Got a check engine light while driving yesterday. Today I took it to mechanic and he got two errors. 1. O2 sensor faulty (the less important one?) and 2. Cylinder 2 is misfiring. Two questions: How much is O2 sensor and where is it? And second, what is the most common and some less common problems for a cylinder to misfire? Is it a spark plug problem usually? Thanks for the info.

PS: I don't think I can attribute these two problems to DRIVING HARD, necessarily. >:)

The oxygen sensor is the most likely cause of the check engine light. Even if you do not drive the car too hard, the sensor is a general maintenance item at about every 50K miles or so. The oxygen sensor looks similar to a spark plug, and is on the exhaust manifold or pipe. It is a very simple repair. Purchase a new (Not used!!!) sensor (about a $100 for my car), remove the old one w/ a wrench, and screw in the new one. The hardest aspect is to trace the wire from the old sensor and plug in the new wire at the same location. Voila! You are done. The best place to buy a sensor is The Benz Bin. They are by far the cheapest that I have found.

Raghu
91 300SEL

You might try a request through PartsShop, above. Their part prices are usually about as good as it gets, and it supports this site. BTW, Hottee18m, maybe your plug loaded up from all the slow and conservative driving we all told you to do ;) (joke). Also, check the plug wires and connectors when you check the plug. They could be loose or bad. The bad news could be like on our Legend Coupe 5 spd when cyl 4 was misfiring - toasted valves!

[Edited by JCE on 01-31-2001 at 11:04 PM]

Dear David
I am assuming you are referring to the C280. You must understand that either of your codes are very important and both can cause a check engine light. Our 1995 HFM-SFI 104 engine have a small problem with deteriorating engine harnesses so be very cautious. A Code PO300 or Its corresponding cylinder lets say #2 PO302 can be very damaging to the catatlyst. You should find someone with a scanner and look at the actual values of the firing voltages. Look at the wiring by peeling back some insulation at the coils primary connector and you will be suprise. Os1 (front O2 sensor ) is located in the exhaust pipe in front of the catalyst and usually there is a probe (looks like an O2 sensor) in the rear of the catalyst to test catalyst efficiency. Parts replacing is not a good practice so get some help with a scanner, pull codes and fix the problem at its source and you will be much happier. Well, just got a post on our site to answer so got to go. Good Luck
Tech 1

[Edited by Tech1 on 02-01-2001 at 02:49 AM]

TECH1
 

Your C280 has a problem with a deteriorating engine harness? Is this the wiring harness? If not, what is it?
Secondly, my mechanic did pull the codes and that is how I knew the exact reason for the check engine light being on. Anyway, he reset them for now, and didn't have time to look at it today, but I am taking it in Friday morning. Will looking at the actual values of the firing voltage give you a more accurate idea on what the problem with cylinder 2 is? You also said to look at the wiring by pulling back the insulation? Are you talking about on the plug wires?
Is it safe to run my car without causing damage to the catalyst until Friday? He reset the codes and they have not come back on yet, but I still have a rough idle every 15 or so seconds. Thanks for all the help, I really appreciate it.

What I mean is that the 104 engine is known to have cracked engine wiring harnesses causing lots of codes and problems. You peel back any of the insulation at the coils or at the injectors and you would inspect the insulation on the wiring itself. If its cracked, replace the harness. Also, I know that codes were pulled, but what are they? They are many codes for an O2 sensor, they are some for implausible, short to ground, lazy, they are some for the heater of the O2 and if so the relay which controls the O2 sensor. They are misfire codes which may not be caused by the plug itself. You may have a bad resistor or a bad coil which would then possibly miss on two cylinders. Engine roughness is not so common. A quick test of the intake seals and vacuum hose should find the roughness. There is a lot more about Mercedes engine from 1991 to 2001 that techs seem to not think about. Things have changed and must be checked, verified and thought about before replacing things. For instance, if a code is retrieved for a heater of the OS1, a tech sometimes replaces the O2 sensor thinking OK, that's it, but a current test is needed at the O2 when the engine is dead cold started and the heater can only draw a max of 4 Amps and very quickly decend to .500 Ma to keep it hot. If you get 4.3amps, you get a check engine light for a O2 sensor. Well, there is a relay techs forget about, so one replaces O2, and forget about the relay. One may also replace the O2 relay and not check for high current which may be caused by a bad ground on the circuit. All this seperates the boys from the men and that's why, good knowledgable techs make good money because knowledge in a system comes from education and experience. PS, I Am one of those techs in the six figures a year and I deserve every dime of it. Ps, which shop are you going to in Newport area??
Good Luck
Tech 1
Support team http://www.lmvind.com

Tech1
 

I take it to a mechanic in Huntington Beach. They are called Zito's Auto Care. They can do a lot, but not everything on a mercededes. For example, I need new plug wires, so I have to get them myself, then they will replace them. Real good guys, real honest. Only on a rare occasion do I take it to Fletcher Jones MC NB. I take it there when Zitos can't help me.
Anyway, it was determined the spark plugs were bad, as were the plug wires. They are changing the spark plugs with OEM Bosch plugs, but I have to get the wires (I was quoted at $106 for the whole deal, is this a good price?), and they will install them. Where do you work ?

Everything runs beautifully!
 

Took my car in for service this morning. And when I got it back, it ran so smooth! Well I just wanted to update everyone as to the result of the problem. They checked spark plugs, they were pretty worn, but not too bad. THey replaced spark plugs with OEM Bosch. I also told them to go ahead and change the fuel filter, because I thought that this MIGHT be part of the problem, and I have no record of when it was changed last. Anyway, they changed the fuel filter (2 hours later... damn thing kept leaking) and said that the old filter was really bad. So spark plugs were changed and new fuel filter was put in. Check engine light was still on, so he ran codes, I believe he got P0130 100 (I could be way off), which was the 2nd O2 sensor. He tried to clear it, but it wouldn't clear. So the rough idle was gone and the car now purred, but still bad O2 sensor (according to Scanner). I told them I would have to deal with that another time because I was already late for a class. As soon as I drove away, Check Engine light turned off and everything is perfect! BTW, I got new tires today, and boy is it nice to be able to floor it from a stop and be making a turn without ANY slippage!
Well on top of my update, I had one question. The rough idle was obviously attributed to the old fuel filter and the not-so-good spark plugs, which in turn, caused a misfire within then engine (registering in cyl. # 2). Problem fixed. Could the rough idle have somehow caused a fualt with the 02 sensor without the sensor actually being bad? Cylinder 2 misfire cleared itself, but o2 sensor did not until short while later. If my o2 sensor is indeed bad, I should expect Check Engine light agian soon, right? And if it's actually good, then I'm just good to go?
I hope I didn't lose anyone on this! WOuld appreciate any help! Thanks guys, you're the best.

Hot
 

you should be good to go now. It was my understanding that the car has to run for a short time AFTER resolution of whatever the problem is before the check engine light turns off. It is my experience that O2 sensors should last 80-90k miles. They are a relatively easy replacement on most every car I have ever changed one on. 50k miles is probably stretching the life out of copper plugs but I have been known to run them up to around 70k on a Honda.
Jim
'95 E320
'97 CRV