sorry to say, but this is waste of time without knowing the actual code.
It can be as easy as replacing fuel cap to vacuum leak, CPS, etc...

I've replaced all 4 O2 sensors (2 post, 2pre) bought from Amazon for $200.
When O2 sensor goes bad, it will shoot out very specific CEL code to indicate which one to replace.
If you are planning to DIY, make sure to have O2 socket tool.

As for vacuum leak, your best friend will be the smoke tester. Without that it's just guessing game.

[oldskewel]

Originally Posted by white46

...When O2 sensor goes bad, it will shoot out very specific CEL code to indicate which one to replace....

So you would not recommend replacing O2 sensors without codes, even at high mileage?

My new-to-me 169k mile X5 with lots of repairs and not so much maintenance shows no record of O2 or spark plug changes. I'm doing the plugs (just like I'm on my way to doing all the fluids), but I usually only replace O2 sensors when there's a code. No running problems except that gas mileage on our first tank was only about 12 mpg (all city, 2001 3.0i). If I knew for sure that the O2 sensors were original I'd replace them. But there's a good chance they were replaced already and I just don't have the records. Also, the car has blown extremely clean smog check results for all of the 3 tests that I have records of.

[g300d]

Yes, read the codes first, it could be any single or combination of a big number of parts from the many subsystems that can trigger that light.

My SES just turned on as I type this, I just finished with the secondary air system and dmtl code...god knows what it is this time!

[StephenVA]

Preventive spark plug replacement at 100 k is always a good thing. O2 sensors also have approx 100k/10 year life.
As has been posted already, the first step is always, pull codes and perform the repair tree diagnostics aligned to any code that reappears after clearing the codes (record first then clear) and a short road trip. Hard failures will appear right away,depending on the part/ circuit.
Most poor economy, poor performance issues will be in the air intake, or ignition side of an engine. Plugs, o2 sensors, and CVCC systems are part of those areas. So the first step is good visual inspection of those areas, pull codes, and then look at specific parts/connections. If you find bad or lazy o2 sensors, one small tip is to hose them down with a WD-40 like product 24 hrs before you try unscrewing them from the exhaust system. If you have o2 codes it will be most likely to be the upstream (before the converter) sensor(s).

Good luck and post your codes if you need assistance.

[Riggodeaux]

My trusty Peake shows table 19, codes F5, F6, secondary air system flow too low, cyl 1-3 and 4-6. Autozone code reading shows codes PO491, 492, same description. I'll do a search for these codes, but any suggestions for the diagnostic tree to narrow this down? I'm leaning toward replacing the O2 sensors while I'm doing the plugs, even though I have no performance issues, as a 100k+ miles PM effort, but of course want to identify and fix the actual cause.

[Riggodeaux]

I found a pretty good DIY thread in another forum linking to a 2008 BavAuto newsletter article describing diagnostics for the Secondary air pump system, which generates these codes. I had to replace this 'smog pump' on my 1997 E36 convertible, so I've been here before. Hopefully, just a vacuum line fix, and if not, the check valve or [hopefully not] the secondary air pump itself. I would appreciate any links or info anyone can provide and I'll post my diagnostic process with links once done.

[g300d]

Ah, just went through this.

The bavauto link shows the earlier version of the secondary air system, wherein the air valve has a separate vacuum actuator system. Our trucks (2004-up 3.0) have the simpler version where the valve is simply pushed open by the secondary air pump air flow.

Usually if the pump is working (which you can check via the process in the bavauto link), the problem is the valve.

What is not commonly discussed online is at least one person has reported success in cleaning the valve.

Remove the valve from the exhaust manifold, spray some carb cleaner into the exit port of the valve, and allow to soak for a bit to work on the carbon. Then spray some more to rinse out. Repeat as necessary till it looks clean.

Re-install the valve (many people online reuse the gasket, its metal), reset the code, and hopefully that does the trick.

Sometimes people still have the code pop up after valve replacement even if the pump is working, which is resolved by a new pump. My guess is that the older pump has weakened enough to not push the needed amount of air. No sure way to diagnose as I've seen, till you are sure the valve is ok. Sort of a trial and error process. Most start trial and error with the valve because it's cheaper.

As I understand, you dont need to include the secondary air mass sensor and air cleaner in this troubleshooting process because since the code is a report that airflow is low, the AMS is working.

Good luck!

[Riggodeaux]

Now I feel like an idiot. After researching the codes Saturday, this morning I stopped to fill 'er up and, sure enough, I had lost the gas cap . . . duh. Light still on after I stopped by the BMW dealer en route to work [$48 replacement cap, but it does have the tether and 'bmw' stamped on it ....]. Hopefully, the missing cap was all it was throwing the codes. We'll see on the code reset. The good news: premium fuel down to $2.569 per gal at the Kroger with the Kroger 'frequent flyer' .03 discount. The teaching point: never underestimate the possibility of user error. Back in the day, the motor pool sergeant would call this a 'headspace and timing' problem.

[white46]

Originally Posted by oldskewel

So you would not recommend replacing O2 sensors without codes, even at high mileage?

My new-to-me 169k mile X5 with lots of repairs and not so much maintenance shows no record of O2 or spark plug changes. I'm doing the plugs (just like I'm on my way to doing all the fluids), but I usually only replace O2 sensors when there's a code. No running problems except that gas mileage on our first tank was only about 12 mpg (all city, 2001 3.0i). If I knew for sure that the O2 sensors were original I'd replace them. But there's a good chance they were replaced already and I just don't have the records. Also, the car has blown extremely clean smog check results for all of the 3 tests that I have records of.

Well... That's your choice. I guess with new O2 sensors, it will improve mpg little bit, but will it improve enough to justify cost of O2 sensors? I dunno.

Of course it's always good to perform preventive maintenance, but personally I wouldn't worry about them, especially with clean smog results.