testing oxygen sensor

I have codes P0171 and P0174.

I have measured the resistance of the three O2 sensors and get:

3.7 ohms on the one behind the convertor. This is identical to the one sold to me from the parts store, so I'm not gonna replace it.

The two before the cats read 8.3 and 9.7 ohms. Is this within spec?

2001 Ranger 4.0 liter V6.

Thanks,
Denver

I have the same codes ,I don't think you can test the sensors by resistance. They can be tested in operation with a digital high impedance multimeter. The voltage switches if the mixture is to lean or rich.
I still think it may be a vacuum leak but my engine runs OK. I never got good gas miliage (around 15mpg). Have put off looking for problem due to holidays and it still running. If you find the problem let me know and I will do the same,thanks

essikr,

I cleaned my MAF sensor about 10 days ago and cleaned the connections to the O2 sensors. I reset the codes and they stayed away until yesterday. I haven't had them read yet, but will soon. As with you, the truck is running fine. I'll keep you posted if I find anything.

Denver

Watch the data stream on your scanner the manual gives guidelines on this.

We have some unseasonal warm weather, so today I connected a vacuum gauge to the engine. It read a steady 21 in at idle.I used a maifold so I could check with all lines connected in case the leak was downstream. With the vacuum hose disconnected there was a slight change in the idle but not much.Now I'm doupting if I have a vacuum leak. Any news from your end?:confused:

P0171 and P0174 I believe meant lean indication on both banks. For both pre-cat sensors to go bad is not impossible but unlikely. I had the same codes in my '99 Ranger 4.0 and it turned out to be the result of a vacuum leak around the upper intake manifold. While idling, try squirting carburator cleaner around the upper intake manifold and around suspected vacuum affected areas.

I'll need time to verify that the CEL doesn't return, but I found a split hose on the back-end of my PCV valve that could have been the source of a vacume leak.

I'll post back in 7 to 10 days to let you know if the codes return.

I've also found a load of info on the explorerforum.com with photos of how to replace the intake manifold gaskets if I have to go there.

Denver

Denver:

If your P/U is anything like mine, the upper intake manifold has three "O" rings so if you need to, don't buy the kit because it includes the gasket for the lower intake manifold also which you don't need. It'll save you money. That PCV hose leak could be you problem.

Another forum to try is the Ford Truck Enthusiasts (www.ford-trucks.com). There are alot of posts in the Ranger forum about the 171 and 174 codes. i had the same problem on my '99 4.0L and cured it with the new gaskets. Does your truck idle rough at startup on cool mornings? If so, it is more then likely the gaskets.

Well ,after reading all the posts about the manifold gaskets ,I replaced mine.
Unfortunatly the CEL came back on the next day.Still have the lean codes on both banks.
I have done all the suggested checking and cleaning except for the o2 sensors. The engine runs and idle's fine ,gas miliage has allways been around 15mpg :uhoh: .
Hard to believe both o2's are bad,I guess my next task will be to pull and try cleaning them. I will check the site on the previous post,thanks. :confused:

I would try testing the fuel pressure it could be either a bad regulator or fuel pump. The testers look like this one in the link.
http://www.autopart.com/tools/toolsmain/tool/T_3440.htm
Low pressure will lean out the mixture on both banks. Look at AutoZone or Advanced and check if they loan these type of tools or borrow one. Basically you check the pressure and then blank the return and check if the pressure comes up.

If completely stumpped, might b worthwhile to get a diagostic.