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Check Engine light - computer threw two codes...what do they mean?

I have a '98 For Ranger. After 256,000 miles, I recently had a new (rebuilt) motor put in my truck. After about 2 days of driving with the new engine, the Check Engine light came on. I went to Auto Zone and had the codes checked, and these two codes came up:

“Misfire monitor malfunction”

“Oxygen circuit slow response bank 2 sensor 1”

The guy at Auto Zone couldn’t tell me what they meant. Does anyone know - and is this something serious that needs to be checked out immediately or have anything to do with the new motor that was put in?

The first thing to assume is that some sensor wiring got damaged or dirty or left loose or whatever when the new engine went in.

Its best if you report the specific codes in the format “Pxxxx”.

I’ll assume that the “misfire monitor malfunction” is Ford’s P1309. As far as I know, it means that the computer can’t tell where the engine is in terms of stroke. This would point me to checking wiring for cam and cranks sensors.

I’ll assume the slow O2 sensor is P0153. So the first thing to do is check the wiring & harness for that O2 sensor. The next thing is to look for an exhaust leak (perhaps manifold left loose/damaged). Next clean the MAF sensor and look for vacuum leaks.

But do verify the codes by number.

If the old engine failed because of a coolant leak into a combustion chamber, and the old O2 sensor is reused, it could be contaminated from the coolant leak from the old engine.

If that’s the case, then I would replace the O2 sensor and see if the misfire code disappears.


Post the codes and I’ll be glad to look them up and define them. I might have ideas on the problem and may not. If I don’t there are others on this forum that are smarter and more experience than I am that might be able to give you some ideas.

I’m also having a problem with one of my cars giving me a PO304 trouble code (misfire on cylinder #4). Off and on as I have time for the past few days I’ve changed the plugs, swapping plug wires between cylinders hoping the code would move to another cylinder which didn’t happen, tested the coil which tested good and still have other sensors to test, but haven’t found the problem. If I stick with it long enough I think I’ll find the problem. If I don’t at least when I have to take it to a mechanic I can give him a list of things that I’ve ruled out and may give him a better idea of what to look for and won’t have to pay him labor hours for replacing spark plugs and troubleshooting things I’ve already ruled out.