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Old 11-20-2002, 01:29 AM
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Join Date: Jul 2002
Posts: 30
W124 O2 Sensor Problem

Here is one for the smart minds: Computer tells me there is an O2 sensor problem. Two codes: 'Incorrect voltage from 02 sensor' and 'Heater amps too high/shorted'.

There is _no_ current to the O2 heater, and a continuity check finds an open circuit from pin 7 of the fuel pump relay, where the schematic says the O2 heater is connected. That would lead me to think there is a break in the wire, but that seems _very_ unlikely.

Meanwhile, the 02 sensor itself is reading only 0.1V, even after being heated up by exhaust. Normally I'd say its a bad sensor, but I just replaced it in April and it has less than 10K miles on it. Its an OEM sensor.

Any ideas? Is there a fuse on the heater wire that might have blown? I didn't see anything during 3+ hours of inspection.

The car is a 93 300E.
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Old 11-20-2002, 11:08 PM
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I want to keep this question of yours alive by responding to it (puts it back on the front page where everyone sees it). Can you tell me exactly what codes you are getting and perhaps some symptoms?

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Old 11-21-2002, 01:52 PM
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Join Date: Jul 2002
Posts: 30
Thanks. I was about to do that myself. The codes (on pin 8 of the diagnostic box) are:

9 - 02 sensor voltage too high
11- 02 sensor heater amps too high/shorted

I've attached a pic of the connector under the passenger seat. Does anyone know what the pinouts for the heater connector are supposed to be? I get a continuty to ground on the brown wire, 96 ohms to ground on the black wire and 4 ohms to ground on the blue/green wire. As I mentioned, its an open circuit to the same blue/green wire on pin 7 of the fuel pump relay (where the heater is connected), even though that pin also has 9 ohms of continuity to ground.

Attached Thumbnails
W124 O2 Sensor Problem-o2-wire-sensor.jpg  
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Old 11-21-2002, 03:54 PM
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ok, I just checked mine and here's what I get:

blue/green - open
brown - 0 ohms (ground)
black - 22 ohms

Quite different than what you got, you are getting your figures with power off right?

With power on I get:

blue/green .36v
brown - 0v (ground)
black - .08v

I'm not sure if this will help you any or not because I really don't know what's causing the differences we have.

I'll keep thinking on this one, anyone else out there have any ideas???

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Old 11-21-2002, 05:26 PM
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Join Date: Jul 2002
Posts: 30
Just talked to another forum member on the phone, and got it cleared up. Here is how it works:

Two white wires (sensor side) are the heater circuit. They should be getting 12V from the brown and blue/green wires. (The brown is the ground; the blue/green is +12V). On my car this isn't working. The two white wires should NOT have any continuity to chasis ground when the O2 sensor is unplugged. If they do, this means there is a short.

The black wire (sensor side) that goes into the single plug is the signal from the sensor itself to the computer. This is what varies from 0V to 1V. The ground for this signal is the gray wire. This ground is seperate from the chasis ground. Mercedes appearently did this to help with signal integrity; the chasis ground can have a lot of noise.

My O2 sensor is in fact working when I test from the black wire (sensor side) to the gray wire (sensor side).

Hope that clears things up for people. Now I need help solving the heater problem: I'm getting 0V to the heater. Anybody know if there is a fuse for this?
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