Can the Post cat 02 sensors have anything to do with drivability? The other day, it was like no gas at all was pumping when I stepped on the gas, I had to really floor it to get it to move.

The dealer wants $1125 to change both of those 02 sensors.

Car has 102,000 miles, I imagine the fuel filter could be part of it also, but they won't check anything else until the sensors are changed.

A local indy shop diagnosed it as a failing throttle actuator, but said they didn't have the tools or something that the dealer did. They told me the sensors after the cat have nothing to do with drivability.

Dealer says the sensors failed because a Transmission connector is leaking fluid onto the wiring for the sensors- I had them fix the leak, but could the sensors be cleaned, or do they need to be replaced?[/align]

 

Hi,

The rear O2 sensors only come into play under certain conditions usually when the vehicle is cold. If the O2 mix is incorrect at the rear sensors it starts up the secondary air injection system thus weakening the mixture and ensuring tailpipe emissions are within legal limits.
So IMHO rear sensors have nothing to do with driveability.
A good garage will be able to access the signal coming off the sensors and confirm that they are working correctly.

So the real question we need to ask is what codes were set in the OBD?
How did your first garage come to the conclusion that the rear sensors need to be changed?

Can't for the life of me see the connection between the trans fluid leak at the gearbox connection (very common but usually insignificant) and the rear O2 sensor wiring. Anyway oil is usually a good insulator so I wouldn't expect problems. However water is a totally diofferent matter. The working part of the sensor is within the exhaust pipe so is by definition away from any oil from the gearbox. Yes the wiring COULD be exposed to this oil but don't think this is relevant. A simple inspection / wipe / clean is all that is required up to the connector which is adjacent to the gearbox flange.

Many things could cause driveability issues but the OBD code will point us in an appropriate direction. At 102k miles an EA Electronic Accelerator is could well be the problem. Your MB has "fly by wire" connection between your foot and engine. i.e. No throttle cable! The potentiometers which determine throttle position are mechanical and dowear out.

Stuart.

 

Thanks Stuart- Your diagnosis of the Electronic actuator is the same I got from an independent, but they said it would have to go to the dealer, they didn't have all thediagnostic toolsthey needed and didn't want to go throwing parts at something they weren't POSITIVE of. It IS throwing codes for the rear 02 sensors, and I guess the dealers point is, "we're going to fix everything with this car". I guess I will change the 02 sensors first, just so we won't have to keep looking at the check engine light.

I have a more competent dealer I can take it to, though further away, and I'm sure they'll want to fix the 02 sensors as well.

How do they test the Actuator?

 

So you have the CEL on!
You also state other codes are showing. What is the full code picture? Which ones reappear after a reset/clear. Do they reappear at ignition on or after one / many drive cycles (ign on-start-off?)
So what is the full picture? I sense you have only given part of the history of this problem.
I have no idea re USA costs but 1125 USD seems a bit heavy. Not sure of the exact MB time for changing two rear sensors but would guess 30 mins max. So what is the breakdown of the 1125 into parts, labour and tax?
I fully agree don't throw money at an MB, or any other modern car. Diagnose then confirm what the problem is before change parts as necessary.
OK 102k is a good mileage but an MB will run 200 / 300k+. Why would just the rear sensors go. Sounds fishy to me.
Anyway you test the sensors by looking at the voltage and waveform that comes from them. Yes you need specific equipment but this is not special MB stuff. The special MB is the best Star Diagnostics SDS but if not readily available a lot of other equip will do the job.
Fault codes do not say...Change the sensor. They state O2 sensor circuit malfunction (bad connection or similar), O2 sensor heater circuit malfunction etc etc. An incorrect voltage off a sensor causes the ECU to meter the incorrect fuel quantity which then results in a code for Misfire or similar since incorrect fuelling result creates a misfire...Get the idea.
This is why the code signature is so important. If they have done this test properly, especially on the MB SDS equipment they should be able to give you a hard copy report with your VIN, Time date, codes etc on the report. Such a test/report is usually chargeable 50 USD? if conducted separate to the actual repair since it does take 10mins or so to complete.
However, I consider it the starting point for doing business with somebody. i.e. should be free. Hope this helps.
Stuart

 

P0136 and P0141 were the codes it throws. I'm going to change them both, then have the dealer check the actuator, unless you suggest otherwise.

 

P0136/P0141 are indeed rear O2 sensor CIRCUIT malfunction.
I still don't have the whole picture...you haven't even identified what model of vehicle but from your personal details I believe you are referring to a 01 320 SLK (M112)

Although I do not rule out the RH system rear sensor (note singular) the code suggests a circuit failure rather than a sensor failure. Of course the sensor could go open circuit or similar but I would ensure the connection at the trans casing is indeed clean, uncorroded and correctly connected before changing parts.
With respect to changing multiple sensors this is just spending money unnecessary.

So why doesn't the garage just look at the signal coming off the sensor. Standard practice! This will confirm what is going on.

I recommend using genuine sensors but others find after market ones do the job at a fraction of the price.

Since I loathe spending money on parts when I suspect they may not be at fault why not change the LH and RH rear sensors over. If you circuit problem moves its the sensor, if not its the wiring / connector. Changing this sensor over is a straight forward DIY job if you are that way minded. No special tools required etc.

As I have stated previously the rear O2 sensor (which is before the rear cat) only comes into play if the system senses that the pre-cat (close to manifold) has not adequately cleaned the exhaust. No sampling is done post the rear cat. This sensor is unlikely to impact driveability.

If you have a severe "driveability/response" problem we are back to the Electronic ACCELERATOR sensor / transponder / actuator EA.

Stuart

 

Will the electronic actuator give off a code if it starts to fail, or will it just fail and leave me stranded? I'm taking it to a different dealer this time, I'm sure they won't even look into the Actuator until the 02 sensors are taken care of first- the first dealer claims that the rear 02 sensors complete some circuit to the front, and could indeed cause drivability problems.

 

Not sure about that. (Will the EA set a code.)

Did you pick up the point I made. The DTC's you havre stored are only on one side Bank 1 Sensor 2.
Bank 2 Sensor is OK so I repeat I would swap the sensors over.

Not sure I can help much more. Changing both sensors will at least eliminate the sensors being at fault but at 1125 USD, rather you than me.

I can't think why everybody is so keen on changing parts without the supporting evidence.

Stuart

 

I replaced both 02 sensors, now all the codes are gone, engine light is gone. I'm going to drive it some more to see if I have a problem with the throttle anymore.

At 100,000 miles, what needs to be done as far as preventive maintenance?

I change the Tranny fluid and filterevery other oil change, the coolant has been done last year, the spark plugs, air filter, etc.

I'm wondering about the belts and hoses- Is this something I want to play with??

Water pump? Timing Chain? Fuel filter?

 

Changed them myself, only cost me $291 USD- They claimed to have tested them, so I rolled the dice. Figured I'd do both while I was under there.