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98 explorer no voltage from all o2 sensors

Peter Foley

New Member
January 3, 2019
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City, State
Year, Model & Trim Level
1998 Ford Explorer Sport
Hello, 1st post here!

I have an explorer 4.0 ohv that has 2 codes on it: P0171 and P0174. They are for "system too lean" for banks 1 and 2. When looking at the live data from my obd2 scanner, I see 0 volts and short and long term fuel trims at -100%. I have tried this obd2 scanner on other cars and it does read oxygen sensor voltage and fuel trims correctly. Strangely enough, when I put a piercing multimeter on the signal wire as it comes out of the o2 sensor, I get a reading. I have tried to visually inspect the wiring as far as I can see it and it looks in good shape. I have also, replaced both upstream o2 sensors, but to no avail, still no voltage reading from the obd2 scanner. So, my questions.... Is the no voltage situation causing the lean codes, or is there a lean condition that is causing the o2 sensors (which could be functioning properly) to read 0 volts because the engine is running so lean. Is this possible? Also, I did do a smoke test, and there was a tiny bit of smoke coming out of the intake gasket, is this vacuum leak bad enough to make a properly functioning oxygen sensor read o volts? Thanks for any help and suggestions!

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Peter Foley

Welcome to the forum, and thanks for starting by clarifying that you have an OHV.

It's pretty important for engine related questions and perhaps you might want to add that info to your Avatar (Year/Model/Trim Level area).

First & foremost; fix the vacuum leak on the upper intake manifold by replacing the three rubber gaskets.

The computer is very sensitive to vacuum leaks and they cause all sorts of Check Engine Lights/Codes/Issues...

Clear all Check Engine Light (CEL) codes by disconnecting the battery for at least an hour.

When the upper intake manifold is off, check for any splitting between the top and bottom parts of the PLASTIC assembly, along the "mating" seam. This splitting will cause intake manifold vacuum leaks. If splitting is present, clean split area of all oil/grime and repair with black rtv silicon.

Check the connection of the big square wiring block, located on the driver side of the engine compartment. This is the block with one big bolt going through the center, and plastic hold down clips on the side. It connects the engine compartment wiring loom to the cabin wiring (and OBD2 port). Sometimes the bolt becomes loose, sometimes there is grime that works it's way into the block and affects the pins & pin connectors, depending on where you live - road salts can get in there and cause problems (are you in Seward AK?). Open it up and take a looksie what's going inside there. Use dielectric grease on the rubber body seal when re-assembling.

Report back when re-assembled - hope that helps & Happy New Year!

P0171 and P0174 on a SOHC engine is leaky intake gaskets/O rings
You will not see voltage from those 02 sensors on your scanner in open loop
You have a lean condition on both banks, it is being caused by un metered air entering the intakes between the plastic and metal where the intake plenum O rings are.

There is a trivial fact that 97-99s (some years, some engines) aren't compatible with standard OBD2 readers They display nothing. Can you see other OBD parameters?
In 2019 though, there is a great free app called forscan, and you can access the ECU directly like the dealer tools. The reader cost a few bucks ($10-$20),. there is really nothing else to use because you can troubleshoot things like the 4x4 system, ABS etc.

Thanks for your responses!

I will try to replace the intake gaskets as soon as possible, but that could take a while as I am a college student right now. (I'm in Nebraska, btw). Anyways, I was just wondering, The intake leak is not that bad, as the engine runs fine except for a little bit rough idle. Anyways, I find it hard to believe that a vacuum leak that small could make the oxygen sensors read 0 volts all the time, even when I rev the engine or spray extra fuel in the intake to make a rich condition, it still reads 0 volts. It seems like there should be SOME reading, even if it is low indicating a lean condition. Anyways, thanks for your help and any suggestions you may have.

and yes, sorry, the obd2 scanner I am using works well and reads all other sensor data correctly, and will read o2 sensor correctly in other cars that are working properly. so I don't think the problem is in the scanner, but thanks for the ideas!

and yes, sorry, the obd2 scanner I am using works well and reads all other sensor data correctly, and will read o2 sensor correctly in other cars that are working properly. so I don't think the problem is in the scanner, but thanks for the ideas!

Ok, you see I've been on the forum a bit. I've seen an issue where the sensor wiring leans on the exhaust, burns out and shorts out. Now, there are usually other codes and devices affected but worth to inspect wiring.

I also have an OHV and yeah, it could have shrunken upper intake gaskets, but I've never heard of the leak being that severe. It is a failure point but not a common one. The SOHC on the other hand....yeah. But even so, having a flatline could be something else. Pull the PCM harness and check the wiring to the sensors.


Thanks for your response and sorry I was unable to get back to you faster. I think you are correct that checking the wiring is a good idea. I'm thinking I could pull the pcm wiring harness and check the pins that correspond to the 02 sensors. The only question then is... Which pins are the right ones? Do you or anyone have a pinout diagram for the pcm wiring harness. Since I think they are all a bit different I will be very specific in restating that it is a 98 Ford explorer 4.0 ohv with a manual tranny if that makes a difference.
Thanks to anyone who can help!

Ah, Concordia university?

Have you tried this simple fix? Sometimes the craziest problems are caused by a plain ol bad connection.

2nd gen issues? Try this first!

Sometimes the craziest problems are caused by a plain ol bad connection.

Interesting thought... will check as soon as I get the chance... I suppose it is best to check all simple fixes before doing anything too crazy!

Thanks for your help!

Sometimes the craziest problems are caused by a plain ol bad connection
also, on second thought, while I have those connections open, it would be a good time to check for continuity between the pcm and the wires coming out of the sensors. Would you happen to have the pcm pinout diagram so I know which pin (or pins?) I should check that connect to the signal wires coming from the o2 sensors? I have looked on other forums and it seems that every model has a different pcm pinout diagram, so it would have to be exactly for the ford 1998 4.0 ohv vin x... manual tranny if that matters. Thanks for any help you may be able to provide!