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Dan, Technician
Category: Mercury
Satisfied Customers: 23941
Experience:  Independent Shop Experience, 2 Year Automotive Diploma, & Access to Information Database
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How do I stop code P0420 on 97 Mercury Grand Marquis?

Resolved Question:

How do I get my CEL to stop coming on for a weak catalytic converter? I don't give a hoot about emissions, so replacing it is not what I have in mind. I've heard you can splice a diode into the signal wire to trick the computer into seeing a normal CC, but I can't figure out which wire is the signal wire. Also, I can't find a referance voltage on any of the wires...just around .02V on the black wire and grey wire with the engine running and the sensor unplugged, and 12V and Ground on the two whites (these are the heater circuit right?).  I've already tried a new O2 sensor. Thanks.
Submitted: 5 years ago.
Category: Mercury
Expert:  Dan replied 5 years ago.
Welcome to JustAnswer, my name is XXXXX XXXXX I'm here to help get your problem resolved. There is a PCM reflash available for this problem and is around $100 to have done at dealer..If that doesnt help then converter will need to be replaced..Only option besides replacing converter if program doesnt help is installing a oxygen sensor bypass box on rear sensors and/or trying to burn clean converter by disconnecting a spark plug wire and run the enginearound 2,500 rpm for 5 minutes allow to idle for 1 minute, then shut down vehicle, reset codes, and see if it comes back...Let me know if you have questions, thanks...
Customer: replied 5 years ago.
I was thinking more along the lines of something I can do without involving a stealer or buying anything expensive. I like your idea of burning out the converter by disconnecting a spark plug. I will try that first. If that doesn't work, can I ask you, how does the O2 sensor circuit work? My understanding is that the PCM provides a reference voltage to the sensor of around .5V, and then the sensor returns a signal of somewhere between .1V and .5V depending on oxygen content in the exhaust. But I could be wrong about this for two reasons: First, I got this explanation from someone who works on GM cars, not Fords. Second, I can't find a wire that provides a reference voltage.
Expert:  Dan replied 5 years ago.
The 02 sensor produces voltage and it needs to be constantly moving so its not as easy as putting in a fixed resistor because then voltage stays the same and computer see's that as a problem...I can give you a wire diagram if you would like but trying to bypass without a actual "bypass box" is going to be tuff if not impossible...I would definitly start with the burn procedure as it does help alot of vehicle still runs good. Let me know if you have any questions, thanks.
Customer: replied 5 years ago.

So from your explanation these O2 sensors work differently than what I was told by the GM guy. These actually produce their own voltage signal. If I could figure out what those voltage ranges are that the PCM is looking for I'm certain I could build my own bypass box. I'd imagine that if a certain constant voltage is looked at as a bad sensor, that maybe an oscillating voltage within a certain range should trick the PCM into thinking everything is okay? I'm pretty sure cars from the '90s don't use the back sensor for anything besides determining CC health, so I wouldn't have to worry about it affecting anything else.


When I have a problem like this I like to gather as much info as possible and fix it myself rather than buy an off-the-shelf fix someone else designed without understanding how it works. Then I can see afterwards if my design works better/worse than whats on the market, and if mine is cheaper/better/easier I can post the results for others to try. I don't need you to design a box for me, I just need to know what the PCM is looking for from the rear sensor and preferably which wire/wires its looking at. At that point I can create a box that does pretty much anything.


I will start with the burn procedure though. If that fixes it I'll just call it good and wait for some other interesting issue to break out my soldering tools and PC boards :)


I would appreciate that wiring diagram for the O2 circuit. Especially if it references what each of the 4 wires do that go to the O2 sensor and what color wires they should be. The colors I mentioned before (black, grey, white, and white) are from the sensor pigtail, and the car side of the connection uses other colors. I doubt it would help, but I could look at what colors they are tomorrow if it helps to locate them on a diagram.


No big hurry, I'll be tied up with Easter and family activities here for the next few days. Happy Easter!

Expert:  Dan replied 5 years ago.

If burn off procedure doesnt work let me know and I'll see if someone else has some suggestions...Maybe someone else has built one but I havent so I woulndnt be able to tell you how..Let me know, thanks.

Dan and 3 other Mercury Specialists are ready to help you
Customer: replied 5 years ago.
Hey, the burn out worked. Good idea. Thanks.
Expert:  Dan replied 5 years ago.
Good to hear, thanks for update...Have a great weekend.