Kind of long but I am just trying to get the details in:

Shortly after I bought my 99 Montero the P0421 code appeared and for a couple of years I rode with it getting less than par gas mileage. Recently I took a trip and got around 14.5 MPG with stock size A/T tires all highway cruising at 72MPH. I then took temperature readings pre and post pre-cats (Cali model has 4 O2 sensors, 2 pre-cats and one main cat) and determined the bank1 pre-cat was clogged. I removed and gutted both (still being 49 state legal, a new one is over $500) and put two spark plug non-foulers on each of the downstream O2 sensors so they would read as if the pre-cats were still operating correctly. My P0421 code never popped up again but a P0135 code did (bank1 sensor1). I drove home 600+ miles with the 0135 code and my gas mileage did improve slightly but with this code, from what I understand, the computer tries to compensate by dumping more fuel into the engine. After the long drive and a few trips around town my truck started to misfire and produced a P0300 code, multiple random misfires. I read that the faulty O2 sensor can cause the misfires so I replaced the O2 sensor with a Bosch ($100) and all was well for 1 day with no codes! However, my truck misfired again and the 0300 popped up again. I tested each coil pack with a plug wire and spark plug to diagnose the problem and found that the coil for cylinder 3/6 was not firing. I replaced the coil ($74) and still no fire. Also after further review I realized that the O2 sensor I replaced upstream is the incorrect part number, it is an O2 sensor for the federal model(which only has 2 O2 sensors, before and after main cat) and not a Cali model. Even if I erase the codes and crank it up it misfires right away which leads me to believe the O2 sensor is not the problem. I'm stuck!
They do have a Pull-A-Part nearby with a 98 Montero that I will visit Friday and retrieve an igniton control module to see if this is the culprit. Any ideas or knowledge of this dilemma? Could a wrong heated O2 sensor cause this? Luckily my friend is the manager at the autoparts store and will refund these electrical components.
On another note: the passenger side pipe which contains the pre-cat is ridiculous to remove and just so happens to be in the way of removing the starter, which would make changing a starter on a 99 Cali model montero an all day job! <img src="/forums/images/graemlins/angry.gif" alt="" />

There is a weird method to check the spark plug firing on a waste spark like yours. I think you have to put the two plugs together with a hose clamp, threads to threads, and ground the pair, or something like that. I think the method was on the sds.com web site. If the plugs are in fact not firing, I think your tach would be reading 1/3 low if it's the ign module, because the tach would not be seeing the coil ground path break.

I took a trip the the junk yard and picked up a "power unit" or ignition control module and it solved the problem! It has only been one day but so far so good. The spark plug non foulers on the downstream O2s works as well and no check engine lights!