No idea how a multi meter would be used but a search suggests it might be possible although not easy - as an example https://www.pistonheads.com/gassing/top ... &t=1371347 .
They are normally checked on the car with a code reader using live data. With the engine running and once up to temperature their values are watched in live data. The one before the cat should be frequently changing as the fuel mixture is varied so the emissions are correct. Code readers with live data are not expensive such as this one from Gendan https://www.gendan.co.uk/product_FXNT200.html
The sensor before the cat is the one that adjusts the mixture, the one after the cat is to check if the cat is working so can probably be ignored to start with. However, its value should be fairly steady but if switching the same as the inlet one then it suggests the cat is not working.
Emissions
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ozvtr
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- Joined: 13 Oct 2020, 01:11
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Re: Emissions
I don't know what engine you have. So I'm not sure what type of O2 sensors it might have. If the O2 sensor before the CAT has 5 wires, this is a Lambda sensor and I'm not good on those. If it has 4 wires then read on.
Its very hard to test the O2 sensors with a multimeter, but you could do it. If the sensors have 4 wires. Typically 2 white, a grey and a black. The 2 white wires are the heater wires, your not interested in them. The grey and the black are the sensor wires. You need to "back probe" the electrical connectors while they are still plugged together. This can be difficult as they usually have a moisture seal around the wires and they are hard to remove. You need to get to the metal contacts. You cant disconnect the O2 sensor, neither physically nor electrically. Set the meter to DC and you are expecting about 1 Volt. So set the meter appropriately.
IF you can get the multi meter connected to the upstream (pre cat) O2 sensor, with the engine fully warmed up and idling, the voltage reading should oscillate between approximately 0.2V and 0.8V. It should oscillate about once a second. However the engine ECU has not thrown a code for it, so I would expect it to be OK.
The post CAT should be reasonable steady at about 0.5V. It can swing a bit but the wider it swings the poorer the CATs performance. I can't tell you what "bad" is because that's set by the MOT (see my comment at the bottom).
If either is "stuck" at 0.45V then the sensor is dead. However the engine ECU would throw a code telling you that, but it hasn't.
After saying all of that, I would rather recommend a cheap OBDII (EOBD) scanner (as PaulC5 suggests). Preferably one that graphs O2 sensor outputs. For 15-20 pound it's worth the money. If you do decide to get one, make sure the seller guarantees it will work with your car. European cars are a bit "iffy" when it comes to generic OBDII scanners. Most will give the the codes but might not give you "live" data. You need live data.
Given all the symptoms, I would say the CAT is working, as you don't have any codes, but it's not good enough to pass MOT.
Where I live we don't have MOTs. So that's out of my wheelhouse. As far as I know there are different emissions levels for different engines and years. I can only suggest you find out the requirement for your car and get a CAT that conforms.
Its very hard to test the O2 sensors with a multimeter, but you could do it. If the sensors have 4 wires. Typically 2 white, a grey and a black. The 2 white wires are the heater wires, your not interested in them. The grey and the black are the sensor wires. You need to "back probe" the electrical connectors while they are still plugged together. This can be difficult as they usually have a moisture seal around the wires and they are hard to remove. You need to get to the metal contacts. You cant disconnect the O2 sensor, neither physically nor electrically. Set the meter to DC and you are expecting about 1 Volt. So set the meter appropriately.
IF you can get the multi meter connected to the upstream (pre cat) O2 sensor, with the engine fully warmed up and idling, the voltage reading should oscillate between approximately 0.2V and 0.8V. It should oscillate about once a second. However the engine ECU has not thrown a code for it, so I would expect it to be OK.
The post CAT should be reasonable steady at about 0.5V. It can swing a bit but the wider it swings the poorer the CATs performance. I can't tell you what "bad" is because that's set by the MOT (see my comment at the bottom).
If either is "stuck" at 0.45V then the sensor is dead. However the engine ECU would throw a code telling you that, but it hasn't.
After saying all of that, I would rather recommend a cheap OBDII (EOBD) scanner (as PaulC5 suggests). Preferably one that graphs O2 sensor outputs. For 15-20 pound it's worth the money. If you do decide to get one, make sure the seller guarantees it will work with your car. European cars are a bit "iffy" when it comes to generic OBDII scanners. Most will give the the codes but might not give you "live" data. You need live data.
Given all the symptoms, I would say the CAT is working, as you don't have any codes, but it's not good enough to pass MOT.
Where I live we don't have MOTs. So that's out of my wheelhouse. As far as I know there are different emissions levels for different engines and years. I can only suggest you find out the requirement for your car and get a CAT that conforms.