What is the purpose of the four oxygen sensors, and what problems could I expect if some of them were not working ? - Any advice much appreciated.

 

Gordy, please keep in mind that you have a front (bank 2) and back (bank 1) bank to the engine. So, your 4 O2 sensors are really what are referred to as the Bank 1 upstream sensor, the Bank 2 upstream sensor, the Bank 1 downstream sensor, and the Bank 2 downstream sensor. The upstream sensors are essentially on the output of the cylinder head exhaust ports and they are measuring to see how much unburnt fuel (by measuring the amount of oxygen that is left over) is leaving the cylinder. The computer ideally wants to see a very small amount of oxygen in the system (ideal), if not zero oxygen. This is done so the computer knows how much fuel to add to the cylinder to get the most power out of the available oxygen yet not be wasteful by running the engine too rich. It also does not want to run the engine too lean as the cylinder temperatures get extremely high (leading to components being damaged). So, there is a fine line between too much, too little, and just the right amount of fuel.

The downstream sensors are looking at the exhaust gases leaving the catalytic converter. It is looking to see that there is a small amount of oxygen left in the exhaust at this point (keep in mind that a small amount of oxygen is admitted into the cat to allow any unburt fuel to be burnt at this time). The computer is going to try and maintain a finite amount of oxygen in the exhaust at this point. When your cat starts to fail, the cat doesn't burn the unused fuel and this results in oxygen levels going up or the fuel actually causing the sensor to think that there is too little oxygen. If this level remains too low, it will trigger a check engine light to tell you that there is a problem with your cat.

If the O2 sensors are not working, you can experience something as minor as a simple warning light telling you that the computer doesn't like something, to a significant drop in mileage, to worst case, engine damage from either running the engine too lean (leading to cylinder damage) or too rich (leading to cat failure/exhaust blockage).

 

Many thanks Thermo, - This has been of great assistance. - I have been advised that I need to replace three of them, and I suspected that the motor was running lean until it got up to temperature, as if I started off down the road from cold , as the motor approached 2000 rpm, it would almost die, and then take off again intermittently until it got to operating temperature. I suspected a vacuum leak, but nothing can be found, so hopefully this will fix it. Thanks once again.

 

Gordy, before you get to far into the O2 sensors, lets start with the obvious issues. First check the big 3 vacuum leaks (brake booster line, IMT o-rings, and PCV hose). One of those cures about 99% of the issues like you are seeing. After that, I would recommend finding a cigar (or a friend willing to smoke one for you) and start up the car (with the A/C-heat off and the motor cold). At this point, gently blow the smoke over the engine and see if you can spot where it is entering the engine. If you can't get smoke, then you can use starter fluid and spray all the joints on the intake and listen for an uptake in the engine's RPMs. If the engine RPMs go up, there is your leak.

If you truely have a bad O2 sensor, then a simple diagnostic machine will see this as the sensors will be giving off goofey readings and any mechanic will be able to narrow it down to a single one. For that matter, if you have an android based cell phone, you can get your own code/parameter reader for around $15 USD. Look up "ELM 327" for your year jag. Then you can download a free app called "Torque". You can then look at the parameters, read codes, clear them, etc. A little bit of research on the internet and you can find most of the answers that you need.

 

Thank you so much Thermo, -- I will certainly take your advice. Cheers.