Where are my lambda sensors?

[veedweeb]

It's a '98 W202 C240, and a recent diagnostic revealed the left hand upper and lower sensors had failed.

Mercedes parts lists 4 sensors: upper and lower on each side

GSF lists 4 sensors too. I bought a pair today and have replaced the upper one quite easily (It's the one on the downpipe, just about in line with the back of the front wheels), but I'm buggered if I can find the other.

It's like a fat sparkplug with about 6" if wire, but there is nothing coming from the exhaust anywhere

Anyone got any experience of this job?

 

[Uncle Benz]

If your car has 2 sensors per bank they are fitted either side of the cat. These systems changed around 98/99, from memory, from a single sensor in each downpipe before the cat to the 2 sensor system. The idea is the second sensor monitors cat performance to check it is doing its job. You may have the earlier system with just one sensor per bank if you can't see a sensor after the cat.

 

[veedweeb]

That sounds about right then, but I'm still a bit confused...

The diagnostic test (I saw the laptop screen with the error codes) said upper and lower sensors had failed, and the MB Main dealer parts department took my chassis number and said it needed 4 sensors, but on the car, I could definitely not see any sensors after the cat.

I did think that maybe it's a "4 sensor" car, with an earlier "2 sensor" exhaust fitted, but there's nowhere for the sensors to plug in either

Very strange... I took it for a test last night after changing the only one I could find, and it seems OK now.