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="" />
