1996 Mercury Mystique Mercury Codes P0171 P0303 P0420

MERCURYGIRL79

MEMBER
1996 MERCURY MYSTIQUE
4 CYL
2WD
AUTOMATIC
14,400 MILES

Hello I have a Mercury Mystique with 144,000 miles. I purchased it used in June of 2009 so I don't know what work has been done on it(if any). It is a 2.0 automatic it has overdrive if that helps. When I bought it the check engine light came on and code P0420 our mechanic said it was the gas cap they did not erase code. Then just recently on a thirty mile highway trip the car acted up on the way home it was surging at 55mph {lower speeds did not affect it } so I just drove slow and then we started experiencing staling at stops and we started putting the car in park at stop lights then place it in drive when it turns green and we have no problems. I had it scanned with OBD2 reader and got codes P0171 P0303 and same P0420 we recently changed spark plugs and wires it drove fine for a day and now its stalling at stops again Please any advice will help !

1 Reply

KHLOW2008

MECHANIC
41,815 POSTS

Hi mercurygirl79,

Thank you for the donation.

P0171 is vehicle is running lean and the PCM is correcting it to make it run rich.

This should be the cause of the surging and stalling.
Possible causes for this problem are :

1. Fuesl system malfunction. Check if the fuel filter is clogged or the fuel pump is weak.

2. EGR system malfunction. This normally should be accompanied with a trouble code which you did not retrieve.

3. Air intake or system vacumn leakes. Check for vacumn leaks and the PCV valve.

4. Engine oil level too high.

5. Excessive internal engine wear. This would be indicated by excessive blowby gases.

P0303 = Misfiring in cylinder # 3. Replacement of the spark plugs should have rectified this problem.

P0420 = Catalyst system.
It could be a fault with the catalytic converter or due to P0171.

Try resolving P0171 first to see if the problem goes away.
3.

1996 Mercury Mystique Mercury Codes P0171 P0303 P0420

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