SL55/63/65/R230 AMG: Persistent oxygen sensor problems
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2004 Mercedes-Benz SL 55 AMG
Persistent oxygen sensor problems
Hello, I have a 2004 SL 55 AMG with persistent pre-cat errors on both banks. Have replaced sensors twice but I am still getting the P0134, P0135, P0154, P0155. Makes me think that it is an issue with the heater circuit, but sometime, I don't get the heater circuit errors.
The car eventually switches to open loop and stays there. In one instance, I was able to set all the monitors without a check engine light. This involves several cycles of clearing the error code and revving up the engine it really high rpms. However the check engine light and the error codes for both oxygen sensors have since returned. There's a lot of black suit in the tailpipe. I've checked fuses and relays generally and have even measured with an old meter that the heater circuit is getting power. I also can see that the o2 sensor signal is switching although slowly.
Some ideas that I'm exploring are poor ground and leak in the exhaust system. Although there's oil in the harness around the oxygen sensor I have cleaned it with no change. Curious why both pre-cat oxygen sensors on both banks have the exact same issue. Also, I have a parasitic draw that's draining the consumer battery relatively quickly overnight but I don't think this is causing problems with the oxygen sensor. Further, it looks like my charcoal canister is full since it takes me a long time to pump gas.
Does anyone have any other ideas? Does anyone know where the grounding points for the heater circuits are? My understanding is that the positive for the heater circuit is supplied by the ECM. Thank you so much.
The car eventually switches to open loop and stays there. In one instance, I was able to set all the monitors without a check engine light. This involves several cycles of clearing the error code and revving up the engine it really high rpms. However the check engine light and the error codes for both oxygen sensors have since returned. There's a lot of black suit in the tailpipe. I've checked fuses and relays generally and have even measured with an old meter that the heater circuit is getting power. I also can see that the o2 sensor signal is switching although slowly.
Some ideas that I'm exploring are poor ground and leak in the exhaust system. Although there's oil in the harness around the oxygen sensor I have cleaned it with no change. Curious why both pre-cat oxygen sensors on both banks have the exact same issue. Also, I have a parasitic draw that's draining the consumer battery relatively quickly overnight but I don't think this is causing problems with the oxygen sensor. Further, it looks like my charcoal canister is full since it takes me a long time to pump gas.
Does anyone have any other ideas? Does anyone know where the grounding points for the heater circuits are? My understanding is that the positive for the heater circuit is supplied by the ECM. Thank you so much.
Last edited by Brajan Kola; Apr 12, 2021 at 05:12 PM.
