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