How JustAnswer Works:

  • Ask an Expert
    Experts are full of valuable knowledge and are ready to help with any question. Credentials confirmed by a Fortune 500 verification firm.
  • Get a Professional Answer
    Via email, text message, or notification as you wait on our site.
    Ask follow up questions if you need to.
  • 100% Satisfaction Guarantee
    Rate the answer you receive.

Ask Dan Your Own Question

Dan
Dan, Technician
Category: Mercury
Satisfied Customers: 21476
Experience:  Independent Shop Experience, 2 Year Automotive Diploma, & Access to Information Database
17871496
Type Your Mercury Question Here...
Dan is online now
A new question is answered every 9 seconds

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: 2 years ago.
Category: Mercury
Expert:  Dan replied 2 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 2 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 2 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 2 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 2 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, Technician
Category: Mercury
Satisfied Customers: 21476
Experience: Independent Shop Experience, 2 Year Automotive Diploma, & Access to Information Database
Dan and 5 other Mercury Specialists are ready to help you
Customer: replied 2 years ago.
Hey, the burn out worked. Good idea. Thanks.
Expert:  Dan replied 2 years ago.
Good to hear, thanks for update...Have a great weekend.

JustAnswer in the News:

 
 
 
Ask-a-doc Web sites: If you've got a quick question, you can try to get an answer from sites that say they have various specialists on hand to give quick answers... Justanswer.com.
JustAnswer.com...has seen a spike since October in legal questions from readers about layoffs, unemployment and severance.
Web sites like justanswer.com/legal
...leave nothing to chance.
Traffic on JustAnswer rose 14 percent...and had nearly 400,000 page views in 30 days...inquiries related to stress, high blood pressure, drinking and heart pain jumped 33 percent.
Tory Johnson, GMA Workplace Contributor, discusses work-from-home jobs, such as JustAnswer in which verified Experts answer people’s questions.
I will tell you that...the things you have to go through to be an Expert are quite rigorous.
 
 
 

What Customers are Saying:

 
 
 
  • AlvinC answered a question about my Ford truck that only someone with an in-depth knowledge of his subject would have known what was going on. Rich D USA
< Last | Next >
  • AlvinC answered a question about my Ford truck that only someone with an in-depth knowledge of his subject would have known what was going on. Rich D USA
  • Because of your expertise, you armed me with enough ammunition to win the battle with the dealer. They are installing a fuel filter and fuel pump at no charge to me. Molly USA
  • I needed help with my car on Saturday morning....got a response in 5 minutes, and it was the perfect solution. Thanks again to your service. Jason V. Kirkland, WA
  • I do know, after going though this with JustAnswer, that I can somewhat trust my mechanic but I will always contact you prior to going there. BR New Jersey
  • Wonderful service, prompt, efficient, and accurate. Couldn't have asked for more. I cannot thank you enough for your help. Mary C. Freshfield, Liverpool, UK
  • This expert is wonderful. They truly know what they are talking about, and they actually care about you. They really helped put my nerves at ease. Thank you so much!!!! Alex Los Angeles, CA
  • Thank you for all your help. It is nice to know that this service is here for people like myself, who need answers fast and are not sure who to consult. GP Hesperia, CA
 
 
 

Meet The Experts:

 
 
 
  • Ron

    ASE Certified Technician

    Satisfied Customers:

    1601
    ASE Certs and Ford Certs
< Last | Next >
  • http://ww2.justanswer.com/uploads/FO/fordguy4u/2011-12-17_222940_HPIM1257.64x64.JPG Ron's Avatar

    Ron

    ASE Certified Technician

    Satisfied Customers:

    1601
    ASE Certs and Ford Certs
  • http://ww2.justanswer.com/uploads/MU/muddyford/2012-6-13_1204_1.64x64.png Chris (aka- Moose)'s Avatar

    Chris (aka- Moose)

    Mercury Technician

    Satisfied Customers:

    1399
    16 years experience with Mercury
  • http://ww2.justanswer.com/uploads/JM/jmcdo28/2012-2-23_01655_resized.64x64.jpg John Mc's Avatar

    John Mc

    Mercury Technician

    Satisfied Customers:

    403
    19 years Ford Lincoln Mercury experience
  • http://ww2.justanswer.com/uploads/wmarti7/2009-4-18_04332_Avatar.jpg Chuck's Avatar

    Chuck

    ASE Certified Technician

    Satisfied Customers:

    287
    33 Years experience,Ford Senior Master,ASE Master,L1 Advanced Engine Performance
  • http://ww2.justanswer.com/uploads/scottz136/2008-12-09_220350_js640_p1010013.jpg scottz136's Avatar

    scottz136

    Mercury Technician

    Satisfied Customers:

    260
    senior master lincoln mercury tech.
  • http://ww2.justanswer.com/uploads/TI/tim37857/2011-10-28_23053_S6301814.64x64.JPG Tim's Auto Repair's Avatar

    Tim's Auto Repair

    mechanic

    Satisfied Customers:

    257
    Have owned a repair shop for 25 yrs.
  • http://ww2.justanswer.com/uploads/DR/drivefast1971/2012-1-12_172422_0009051tQKh.64x64.jpg Ron Z.'s Avatar

    Ron Z.

    - Mercury Tech -

    Satisfied Customers:

    197
    18+yrs experience. State Inspector and Mercury Diagnostics