How do I turn off the annoying "check engine" light?

[AZ Jeff]

I need to get emissions tests done on my XJ soon to renew my registration.
If it passed two years ago, does it stand to reason that it will pass now?

Depending on the mileage then and now, not necessarily.

 

[falcon556]

They arent very expensive and they are very easy to replace. They typically last 80K miles or so but some last longer. When they fail they can cause a few different symptoms. The main one is very poor gas mileage.

The front goes for $60-90 the rear for $120-150.
There are generic sensors that you splice to the existing wiring for about $55.
If you have more than one car to take care of, it can be pricey.
They can be tested but that can cost money. Not sure it is worth doing.

Of the two sensors, which one is more critical? If I can replace one only, which one first?

 

[Atl XJ]

The front one is the one that usually fails. I got mine at Autozone for pretty cheap but I dont remember the exact price, (I know it was a lot less than $60-$90) it was a few years and 60K miles ago. It was Bosch and it plugged right into the factory wiring. The rear one is just there to made sure the cat is functioning properly.

Edit: What year is your XJ? Only the 96+ models have the rear O2 sensor.

 

[falcon556]

The front one is the one that usually fails. I got mine at Autozone for pretty cheap but I dont remember the exact price, (I know it was a lot less than $60-$90) it was a few years and 60K miles ago. It was Bosch and it plugged right into the factory wiring. The rear one is just there to made sure the cat is functioning properly.

Edit: What year is your XJ? Only the 96+ models have the rear O2 sensor.

A '96 and a '99. A little research shows that between the two XJs there are 3 part numbers for the four sensors. OTOH if I was to use the generic type, one size fits all.
Basically they are all the same only difference being wire length and connectors.
So, fuel economy and performance depend on the front, I don't see any reason for replacing the rear.
Thanks.