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Brad, ASE Certified Technician
Category: Mercury
Satisfied Customers: 2366
Experience:  ASE Certified Master Technician
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98 mercury mystique: code reader..power to the coil block..restart

Customer Question

Have a 98 mercury mystique after the codes po 135,141,155,161
shows on a code reader the engine does not start.
no power to the coil block. will restart if battery is disconnected
for a short time is the computer programed to do this.
or do I need a new computer.
The problem is this does not do the power shut down on a regular time.
It does this after 50 kms or 500kms when the engine is shut down
and you try to start the engine after it has been shut off for a while
Submitted: 6 years ago.
Category: Mercury
Expert:  Brad replied 6 years ago.
Hello and welcome to!

P0135 means there is a problem in the electrical circuit for the Oxygen Sensor Bank 1, Sensor 1.
P0141 means there is a problem in the electrical circuit for the Oxygen Sensor Heater Bank 1, Sensor 2.
P0155 means there is a problem in the electrical circuit for the Oxygen Sensor Heater Bank 2, Sensor 1.
P0161 means there is a problem in the electrical circuit for the Oxygen Sensor Heater Bank 2, Sensor 2.

Since the possibility of all of these sensors going bad at once is very slim then I'd say you have a problem with that circuit somewhere. Since other parts may be powered by that same circuit that would explain a lot.

I'd like to study the diagrams to try to find a point of failure but first I need to know if you have the four cylinder or if you have the six cylinder engine? Thanks!
Brad and other Mercury Specialists are ready to help you
Customer: replied 6 years ago.

It is a v6 2.5 litre engine.

I had a code po 401 once.

The car is being used now.

driven on short trips.

Just used the car on a short

round trip of 50 kms and no engine check lite.

Amazing hope you can help with this


Expert:  Brad replied 6 years ago.
OK fuse #13 in the underhood fuse box controls the heater circuits for those oxygen sensors. This is powered up by the PCM (engine computer) relay in that same fuse box.

Every time you go to start the car I need you to pay attention. When you first turn they key to ON (before you try to start it) the CHECK ENGINE light should come on. I need you to see if this light is coming on when it won't start. If not then it means that the computer is not powering up and this goes back to a problem in that relay, the fuse box or possibly even a fuse box.

I have a feeling that either that relay is sticking, or there are poor connections in the fuse box itself. I'm thinking the latter because these are notorious for having electrical problems in the fuse boxes. I don't think it's the computer itself. I suppose a bad ignition switch could also cause the problem as well.

I will have to warn you. In the industry we call this car the "Mercury Mistake" because they can easily become an expensive electrical nightmare. Apparently the quality of the wiring and connections isn't so great. There's a really good chance you'll need a professional to help diagnose and fix this problem.

As for the P0401.. that's a problem with the EGR sensor (called the DPFE sensor). A replacement Ford or Motorcraft sensor will usually fix this but let's not worry about this just yet as it has nothing to do with the vehicle starting and running