Multiple Cylinder Misfire

[cburwell at comcast.net]

I am trying to hunt down the source of what my ECU says is a misfire problem.

I have pulled the following codes from my ECU: P0135 (Front O2 sensor heater), P141 (Rear O2 sensor Heater), and P0300 (Multiple cylinder Misfire). I also recieved error code P0304 (Misfire on cylinder 4), but when I checked my sprak plugs they looked fine (White/brow/grey, which I presume is normal). When I reinstalled my plugs I scattered their order and now I do not seem to get a misifire code that pinpoints a cylinder but rather just the P0300 code.

I know my front O2 sensor needs replacing, as does my rear one because of the P0135 and P0141 codes which tells me that the heater on both is malfunctioning. So I am currently pricing them for replacement.

What is bothering me is the misfire code(s), because my car does not feel or sound like it is misfiring, and power delivery seems about the same. In the B15 FSM it says that if the ECU detects a misfire that will harm my three way cat (TWC) my MIL will blink, then on the second occurance of a harmfull misfire the MIL will blink again and then stay on from then on. Obviously my MIL comes on right away because of my O2 sensors, but at no point does it blink. This leads me to believe that the second condition listed in the FSM for a misfire is occuring.

The second condition for a misfire is a misfire that is not threatening to the cat, but rather one that will effect emissions. In this case the MIL will come on after the second occurance of this misfire and stay on (at no point will it blink). This leads me to believe that this is the type of misfire I am encountering.

As I said the car does not seem to be misfiring, and my plugs look fine. In the FSM under "Things to check" when a misfire occurs it lists "heated oxygen sensor 1" as an item to check. Could this misfire code be caused by the the malfuntioning of the front O2 sensor's heater? Remember I am only getting a code for the heater on the front O2 sensor, and not the O2 sensor circuit itself.

My car is a 2000 Sentra SE (RR SR20DE) with 34,7XX miles on it. I replaced my plugs at 26,000 miles and my cap and rotor at 25,000 miles (In an attempt to get better gas mileage).

As I mentioned before I do not feel my car misfiring. I assume I would feel my car if it was misfiring. Are there any other reasons why I would be throwing the misfire code? I searched and found a few threads, but no real answer as to what would cause the misfire.

--
Chris Burwell
2000 Sentra SE
http://home.comcast.net/~cburwell