Problems with Mercury Mystique after broken oxygen sensor replaced.

DERPSON

MEMBER
1998 MERCURY MYSTIQUE
4 CYL
FWD
MANUAL
100,000 MILES

I have a 1998 Mercury Mystique with the 2 litre Zetec inline 4.
The oxygen sensor broke off, and it ran fine for a few days, then one morning the car would only start with the pedal pushed most of the way down and shook very bad at idle.
I checked all the mounts, checked the spark in all 4 plugs, replaced the fuel pump fuse and checked fuel pressure, and replaced the oxygen sensor- to no avail.
The ecm is throwing the error code (too much circuit resistance).

I took it to a shop and they basically just started throwing parts at it hoping that it would fix the issue. After a 500 dollar bill I told them to stop and am going to replace the ecm if I don't find out anything, but its a fairly expensive part new and I don't want nothing to change and be out more money.

Is this a common problem (the ecm frying after the oxygen sensor is disconnected) or is there another issue?

1 Reply

CARADIODOC

MECHANIC
33,916 POSTS

You have a basic misfire problem. Start with new spark plugs and wires, but I'd rather have someone diagnose it properly because that will save you money in the long run.

The oxygen sensor doesn't even enter the picture until it gets up to 600 degrees. The Engine Computer will detect a problem with it, set a fault code in memory, and turn on the Check Engine light. I really doubt the computer is the problem. It is responsible for determining when there should be a spark and how much fuel enters the engine. You have spark and fuel so if the computer is doing something incorrectly or at the wrong time, it is more likely there is a sensor problem.

There's a whole bunch of circuits that are monitored by or controlled by the computer. I never heard of a code related to high circuit resistance, but regardless, that code is going to have to be a lot more specific. You might suspect a dirty or failing mass air flow sensor but I'm not going to suggest you replace it until someone verifies it is defective. The least effective way to find this kind of problem is by throwing parts at it. I think you realize that already.