Hi, I know this has been discussed here but I still dont have a solid answer. I have a 96 6 Cyl 4.0 explorer and I got the code awhile ago, gas mileage is great and runs great too. Failed my emmisions test today because of the code.

Bottome line; some people say it is def the cat, but sites listing ODBII codes say it is a possibility that its an o2 sensor. also others say it could be the upstream sensor or the downstream. I have heard more of the upstream than anything but still have heard both, and heard of both being successful.

How do I find out if I have 4 or 2 sensors on mine. And which one should I replace before thinking about replacing the Cat itself.

Thanks,
Kyle

 

Testing O2 sensors that are installed
The engine must first be fully warm. If you have a defective
thermostat, this test may not be possible due to a minimum
temperature required for closed loop operation. Attach the
positive lead of a high impedence DC voltmeter to the Oxygen
sensor output wire. This wire should remain attached to the
computer. You will have to back probe the connection or use
a jumper wire to get access. The negative lead should be
attached to a good clean ground on the engine block or
accessory bracket. Cheap voltmeters will not give accurate
results because they load down the circuit and absorb the
voltage that they are attempting to measure. A acceptable
value is 1,000,000 ohms/volt or more on the DC voltage.
Most (if not all) digital voltmeters meet this need. Few
(if any) non-powered analog (needle style) voltmeters do.
Check the specs for your meter to find out. Set your meter
to look for 1 volt DC. Many late model cars use a heated
O2 sensor. These have either two or three wires instead of
one. Heated sensors will have 12 volts on one lead, ground
on the other, and the sensor signal on the third. If you have
two or three wires, use a 15 or higher volt scale on the meter
until you know which is the sensor output wire.

When you turn the key on, do not start the engine. You should
see a change in voltage on the meter in most late model cars. If
not, check your connections. Next, check your leads to make sure
you won't wrap up any wires in the belts, etc. then start the
engine. You should run the engine above 2000 rpm for two
minutes to warm the O2 sensor and try to get into closed loop.
Closed loop operation is indicated by the sensor showing several
cross counts per second. It may help to rev the engine between
idle and about 3000 rpm several times. The computer recognizes
the sensor as hot and active once there are several cross counts.

You are looking for voltage to go above and below 0.45 volts.
If you see less than 0.2 and more than 0.7 volts and the value
changes rapidly, you are through, your sensor is good. If not,
is it steady high (> 0.45) near 0.45 or steady low.

Testing O2 sensors on the workbench.

Use a high impedence DC voltmeter as above. Clamp the sensor in
a vice, or use a plier or vice-grip to hold it. Clamp your
negative voltmeter lead to the case, and the positive to the
output wire. Use a propane torch set to high and the inner blue
flame tip to heat the fluted or perforated area of the sensor.
You should see a DC voltage of at least 0.6 within 20 seconds.
If not, most likely cause is open circuit internally or lead
fouling. If OK so far, remove from flame. You should see a
drop to under 0.1 volt within 4 seconds. If not likely silicone
fouled. If still OK, heat for two full minutes and watch for
drops in voltage. Sometimes, the internal connections will open
up under heat. This is the same a loose wire and is a failure.
If the sensor is OK at this point, and will switch from high to
low quickly as you move the flame, the sensor is good. Bear in
mind that good or bad is relative, with port fuel injection
needing faster information than carbureted systems.

ANY O2 sensor that will generate 0.9 volts or more when heated,
show 0.1 volts or less within one second of flame removal, AND
pass the two minute heat test is good regardless of age. When
replacing a sensor, don't miss the opportunity to use the test
above on the replacement. This will calibrate your evaluation
skills and save you money in the future. There is almost always
*no* benefit in replacing an oxygen sensor that will pass the
test in the first line of this paragraph.

 

Thanks for your reply. Seems like the bench method may be the best because it will be difficult to get under there with a meter. Would one that runs on batteries work ok even though it is a cheap meter? I would think since it runs off of its own power, not on the power it is trying to test???

Also, I dont want to go into this blind. How do I find out if my car has 2 or 4 sensors, and should I start by testing the upstream or downstream sensor due to the code I am receiving.

Any help is greatly appreciated

Thanks

 

96 should have 2 upstream of the cats and 1 downstream. You will be checking voltage produced by the o2 sensor.

 

Been waiting for the O2 sensor bible...getting a little deprived lately. Thanks ex-aqua guy!

 

The code you mentioned I believe indicates a fault in the CAT system. If you haven't received any other faults, i would try clearing it and returning to the emmissions testing station. I have had several P0420 faults on my '96 4.0, but in combination with a misfire or other O2 sensor related fault. After getting rid of the garbage Bosch plugs (one with center electrode gone in cylinder #3), I had a Po420 fault about 2 weeks later. Cleared it, and hasn't returned in two months.

 

Just an update for those with the same issue. I had all the sensors checked at the mechanics and they said they were funtioning correctly. I replaced with a direct fit cat myselft and the code hasnt come back in about 6 months. I was hesitant because of the cost, but it seems that code by itself was deinitely it for me. Good luck.

 

Ok, listen to me on this one. I am a catalytic converter distributor.

The P0420 code is misleading especially if it is the only code. Catalytic converters do not fail by themselves unless it was a defective design to begin with. The Explorer does not have a defective design.

The first thing to look at is the freeze frame data at the time the code was set. The specific information you are looking for is the fuel trim data at that specific time. If the computer is having to change the mixture more than a certain amount, then there is a fuel management problem somewhere. If it is enriching the mixture more than a certain amount, possible suspects could be:
Bad O2 sensor(s)
vacuum leak(s)
clogged injector(s)
fouled plug(s)
tune up required
any other condition that can cause a high level of unburned oxygen into the exhaust system.

If the computer is leaning out the mixture more than a certain amount, look at:
Bad O2 sensor(s)
faulty fuel pressure regulator
Leaking injector(s)
clogged fuel filter
faulty fuel pump


Since you have the P0420 code chances are you will need a new converter, but I have never in my experience found a vehicle that had a properly designed catalyst that didn't have another problem that contributed to the original converters failure. If this original problem is not corrected, the replacement converters will eventually be damaged in the same fashion. This may not happen quickly, but it will eventually destroy the new converter. There is a possibility that if you find and correct this problem, that your original converter may begin functioning well enough to keep the code off.

In any case, I'm reasonably certain that we have a new converter for that model in stock, so if you click on us in the sponsors list, and fill out the contact form, we can get you more information on the pricing for your vehicle. I apologize that our online store is not up and running yet. We hope to have that working by this weekend.