Had the same symptoms last week on my '99 ML320 (90,000 km/55,000 miles). Got a new CPS and it hasn't let me down since. One thing though, 2 days after the sensor change, I got a CEL, and have reset it 3 times now, same code 170 and 173. It runs great, just sends a code every second day or so. I am told it's the MAF, but wouldn't that cause it to run /start crappy? I have no issues about performance. I am also told it's an O2 sensor, but wouldn't I get a different code? How do I know what part to buy or how to check?