A Useful X300 Lambda (Oxygen) Sensor Modification

Every once in a while we are asked to diagnose faults with X300 cars giving unexpected or odd Lambda readings. Sometimes replacement of the sensor(s) is the cure, sometimes it is clear the sensors are working but not giving the readings you'd expect. In a case like that we have to consider whether the Bank One and Two sensors have been mixed up and then connected incorrectly, an easy mistake to make because both sensors have identical plugs. To prove or disprove this theory it is necessary to disconnect one Lambda connector to find out which sensor is reading on which Bank whilst monitoring the sensor outputs in OBD, then trace the remaining Lambda wires back to see which sensor (B1 or B2) is still connected and actually giving the reading.

X300 Lambda connector plugs are very awkward to get at....

And there lies a problem....X300 Lambda (Oxygen) sensor connector plugs are very difficult to reach and disconnect. They can be even harder to re-connect. Squeezed right between the cylinder head and bulkhead, placed just above the gearbox bell housing, things can get really tight. Grazed and scratched hands are practically unavoidable if you can reach the connectors at all. After struggling with a particularly tight pair of plugs on one of my own cars I decided to find a solution. It turned out to be a great success, so I thought I'd describe it here;

Disconnect both sensor plugs in any way you can (very long nosed pliers can be helpful) after marking both sides of one connector plug with 'Tippex' or similar so you won't get the sensors mixed up. Now carefully cut the binding tape on the covered loom that the sensor plugs come out of. This is shown in the second picture and is fixed to the bulkhead, crossing from one side to the other. Next, peel back the split corrugated plastic sheathing to expose the wiring loom. The wiring to the Lambda plugs can be teased out of the bundle and moved to the driver's side (RHD cars). Next thing to do is to free the Lambda wire tails from the clips that hold them to the top of the gearbox bell housing (if the clips are still in place).
The Lambda sensors can now be reconnected and secured on the driver's side of the engine (RHD cars) taking great care that the tails cannot come into contact with the exhaust system. The wiring that goes to the ECU side of the plugs should be re-covered with a similar type of corrugated plastic tube or failing that, 'Gaffer' or 'Duct' tape. With the lambda wiring and plugs in their new, accessible position, connection /disconnection is a simple matter and it is a lot easier to make sure you are connecting the correct sensor to its corresponding bank, the wires can simply be followed through to their respective sensor from the same side, instead of trying to trace them along the back of the engine. We now perform this modification as a matter of course when doing any work involving X300 Lambda sensors, it really does make things a lot easier.


For anyone finding four thin wires (Brown, Blue, Yellow and Red) in the main loom which appear to have been cut, (Ringed in RED in the picture below) worry not, you haven't made a mistake. These dead-end wires have appeared in every X300 loom I have inspected and have no function I'm aware of in UK cars. I have no idea why these wires were included although they must have been required in cars built for other markets.

I hope that that this article will help owners having difficulty in connecting or disconnecting X300 Lambda (Oxygen) sensors in the future.

Steven P. Sparrow BSc (Hons)


October 2008

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