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Hey guys basically I failed air care because my check engine light is on. So here is the problem that I need help with:

2001 CLK320 Coupe Auto
138,000KM

Check engine light comes on:
- At first ODB11 reader showed only 1 Code-P0141- Bank 1 Sensor 2. The check engine light would come on and dissapear for awhile.
-However recently all 4 Codes have appeared- P0141, P0136, P0156, P0161
I needed to fix these codes because I needed to pass aircare so I assumed I just had to replace all 4 O2 sensors.

- I bring it to a shop and they replace all 4 O2 sensors with OEM BOSCH direct fit sensors. The check engine light does not come on for a couple hours, however the check engine light comes back on and off for codes P0141 and P0161 the same day and thereafter.

-At first I thought it might be faulty O2 sensors however these are brand new 02 sensors replaced by the shop.
- I bring it to another shop- deals with German cars and the mechanic checked everything- O2 sensors, wires, voltage, connectors, MAF sensor and can't seem to figure it out. He came to the conclusion that it might be problems with the ECU. He suggested I take it to the dealership to diagnose this and that the dealership is the only option that can fix the ECU.

What do you guys think?
 

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215 Posts
Do a search for CEL on this forum, there is lots of info out there. In my case, the air mass sensor was to blame. There is also a post out there with all service code descriptions. I would also recommend GP 44K additive, it did wonders for my engine.
 

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2004 CLK 240 Coupe
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11,377 Posts
Hi -sorry, I can't suggest a definite solution for you here...

But I did note that your fault codes P0141, and P0161 both concern the O2 sensor heater. The heaters are powered from the ECU (ME-SFI control unit N3/10, and they are connected in parallel for sensors G3/5 and G3/6 - left & right, downstream of the cat.

The heater is designed to be turned on in a controlled manner initially on cold starts to bring the sensor to its operating temperature quickly, while preventing the sensor being damaged. Then as the engine and the exhaust system warm up, as measured by the coolant temperature, the heater is switched off by N3/10.

If all the O2 sensors have been changed and the fault is still present, the problem might a fault with the sensor connectors or cables.
 

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2003 SL500
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Try you gas cap! Wet the rubber ring on it and stop and start engine a 3-4 times after a few minutes each time. Mine does that so while gassing up sometimes as the gas nozzle shuts off and drips I drip some gas on it then put it back on after 3 or 4 stops no light!
 

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'01-E320 & 02-ST2
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Since you have codes unrelated to evap, may I suggest that you please ignore cadbob's guess; it's an unrelated red herring.

I think Keyhole may be in a closer direction. If those are both the downstream sensors (I'd look up the codes but I'm on a smartphone) they are more exposed to water and road debris, and thus more likely to be damaged or have corroded connections. It's a four-wire sensor and I can't recall the color code for the heater circuit (I believe it's the white wires but I can check later tomorrow).

Can your mechanic pull live data for fuel trim? If that's out of range (or close to it) that can signal a failing MAF, which can also trigger O2 and Cat codes. (An intake leak after the MAF can also upset FT readings.)

That said, while the codes are usually reliable, I have seen a couple of instances where the codes reported were not the actual part. I've never heard that in relation to O2 heater circuits, but it's possible. So I'd still start by definitely checking the harness and connections for the O2 sensors and pulling live fuel trim data if he can.

Good luck.
 

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Oh, and fuel trim data should be captured with the engine fully warmed up, that ensures that the O2 sensors are hot and humming, regardless of the heater circuits. If FT is good when the sensors are hot, it's more likely to be an issue with the heater circuits (and less likely it's the MAF, intake leak, etc.)
 
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