My wife's F-150 is coming up on 100,000 miles. She has the 4.2 V6. I can only afford to replace two of the four O2 sensors right now. My question is which two should I change first? I would think that it would be the two after the cats. Am I right?
Thanks in advance.

 

My F-150 is coming up on 183,000 miles. I have a 4.2 V6. With the O2 sensors it left the factory with.

If you have a reason to change the sensors, the ones in front of the cats is used for calculating fuel. That is their only function. The ones behind the cat are only there for monitoring the catalytic converter. They have no effect on how the engine operates. They are better protected than the pre-cat sensors.

So I don't change them without a reason. If I felt so compelled, I would swap the pre-cat ones (that are at least important) with the post-cat ones (that aren't but are probably in better shape).

 

Unless they are causing problems, dont worry about them.

 

Alll righty. Thanks a lot.

 

At 100,000 miles you should be at least on your second set of front O2 sensors. If you go to the Bosch site you will notice the service interval is 100,000 miles. Personally I change mine every 50K.

JMC

 

If you buy a used truck, buy it from JMC. He obviously takes better care of his than I do.

If your long term fuel trim is low, you know that the MAF and O2 sensors agree very closely. Mine at almost 183,000 are under 1% at cruise. Any scan tool that reads generic ODB2 PIDs can be used for that test. OTOH, my BARO at about 600' above sea level is 26" mercury so the MAF and O2 sensors might be a little off.

An even better way that you can get a very good idea of the condition of the pre-cat O2 sensors with a graphing scan tool or an oscilloscope. This assumes the cat doesn't have secondary AIR. If the post cat sensor isn't always pegged but switches or floats between about 300mv and 600mv, then you have verification that the pre-cat O2 sensors have good fuel control.

Finallly, again with a graphing scan tool or oscilloscope, you can watch to see if the O2 sensor switches cleanly. My HO2S11 sensor tends to hang high more than the brief dips lean. On the other hand, the cat is toast and the HO2S12 shows pretty good switching. Bottom line is that I watch my O2S occasionally and have confidence in them. It would be almost as good to run until you have drivability or mileage issues before doing anything with them. This assumes you would recognize lean symptoms if they occurred.

This attitude is very much like the guys that don't change their oil until one of their regular oil analyses indicate they should. Why change perfectly good oil? Many fleet operators regularly get 30K to 50K out of oil with simple micron filtration and oil analysis. And they're getting at least industry average (400K mile) overhauls.

To each their own. Regular preventitive maintenance is good. Monitoring for failure of components like O2 sensors is also valid. I change oil every 10K or 12K miles, have the factory PCV valve, O2 sensors, and EGR equipment, yet have 183K miles without even pulling the valve covers (I have had transmission and accessory problems). I respect preventitive maintenance but you can't actually argue with my results.
--------------------'t always pegged but switches or floats between about 300mv and 600mv, then you have verification that the pre-cat O2 sensors have good fuel control.

Finallly, again with a graphing scan tool or oscilloscope, you can watch to see if the O2 sensor switches cleanly. My HO2S11 sensor tends to hang high more than the brief dips lean. On the other hand, the cat is toast and the HO2S12 shows pretty good switching. Bottom line is that I watch my O2S occasionally and have confidence in them. It would be almost as good to run until you have drivability or mileage issues before doing anything with them. This assumes you would recognize lean symptoms if they occurred.

This attitude is very much like the guys that don't change their oil until one of their regular oil analyses indicate they should. Why change perfectly good oil? Many fleet operators regularly get 30K to 50K out of oil with simple micron filtration and oil analysis. And they're getting at least industry average (400K mile) overhauls.

To each their own. Regular preventitive maintenance is good. Monitoring for failure of components like O2 sensors is also valid. I change oil every 10K or 12K miles, have the factory PCV valve, O2 sensors, and EGR equipment, yet have 183K miles without even pulling the valve covers (I have had transmission and accessory problems). I respect preventitive maintenance but you can't actually argue with my results.

 

Don't change the O2 sensors until you have a problem with them. Why change perfectly good oil?

Why bother washing your clothes, they're only going to get dirty again.
Why change oil that still passes the analysis? Because it's F dirty! And loaded with metal particles. I don't give a rats *** how much you filter it, used oil has crap floating in it that circulates right past the bearings. You don't want to change it? Fine by me. Are you going to tell me that oxygen sensors don't accumulate byproducts of combustion and wear out (read:send info to the ECM at a slower and/or less accurate rate) too? While we're at it, lets leave the spark plugs in til the motor starts missing, and forget flushing the cooling system. Lets just wait until the entire s F dirty! And loaded with metal particles. I don't give a rats *** how much you filter it, used oil has crap floating in it that circulates right past the bearings. You don't want to change it? Fine by me. Are you going to tell me that oxygen sensors don't accumulate byproducts of combustion and wear out (read:send info to the ECM at a slower and/or less accurate rate) too? While we're at it, lets leave the spark plugs in til the motor starts missing, and forget flushing the cooling system. Lets just wait until the entire system is clogged with corrosion. Hoses? They ain't blown a hole yet, should last another five years eh? Come on people.

SL

 

I don't want to argue, I respect the change them before they need it attitude. A few minor points and I'll leave it alone.

1. I don't know what the oil change interval is on these trucks. But the oil change companies still recomment 3K miles. With today's driving conditions (most never drive on dirt roads), todays engines (so titght my PCV valve is still fresh after 183K miles), and today's oils I think that is absurd. I don't blame oil companies for 3K oil change suggestions, that's profitable for them. I expect that Ford recommends 3K or 4K oil changes for my truck, it's no sweat of their back. If they recommend 3K mile changes now, they should have recommended 500mile oil changes in the 60's.

Your points about oil are all valid. But if the world isn't changing how do you explain my 10K to 12K oil changes not causing excessive wear? I still have never consumed a quart in my 10K-12K mile oil changes. I'm not recommending this, but it really is working for me.

2. For parts that have recommended replacement intervals. For the majority of people, those intervales will keep them out of trouble. If you have the ability to monitor them or verify they work correctly, why replace them until they need it? For O2 sensors, I listed 3 methods that could verify their correct operation. I use all three. I'm an oddball in that regard, but you have to admit, I would be on my 3rd set when these are fine. And Bosch recommending change intervals is like JiffyLube recommending 3K mile oil changes.

3. Finally, in Japan, they don't allow driving a passenger car over 50,000 Kilometers (about 30K miles) for emissions reasons. Here we simply verify the emissions in areas where emissions are high. Would I choose to follow a recommended replacement interval on my truck or would I operate it as long as I could verify it worked well.

What you are doing is fine, it works well, it's proven. I'm glad you have a method that works well for you. Vehicle after vehicle, mine has worked well for me. Drive safe.

 

cantrma,

I would not recommend one of my used, read abused, vehicles. While I maintain it I also "use" it. If you have a way of monitoring the sensors, as you do, then by all means don't change it till you feel it needs it. Is there any way that you could compair the scope of a new one with your current one. I am positive you would see a difference. I would like to see the difference too. As for the rest of us that do not own or have access to a scope the cost of having the O2 sensors scoped would most likely be the same as or close to the cost of 2 new ones. I always wanted to get a scope but never got around to doing it. I spend my $$$ on go fast goodies. If you could recommend a few good ones I would appreciate it. Who knows I might just break down and get one this time.

JMC

 

From my earlier post "My HO2S11 sensor tends to hang high more than the brief dips lean. " Yes, mine aren't new. The question is do they control the mixture well. I can answer that several ways, by monitoring the post cat sensor, by comparing the fuel trims of the two banks, by the magnitude of the fuel trim. Just as valid I can validate them with good milage, running smoothly, no rich or lean symptoms, it runs as well in closed loop as open.

A scope is a nice way to get a graph of the O2 sensors, but a graphing scan tool is probably better because you get to see all 4 at once and compare them on the same graph to throttle position, RPM, MPH, engine load, or if you have an automatic, the gear (for from stop acelleration). Scopes are also a pain to wire up and secure for road tests.

I have an OTC Vision which with 4 channels and an optional secondary ignition head was a hot ticket 6 years ago. But it is definately dated today. Small screen awkward human interface. Today I'd be tempted to get an integrated scan tool/scope like the Genisys or Modis. Both are two or three times what I could spend. I've been watching the OTC 3961(?) handspring Visor based scopes go for under $70 on ebay. I already have a Visor with the OTC scan tool (the one with GM/Ford/Chrys/Toyota proprietary). It sucks as a scan tool but I can add a cheap scope that I would leave in the truck for very little.

 

Lost train of thought. The best buys now are probably the Pico or other PC based scope. That assumes you already have a portable computer, which I don't. If you have to pay for the laptop just as a scope accessory, they're pretty expensive. But those PC based scopes are pretty cool. Tons of storage for waveforms, long term scoping for drive cycles.