We have a 2015 ML350 W166 with about 55K miles that REFUSES to run the O2 Sensor I/M Readiness check for emissions testing (All other tests have run/completed! At no point in this process has there been any check engine lights or DTC codes.

Details of situation (Live in CA)

1. Main Battery died in July 2025, replaced with Platinum Diehard

2. Probably drove 200-300 miles between battery dying and CA smog check which failed due to readiness monitors (primarily O2 not being ready)

3. Got the attached Drive Cycle from wife's indy mechanic and drove it 7-8 times following the directions very closely. (Yes I realize it's for a BMW but he said it is what he uses for MB too). Every time I've done this ALL the monitors set EXCEPT O2 Sensor.

4. Took the car to indy mechanic so he could flash the computer and do the drive cycles - O2 Sensor test still not running

5. Mechanic noticed the engine temperature seemed low, (I had noticed unusual swings in temp as well but hadn't made anything of it) and replaced the thermostat as the engine may have been too cold which may not have triggered tests.

6. Multiple drive cycles (4-5) post-thermostat replacement and test still not running

7. Oct 10: Self-replaced all four O2 sensors, ran drive cycle day after. O2 sensor check still not testing.

Spoke with MB Service manager and she said she had seen this before and replacing O2 sensors fixed it. Maybe I just need to do a few more cycles?



I've seen a few other drive cycles, one from the attached MB service bulletin that appears to be for older cars (and I did try that one before without success), and another one that is attached that I haven't performed yet. Has anyone with a W166 had any luck with these things?

Attachments:

1. Drive cycle performed many times (recommended by indy mech, pic with thumb in it)
2. MB Service Bulleting Drive Cycle (Done several times)
3. Unperformed drive cycle (Should I try this)
4. Screenshot from OBD2 reader I/M readiness status

Car is due to be registered in November so am running out of time and not sure where to go from here. Hate shotgunning these problems...need to get to root cause!

Any suggestions would be greatly appreciated.


This drive cycle has been performed at least 10 times.

 

You have an State emission test ("SMOG") coming up shortly and your O2 readiness keeps not completing or becoming ready. You have replaced various parts including all 4x O2.

Can your request your local MB specialist to
-1- scan the engine computer for live faults?
-2- read the LTFT percentages

This should show why the emissions control fail.

Needless to note exhaust sensors are very specific mix of Lambda and Legacy O2 sensors.

 

Yes I can do this. I guess its fair to assume that the MB dealership has more detailed diagnostics they can perform vs my wife's independent mechanic.

In regard to your comment about the Lambda and legacy O2 sensors - Are both of those part of the oxygen sensor? I'm not familiar with what the difference is.

I did notice that the downstream sensors were NGK brand and I replaced with Bosch. This confused me so I tripled checked that had ones that were compatible. (The upstream sensors were Bosch and I replaced with Bosch). The fact that they were different were odd to me and makes me wonder if they had been replaced in the past (we bought the car used in 2018).

So LTFT might impact the O2 sensor test from running but not necessarily throw a code?

 

Luckily this is dealing with sensors that are closely supervised by the ECU.

From what I understand, it sounds like there are no related faults... yet OBDII monitors remain incomplete.

Swapping further parts without cause can be a generous waste of resource. Your local MB specialist may focus on troubleshooting the root cause with a full system scan.

The long story short is the exhaust sensors are good but the engine likely has an ACTIVE ISSUE such as intake leak or low fuel pressure that's preventing normal engine operation to complete monitors.

You understand why these system monitor tests must set "complete" status. Un-complete calls for attention.

 

I agree and hate shotgunning an issue without knowing root cause. The independent tech said he ran the Mode 6 tests (are these the same as the ECU diagnostics you mentioned) but didn’t see any other faults. Agree there is probably something else going on but wanted to do the sensors based upon feedback from the MB dealer service manager. Gonna do the drive cycle one more time tomorrow and then will take it in to dealership.

 

good idea, somebody needs to troubleshoot this.
It won't take long.
The result will be amazingly simple but yet still hard to focus on data pointing to one detail.

 

UPDATE: After completing the drive cycle yesterday the O2 sensor readiness check has run and the car passed the emissions test today.

Will have to ask the wife if she wants to get it further inspected (assuming not at this point since it appears to be running fine). May take it in the next time she is out of town.

Thanks for your responses!