At least on European cars you can read the O2 sensor voltages over the OBD2 port. $30 bluetooth dongle from ebay and you get the readings graphed on your phone. I just did that yesterday on my '02 Peugeot 307.
When you say the "right" O2 sensor I presume you're talking about the one after the cat? Doesn't that mainly tell you whether the cat is working or not? If the engine is running a bit rich and there is a bit of excess fuel coming into the cat, the cat should burn it up, I think.
And you probably know this, but there is an important difference in the voltage readings depending on whether you have wide- or narrow-band O2 sensors. Wideband is typically for performance cars.
I can recommend a odb2 dongle as well. Very interesting to plug in every once in a while. Do mind most dongles are read only though. You need a good cable to write. Be carefull if you want to tweak de ECU! Theres lotsnof info on the internet on how to tweak stuff.
$30 is way too expensive btw. On aliexpress i bought a wifi (because of iphone) dongle for $9,99 incl shipment.
When you say the "right" O2 sensor I presume you're talking about the one after the cat? Doesn't that mainly tell you whether the cat is working or not? If the engine is running a bit rich and there is a bit of excess fuel coming into the cat, the cat should burn it up, I think.
And you probably know this, but there is an important difference in the voltage readings depending on whether you have wide- or narrow-band O2 sensors. Wideband is typically for performance cars.