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

Sajeev Mehta
by Sajeev Mehta
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)

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2 of 35 comments
  • Andy D Andy D on Feb 14, 2010

    Maybe apples and oranges, but my 88 BMW has a relay that powers the O2 heater. Just for grins I would swap in another relay that has the same pinout. In fact, I might do that to Marina's car as a WAG to fix it's cold stumble. My 350k mile BMW has its original cat

  • Paul Paul on Feb 18, 2010

    It seems to me the easiest an most logical thing to do is to put a oil pan heater on the transmission pan. It won't fix the probem as some tranny fluid will still be in the cooler, tourque converter and lines but it will make warm up quicker

  • Kat Laneaux Agree with Michael500, we wasted all that money just to bail out GM and they are developing these cars in China and other countries. What the heck. I understand the cheap labor but that is just another foothold the government has on their citizens and they already treat them like crap. That is pretty disgusting to go forward to put other peoples health and mental stability on a crazy crazed, control freak, leader, who is in bed with Russia. Thought about getting a buick but that just shot that one out of the park. All of this for the greed. They get what they lay in bed with. Disgusting.
  • Michael500 Good thing Obama used $50 billion of taxpayer money to bail them out and give unions a big stake. GM is headed to BK again with their Hail Mary hope of EVs. Hopefully a Republican in office will let them go BK the next time, and it's coming. The US economy is not related/dependent on GM and their Chinese made Buicks.
  • MaintenanceCosts "Rural areas hardly noticed COVID at all."I very much doubt that is true in places like the Navajo Nation or the Kenai Peninsula in Alaska, some of which lost 2% or more of their population to COVID.No city had a death rate in the same order of magnitude.Low-density living is a very modern invention. Before cars, people, even in agricultural areas, needed to live densely to survive.
  • Wjtinfwb Always liked these MN12 cars and the subsequent Lincoln variant. But Ford, apparently strapped for resources or cash, introduced these half-baked. Very sophisticated chassis and styling, let down but antiquated old pushrod engines and cheap interiors. The 4.6L Modular V8 helped a bit, no faster than the 5.0 but extremely smooth and quiet. The interior came next, nicer wrap-around dash, airbags instead of the mouse belts and refined exterior styling. The Supercharged 3.8L V6 was potent, but kind of crude and had an appetite for head gaskets early on. Most were bolted to the AOD automatic, a sturdy but slow shifting gearbox made much better with electronic controls in the later days. Nice cars that in the right color, evoked the 6 series BMW, at least the Thunderbird did. Could have been great cars and maybe should have been a swoopy CLS style sedan. Pretty hard to find a decent one these days.
  • Inside Looking Out You should care. With GM will die America. All signs are there. How about the Arsenal of Democracy? Toyota?