Having recently failed the MOT on the lambda reading, and not being able to find a how to guide to replace the two lambdas, I thought I'd do a quick write up.
The lambda reading on modern cars basically shows whether the fuel:air mix is optimal (known as the stoichiometric value). When lambda=1, the fuel to air mix is optimal.
The lambda / oxygen sensors in our cars are used to adjust fuelling on the fly. If the lambda sensors in the exhaust (pre-cat) senses that there is too much oxygen remaining after combustion, it will increase fuelling; too little and it will reduce fuelling.
In the MOT, cars are allowed a variance of 3% i.e. lambda value of 0.97 or 1.03. A value of less than one indicates that there is fuel which has not been burned existing in the exhaust. A value of more than 1 indicates that there is too much oxygen in the exhaust (hence known as an oxygen sensor!)
Should your M fail the MOT, however, don't simply assume that the lambda sensors are in need of replacement. The reason is that the MOT lambda reading is taken at the end of the exhaust, and therefore the following steps should be taken first.
1) Ensure there is no leak in the exhaust. An exhaust leak after the lambda sensor could draw in air from the outside, hence there will be excess oxygen.
2) Check vacuum hoses around the engine block.
3) Read your codes!
Right then, onto the tutorial!
Tools needed:
Philips Screwdriver
10mm socket
8mm socket
Lambda sensor removal tool: LINK
2x Lambda Sensors (different length cables):
Part number 11781406621
Part number 11781406622
Lump hammer
Penetrating Spray
Four post lift / pit in your garage. I wouldn’t attempt this by jacking up the car and crawling underneath, as access is extremely limited
Step 1 – Remove the undertray
There are 2x10mm bolts at the front, 6x8mm bolts around the other three sides and 1x screw in the middle. Below is a picture of the undertray
You will now be able to see the lambda sensors, toward the front of the visible exhaust
Step 2 – Use the penetrating spray!
The lambda sensors have been expanding and contracting for many cycles, so they will be pretty tough to get out. Spray liberally!
Step 3 – Undo the first lambda sensor
Slip the tool over the first lambda sensor (to the left hand side of the car). Turn anti-clockwise (towards the back of the car). At this point it may still be stuck, so I used a lump hammer to “persuade” it. That loosened it! You can probably turn the rest of the way by hand once freed.
The wiring is routed along the heat shield towards the back of the car, across and then back towards the heat shield on the other side of the car. The connectors are shown here:
Step 4 – remove the other lambda!
Again, use the tool and a lump hammer. Make sure you remove this sensor before replacing the other, as you’ll have no chance otherwise.
Step 5 – replacement is the reverse of removal.
Just remember to plug the lambda’s into the right connector
The lambda reading on modern cars basically shows whether the fuel:air mix is optimal (known as the stoichiometric value). When lambda=1, the fuel to air mix is optimal.
The lambda / oxygen sensors in our cars are used to adjust fuelling on the fly. If the lambda sensors in the exhaust (pre-cat) senses that there is too much oxygen remaining after combustion, it will increase fuelling; too little and it will reduce fuelling.
In the MOT, cars are allowed a variance of 3% i.e. lambda value of 0.97 or 1.03. A value of less than one indicates that there is fuel which has not been burned existing in the exhaust. A value of more than 1 indicates that there is too much oxygen in the exhaust (hence known as an oxygen sensor!)
Should your M fail the MOT, however, don't simply assume that the lambda sensors are in need of replacement. The reason is that the MOT lambda reading is taken at the end of the exhaust, and therefore the following steps should be taken first.
1) Ensure there is no leak in the exhaust. An exhaust leak after the lambda sensor could draw in air from the outside, hence there will be excess oxygen.
2) Check vacuum hoses around the engine block.
3) Read your codes!
Right then, onto the tutorial!
Tools needed:
Philips Screwdriver
10mm socket
8mm socket
Lambda sensor removal tool: LINK
2x Lambda Sensors (different length cables):
Part number 11781406621
Part number 11781406622
Lump hammer
Penetrating Spray
Four post lift / pit in your garage. I wouldn’t attempt this by jacking up the car and crawling underneath, as access is extremely limited
Step 1 – Remove the undertray
There are 2x10mm bolts at the front, 6x8mm bolts around the other three sides and 1x screw in the middle. Below is a picture of the undertray
You will now be able to see the lambda sensors, toward the front of the visible exhaust
Step 2 – Use the penetrating spray!
The lambda sensors have been expanding and contracting for many cycles, so they will be pretty tough to get out. Spray liberally!
Step 3 – Undo the first lambda sensor
Slip the tool over the first lambda sensor (to the left hand side of the car). Turn anti-clockwise (towards the back of the car). At this point it may still be stuck, so I used a lump hammer to “persuade” it. That loosened it! You can probably turn the rest of the way by hand once freed.
The wiring is routed along the heat shield towards the back of the car, across and then back towards the heat shield on the other side of the car. The connectors are shown here:
Step 4 – remove the other lambda!
Again, use the tool and a lump hammer. Make sure you remove this sensor before replacing the other, as you’ll have no chance otherwise.
Step 5 – replacement is the reverse of removal.
Just remember to plug the lambda’s into the right connector
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