O2 sensor hell. Max fuel trim DTC

[andrewwynn]

I've been meaning to replace my knock off sensors for a while but money is extra tight and they were working

Ish

I bought a two pack for $45 or something kler deal and they worked fine for most of a year.

One failed so they sent me a pair under warranty. That came in handy when the same one (bank one) failed a few months later.

About a week ago got an error for no heater bank one pre cat. Same damn sensor. Of course days after my tax refund was spent.

Got my "second tax refund" last wed and bought a pair of Bosch from fcp euro. I had to drive my car to and from Chicago burning 12% extra gas (fortunately that's as far as fuel trim will apparently go). Idle pretty rough indeed warmed up and did get a handful of misfires over a couple days.

Got the new sensors installed before my next trip to Chicago and reset adaptations before departure.

To my dismay: 12% fuel trim the whole trip down and the next day also.

So: normal sleuth says it's fuel delivery. Not cool.

Today I go to get some fuel injector cleaner and to my shock, fuel trims are like 3%.

WTF?

Anybody seen this before?

It's acting like there was a clog in the fuel pressure regulator that broke free or something. I didn't get a chance to meausre fuel pressure yet.

I'm hoping it was somehow an unlisted long LONG term fuel trim because of course the long term was reset when I reset codes and adaptations

75 mile drive Monday getting home. Fingers crossed I'll get better than 17 mpg on my trip up.

[crystalworks]

I had nothing but problems until putting all 4 Bosch OEM replacements in. Tried Bosch splice in... Good for a few months. After second splice in failure I went with the aforementioned plan and haven't had a problem since. Hate o2 sensor codes. Hopefully your fuel trims are back to normal though.

[80stech]

O2 codes are rarely caused by the O2 sensor unless you don't have good 'replacement' O2 sensors!

[crystalworks]

Originally Posted by 80stech

O2 codes are rarely caused by the O2 sensor unless you don't have good 'replacement' O2 sensors!

Very true, but at 100,000+ miles. But mine were heater codes and not rich/lean codes.

[andrewwynn]

Well I was not surprised by the wonky fuel trim on the bank with the broken O2 heater but the strange thing that happened was during the course of 90 minutes drive the other bank joined right in and both were slammed max 12%



Today when I first started it ran pretty crapily which I expected. I cleared codes and drove to the auto parts store for injector cleaner and with only 1/4 tank it should do a good job.

Oddly when I checked the trims on the way back from parts store they were normal (last pic). I have a hard time guessing what could have been the cause unless like I said there is some deeper level fuel trim that took maybe five starts to clear.

I have noticed that changing a sensor often won't fix the heater dtc unless you force a reset by running with the O2 in question unplugged for 1-2 starts.

I'm hoping somebody else experienced similar where it took a couple days and half a dozen start cycles before it came back to proper operation else I still need to root out the actuak cause = fuel pump or FPR/filter.

[andrewwynn]

Originally Posted by 80stech

O2 codes are rarely caused by the O2 sensor unless you don't have good 'replacement' O2 sensors!

Unless the code is "no heat" it's for sure the sensor.

You'll notice the codes aren't specific to the O2 sensor just that fuel trims (which are controlled from O2 input) are maxed and DME still not happy that it's too lean.

I only knew it was O2 related because the FIRST time I read code agree SES, I got a no heat pre cat bank one.

Minutes later I pulled up the fuel trim and saw 6/2 then 8/2 then 12/4 then within an hour 12/12.

[andrewwynn]

I just realized I never posted follow up. After about two days of driving the car apparently decided the new O2 sensors were worn in and would use them to control fuel flow. Absolutely nuts. My fuel trims went from max to something semi normal but with a weird caveat of the long term being negative with the short term being positive. I've read that this is indicative of MAF worn out. I've swapped with wife's and no change but no surprise both our cars are same age.

If PUA ever kicks in I will get a couple new MAF. At roughly 185,000 miles probably only 50,000 overdue. I did notice wife's short term trims also are pretty high though she still gets much better mpg than I do. Her car is on the disabled list with apparently some fuel supply problem that i would like to actually determine the actual problem before I use a parts cannon.

[upallnight]

Originally Posted by andrewwynn

I just realized I never posted follow up. After about two days of driving the car apparently decided the new O2 sensors were worn in and would use them to control fuel flow. Absolutely nuts. My fuel trims went from max to something semi normal but with a weird caveat of the long term being negative with the short term being positive. I've read that this is indicative of MAF worn out. I've swapped with wife's and no change but no surprise both our cars are same age.

If PUA ever kicks in I will get a couple new MAF. At roughly 185,000 miles probably only 50,000 overdue. I did notice wife's short term trims also are pretty high though she still gets much better mpg than I do. Her car is on the disabled list with apparently some fuel supply problem that i would like to actually determine the actual problem before I use a parts cannon.

If you add the two numbers together, they should be close to 0. As long as the long term is negative, the short term will be positive to try and balance the fuel trim out. If the long term ever becomes positive, the short term will go negative.

[andrewwynn]

combine to positive though a little tricky since my scanner shows long term in millisecond and short term In pecemt.

Due to age and having replaced all the other similar sensors in think I'll be changing the CPK soon and the MAF with OEM parts.

A year ago I was used to seeing 4-5% for short term trim and now more like 7-8. Fuel consumption is up where I used to get 20-21 highway typically now getting 18-19.

I swapped MAF maybe 15-18 mo. ago between our two X5 as an experiment to see if that was why wife's X5 gets like 10-15% better mpg than mine and made no difference so if they are worn out they are likely very similarly worn out.

[andrewwynn]

The super strange thing and my main point to the follow up is that after replacing the O2 sensors it took two days before the repair "took" even though I reset adaptations right away.

Believe me I was not thrilled to see my fuel trims maxed within 30-40 minutes of driving after replacing the parts that determine fuel consumption.

With wife's FPR apparently shot, and the indication of my car running lean I would not be surprised if I'm also having a weak FPR especially since when using my car as a control I noticed a pretty reliable dip in pressure at high throttle, even if just half that of wife's.

In wife's case the initial prime of the pump only gave me like 42-44 psi, a clear failure of the FPR as with engine not running there is no flow to lower psi from a week flow such as a kinked hose or plugged filter.

Wife has new exhaust and intake CPS but mine only has now five year old intake CPS no indication of failure on the exhaust one yet.