I am really confused, about a year ago, the dealer replaced left rear O2 sensor, the truck had codes 156 and 161. About a month ago, CEL came on and is usuallu on for 2 days then goes off for maybe 1/2 a day and then comes back on again. I checked the almost freshly replaced O2 sensor's wiring, it seems OK. So I thought the MAF might be defective. I swapped the same unit from my other MB and the on/off CEL game continues as before. So if it's not MAF, where should I check next???

And it is '99 ML320.
Thanks.

Here are the codes:
http://www.buellwinkle.com/MBOBD2.html
I would start by checking, to see if there is 12 volts supplied to the O2 sensor Heater circuit.

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2007 C 230 Sport.

Manny, I got the OBD reader and there were acually 3 codes - 156, 161 and 173. I cleared them all yesterday and than ran the car for about 50 miles with the swapped MAF sensor, no CEL but when I read the codes again, it had 161 there again as pending code. So I cleared again and put the original MAF back, ran for 7 miles and no CEL came up. So I think that it is not the MAF, but rather that almost new O2 that was supposedly replaced a year ago by MB dealer. I will get underneath the truck tonight and measure if that Bank2, Sensor 2 is getting the right juice. Could the O2 be perhaps defective??

Sounds like a problem in the 12 volt ( O2 sensor heating circuit ).
Turn the ignition on & see if you have 12 volts on one of the terminals of the sensor connector.
If you do, I suspect there is an intermittent " Open " in the sensor.

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2007 C 230 Sport.

Thanks Manny, I will test it in my garage tonight.

Manny, I cannot separate the connector. It's so hard to reach that I could not figure out whether it is a twist or other type? Do you know?
Also, I put the old MAF back and now it gives PO173 and engages CEL after 2 starts. Before, on the other MAF it only stored PO161 but did not prompt the CEL. Strange.....

You don't have to unplug the connector.
I take a safety-pin, back-probe into the connector ( beside the wire ), clip on my voltmeter lead and read the voltage ( DC scale ).
On a 4-wire O2 sensor, the 12 volt lead should be one of the white wires ( the other white is ground ).

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2007 C 230 Sport.

Manny,

I succeeded to unplug the connector (broke 1 of 2 holding tabs :-(( ) and measured voltage supplied to the 2 white wires. I could not measure 2 at the same time due to lack of space there and poor light, so I measured one by one and used ground elsewhere. It is strange, but I got 12V on either one.
Peter

Well, to be honest, this has me stumped now.
Other than possibly an internal ECM problem, I just don't know why there would be 12 volts on both white wires.
Maybe our local wizard ( Arthur Dalton ) can chime in here with some suggestions.

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2007 C 230 Sport.

Manny,

I was doing some research on OBD II forum and found out that my problem might also be vacuum related, especially when only one code of the 170, 173 codes is showing. If both, it's usually MAF. What do you think?

Well, vacuumleaks have been known to drive a sane man to drink.
Other than a good visual check with a very strong light and possibly spraying some carburetor cleaner at all suspicious places/sources, the only other alternative I can think of is your friendly M-B dealer and his trusty gadgets.

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2007 C 230 Sport.

Yes, I sadly came to the same conclusion. But before I give up, I wanna try to clean the MAF, because when I switched the one from my C240, the reader only read a pending 173 code. Now with the old one in the truck (ML), the OBD reader pulls 173 and 161, none of these is pending. So maybe the system was trying to adjust with the other MAF, therefore only 1 pending code. And I also had no CEL. Oh well, CRC MAF cleaner is the last resort, cause I don't wanna to pull MAF again from the C - big pain in the .....

Code 173 = Fuel Trim Malfunction on Bank 2.
You may want to focus on that area.
Just a couple more guesses.
This code ( as I see it ) could be the result of O2 sensor problems, injector problems, or as stated above, indeed " just " a vacuum leak.

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2007 C 230 Sport.

So could the new O2 sensor (B2 S2) fail after just 15 months. Again , OBD code 161 is for heater circuit failure.