Piston Slap: Cracking the Code, Sans The (OBD-II) Code?

Jason writes:

Sajeev, I have a 2002 Mercedes C320. I replaced the pre-cat O2 sensors at 94k miles as recommended by my service technician. This was in the summer of last year, so roughly a year and a half ago. Since then I have had the CEL twice for O2 heater circuit malfunction on the pre-cat sensor on the driver’s side. The first time I replaced the sensor and that was six weeks ago. Yesterday I start the car up and it’s back (verified at the Advance store with OBDII). Have I received two bad O2 sensors in a row, or could I have something going on with the car that is causing the O2 sensors to have problems?

Sajeev answers:

First off, let me express my supreme disappointment with our society’s collective inability to discuss engine troubles in the appropriate OBD-II error code. It adds doubt, mistrust, and possible financial misconduct in whenever a warning light illuminates. Mechanics should tell customers they have a such-and-such code. Customers should hit the forums to learn more, namely the diagnostic tree to find the code’s problem. Forget about World Peace: everyone needs to talk in code.

So we need the code to answer the question, but perhaps it doesn’t matter this time ‘round. Codes relating to O2 sensors don’t always mean the O2 is bad. Sometimes an “O2 bank lean” code stems from a bad vacuum line in an emissions system. Or, even more obscure, a failing connection from the engine to the PCV valve. Yes really, that has happened.

Not that I fault your mechanic, it is wise to replace O2 sensors at this mileage. But there are other items to address, because replacing an O2 sensor is sometimes like shooting the messenger. I will not go into further detail, because websites like OBD-codes.com are a far superior resource. Once you get that precious code, that is.

(Send your queries to mehta@ttac.com)