I know narrowbands can go lazy with time, but what about widebands?

This is a sequel to the cold-start misfire mystery.

It turns out that we were looking at the wrong PID's for the front O2 sensor voltages, assuming they were narrowbands. Probing the wideband sensor PID's for Bank 1, Sensor 1 and Bank 2, Sensor 1 lambdas yielded a proper signal.

This 2009 Mercedes GLK 280 had an intake leak that was fixed by a mechanic. The LTFT's are now very close to zero (-0.8 %). The STFT's, however, are all over the jungle, cycling between -15 % and +20 % every 15-30 seconds.

What I have noticed is that the STFT's are following the wideband lambdas at idle, which cycle very slowly between 0.8 and 1.2 (11.76:1 - 17.64:1 AFR). Post-cat O2 sensor voltages are around 0.75 V, so rich as expected.

The reason why I'm seeing this car again is because of a CEL that manifests itself intermittently when coming to a stop at a traffic light:

  • P0171, P0174 - System too lean

My hypothesis

  • LTFT's are showing the system is airtight
  • STFT's are cycling wildly because of the front wideband O2 signals cycling
  • Widebands need to be replaced


Here are the DTC's and freeze frames

Here are the DTC's recorded

Freeze frame for P0171 - 1

Freeze frame for P0171 - 2

STFT's and Wideband lambdas at idle

This is before clearing the DTC's, but the behavior here didn't seem to change much after clearing them

Vehicle speed, wideband lambda bank 1, STFT's

Both bank's STFT & Lambda

Here is the throttle blip test (if I remember correctly)

Ignore the green line because it isn't Vehicle Speed and smoothing was turned on.

Throttle blip

  • 1
    I like this question. I've heard about this over the years but have never really seen any facts to reflect either way. I can see the factual response falling into either bucket and create a made up explanation for it in mind. Commented Apr 14, 2016 at 22:42
  • What do the sensors show at idle in park? What happens when you press the brake pedal in park? Is it the same as when the car is in gear?
    – Ben
    Commented Apr 14, 2016 at 23:58
  • In my experience when Denso AFR sensors go bad it ends up being a range issue and the sensor under or over reports voltage. If it's not completely shorted. I don't believe I've seen voltage cycle (for no reason) on one.
    – Ben
    Commented Apr 15, 2016 at 0:04
  • 2
    Speaking of vacuum leaks what about the brake booster? If the problem only manifests while coming to a stop that would be a good place to start.
    – Ben
    Commented Apr 16, 2016 at 0:09
  • 1
    Zaid: I like @Ben's hypothesis better. Test for a leaking brake booster. Watch the mixture sensors and fuel trims while pushing the brake pedal. If they go lean disconnect booster hose and repeat the test. Commented Apr 16, 2016 at 5:20

1 Answer 1


We found at least two issues

  1. The air filter housing wasn't forming a proper seal with the MAF sensor

    This engine boasts a quirky design where the MAF sensor sits at the back of the engine. The air filter housing seal is in the middle of this first picture, MAF sensor in the second picture (stills were taken from this video).

    Air filter housing

    MAF location

    Whoever had installed it didn't install the air filter housing correctly, so unfiltered air was making its way into the MAF sensor and causing all manners of problems.

    After seating the housing in the MAF properly, the STFT fluctuations vanished. Curiously, this vacuum leak was actually metered by the MAF sensor. Notice the slightly postitive LTFT's, which weren't registered by the app before.

    No more STFT fluctuations

  2. The brake booster line jiggles a little

    We monitored STFT's upon pressing the brake line and it looks to confirm that the brake booster/line is suspect. I think I can make out a hissing sound near the brake booster vacuum port but didn't have carb/brake cleaner handy to confirm it. Here is the trace when my friend tapped on the brakes three times in quick succession (car was stationary):

    STFT response to pressing brakes

The CEL still returns upon braking. At this stage we feel it necessary to repair the brake line vacuum leak before proceeding with declaring this problem solved.

I will update this answer with details as an when they become available.

Update : Found the smoking gun

And it wasn't the brake booster line. It was this: a crack in the part-load breather hose:

Part load breather hose closeup

Part load breather

The way we discovered it was by blocking the air intake and realizing that there was barely any suction. Because this hose was buried deep under the air filter housing, it wasn't visible but some brake cleaner made light work of uncovering it.

It looks like someone tried to repair it with epoxy.

As a temporary fix crack was sealed up with duct tape and the car taken for a spin. Although I didn't get the chance to log the fuel trims, the car felt much crisper in response to throttle changes.

There was no need to replace the wideband sensors after all!

  • I expect to see a change in mixture when the brake booster is activated, due to the dip in manifold pressure. But not that much motion in the trims. Would need a know good to confirm if this is normal. Commented Apr 21, 2016 at 5:53
  • @FredWilson updated with fix. Thanks for your help!
    – Zaid
    Commented Apr 28, 2016 at 20:33

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