Might be a bad connection in the wire harness.
The terminals need to be nice and shiny.
could be a bad wire or one is touching the exhaust pipe.
The catalytic converter may be no good. The reason for multiple O2 sensors is to measure the the catalytic converters performance. The easiest way to determine if the cat is working properly is to measure the temperature on both sides of the cat. The temp going in should be approximately 100 deg less then what's coming out of the cat. So if the temp is 250* going in to the catalytic converter, it should be about 350* coming out. the best way to measure the temp is to use a laser pointer temp gauge. If the temperature is the same on both ends, I recommend replacing the catalytic converter.
SOURCE: Check engine light on a 1998 Mercury Mountaineer came on.
Only replace the two before tha catalytic converter the post converter O2 sensors just tell the conputer the converter is OK. Go to rockauto.com to order the sensors or to Napa.
SOURCE: 1997 Mercury Cougar light and ruff engine start up
You have a vacuum leak. Check PCV hose on passenger rear of engine. Stuck open EGR valve can also be the cause.
SOURCE: engine light on, idles rough. had engine light diagnosis.replaced
See if you can get the codes. If it is still number 4 only (probably a P0304) it could be the fuel flow through injector 4. Try some Techron cleaner and make sure number 4 plug isn't fouled. Also get the compression checked on number 4.
If not, it could be the ignition control module assuming all other changes have been made. If you get a code it will help. A P0300 would point toward the ICM. If it is number 4 only it could still be the ICM. Expect around $150 for the module if you do it yourself. The ICM may be under the coil pack. If you are getting a P1361 and/or P1362 it could be the Cam position sensor. They are around $40 and are usually near the number one cylinder. Mine is hiding under the water pump pulley. It is a Buick with the 3.8 engine but the principal is the same.
SOURCE: 1997 mountaineer check engine light on again
Assuming everything you replaced was bad,and I doubt it,you would have had a different code for each item you replaced!
What code do you have now?
It sounds as though,your replacing the parts, that are the symptom ,e.g. codes,not finding the cause or problem ,that set them.
SOURCE: check engine light is on.
The OBD2 Fault Codes you get from a Malfunction
Indicator Lamp (MIL) DO NOT TELL YOU WHAT IS
WRONG,OR WHAT TO REPLACE
Study OBD2 Systems.
Common Sense--why would you change all
O2 Sensors or a converter the should last 15 years ?
You take each code and go to a trouble chart & work
each code to find a problem,then come up with a solution,
and then the repair
What you did causes 2.3 Billion Dollars a year Worldwide,
in unnecessary parts purchases.
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