2001 F-150 with a 5.4L and I need to change the O2 sensor, however I am not sure if I need to change the front or rear O2 sensor. The computer scan just stated to replace O2 sensor 1/2. I did not perform the scan myself a friend did. The truck has 77,000 miles and neither O2 sensor has ever been replaced, does one O2 sensor tend to need replacing first, front vs. rear???

Thanks,
A friend of ford builder

 

I just buy some "electrical motor" cleaner and spray them with that. Use a tooth brush as well.
You are basically cleaning the carbon off of them.

 

I reccomend replacing them as cleaning never really works as the sensor is in side the metal tip.

Check out this link.

If you look at Rockledge's link, you'll see that 100K is the expected life span of a 4 wire O2 sensor.

I replaced all four sensors on my truck. The front ones are very tricky to get the electrical connectors undone. On the Drivers side it helps to remove the 4x4 drive shaft and on the passenger side it helps to take the tire off.

When installing mine, I had to cut my 22mm wrench in half and then I used a bottle jack handle with the open end of the wrench I cut up as my tool. When you look at the passenger front sensor, you'll see why I cut my wrench up.

Before busting them loose, I let them soak in PB Blaster overnight. A small grunt and they all popped loose.

If you decide to replace the sensors, I reccomend Bosch, but I will warn you that the Bosch book at Checkers was wrong on my rig (see the link). It may be wise to remove the sensor(s) and then head to the parts shop, for the exact same thing. If you replace all 4, you should walk out with 2 different part numbers. The fronts are a different # than the rears.

 

The "rear" sensors are cat monitor sensors, not O2 sensors. Though they are basicly the same sensor, their function is completely different. Cat monitor looks for a more steady voltage reading (very little voltage fluctuation after the cats have warmed up); while an O2 sensor will rapidly fluctuate from about .1v (lean) to .9 (rich). If the O2s flucuate rapidly and the cat sensors are more stabil the PCM knows that the catalytic convertors are working properly. A bad convertor will cause the cat sensors to fluctuate like the O2 sensor.

 

Generally speaking, If I spend all that time removing, I will replace with new sensors but I have cleaned my sensors before and replaced them and they continued to work for a long time.

 

I don't believe anyone answered your original question of which sensor to replace. I'm not an expert, but I believe O2 sensor 1/2 means Oxygen Sensor in the 1st bank (number 1 cylinder side), 2nd sensor down line (there are two).

 

Replace the upstream o2 sensores ( the ones above the cat) those are the ones that tell the cpu how much fuel or o2 is in the system. As 1toptech said the down stream sensors raely go and only monitor the CAT. the cost is about 59.00 a piece.

Lance