Failed MOT. Lambda?

[Rversteeg]

Now I see why you needed the O2 sensor p/n!

As Classicswede already explained, the lambda value is the relation between the amount of air you need for a full combustion and the amount of air that is actually available. Ideally this relation is 1. Anything below one indicates that the combustion is rich (not enough air / too much fuel), anything above indicates the combustion is lean (too much air / not enough fuel). 1.35 is too lean.

The O2 sensor measures the amount of air in the exhaust gasses, at lambda = 1 the measured voltage over the sensor is about 0.45 V.
The ECU will constantly adjust the mixture to maintain this 0.45 V voltage. As the ECU is constantly adjusting the mixture around lambda = 1, you should see the O2 sensor voltage fluctuate around 0.45 V. Although it is not the sensor itself that causes the voltage to fluctuate, a constant voltage is usually an indication of a faulty sensor ("stuck" at single value). It could also be that the sensor calibration is off, which gives false information to the ECU. The measured voltage would then still be fluctuating around 0.45 V, but the mixture is too rich or lean.
The sensor is considered to be a wear part anyway, so replacing after 500K km is not a bad thing anyway.

[Clan]

Originally Posted by SalvadorP

Correct, he did not give me the figures. He said/the paper says, the reason it failed was the lambda value, not the exhaust values. But I understand knowing those could help diagnose.

In your opinion could this be as simple as replacing the O2 sensor?

No not until you have eliminated an air leak or high fuel pressure for example Guessing can be much more expensive than getting it diagnosed Professionally.

All the Lambda sensor is doing is measuring the Oxygen content of the exhaust gas and a lot can affect that.

[SalvadorP]

Originally Posted by Rversteeg

Now I see why you needed the O2 sensor p/n!

As Classicswede already explained, the lambda value is the relation between the amount of air you need for a full combustion and the amount of air that is actually available. Ideally this relation is 1. Anything below one indicates that the combustion is rich (not enough air / too much fuel), anything above indicates the combustion is lean (too much air / not enough fuel). 1.35 is too lean.

The O2 sensor measures the amount of air in the exhaust gasses, at lambda = 1 the measured voltage over the sensor is about 0.45 V.
The ECU will constantly adjust the mixture to maintain this 0.45 V voltage. As the ECU is constantly adjusting the mixture around lambda = 1, you should see the O2 sensor voltage fluctuate around 0.45 V. Although it is not the sensor itself that causes the voltage to fluctuate, a constant voltage is usually an indication of a faulty sensor ("stuck" at single value). It could also be that the sensor calibration is off, which gives false information to the ECU. The measured voltage would then still be fluctuating around 0.45 V, but the mixture is too rich or lean.
The sensor is considered to be a wear part anyway, so replacing after 500K km is not a bad thing anyway.

Thx. I'll try testing it today to see.

Originally Posted by Clan

No not until you have eliminated an air leak or high fuel pressure for example Guessing can be much more expensive than getting it diagnosed Professionally.

All the Lambda sensor is doing is measuring the Oxygen content of the exhaust gas and a lot can affect that.

I know I have an exhaust manifold leak, but it can't be that, because the last MOT was 3 months ago and the car passed without issues.

[SalvadorP]

I've tested it. Here's a video of it. Can I get some opinions?

+ YouTube Video

https://youtube.com/shorts/wuFj5_RTX...0GZg8E3Qb5bcop

The value seems to fluctuate quite a bit. After the video I disconnected a vacuum line and the value went to zero, and then slowly it increase to the values in the video again, or similar. It definitely reacted to the vacuum leak.

[SalvadorP]

Ordered the lambda, bosch. On the phone guy said 89€. Went to grab it today, guy asks me 109€. "It's 89, plus tax." On the phone they did not say it was 89 plus tax. There was no mentioning of 89 being pre tax.

The oem from volvo is 120€. But Volvo Portugal is a joke. The part had to come from Sweeden and would take at least 10 working days...

I should have just bought the same bosch part from autodoc for 85€.