I have a 2004 with 288K miles on it and this is the first time my van has set a code since I got it at 40K miles in 2007. Though I have already determined the cause of the code I thought I would share my symptoms along with how I diagnosed it.
Some background. About a year ago (about 20K mi) I noticed my exhaust system starting to rattle. I investigated and couldn't get it to reproduce the sound while up on ramps or on jack stands either in gear or park/neutral. The only issue noticed was the flex pipe was starting to deteriorate. This is issue has gotten progressevily worse up to the code being set.
Starting a month and 2K miles ago I started to notice some very slight hesitation under light load before the engine warmed up. This would happen in both drive and reverse. I checked for plugged EGR passages, checked coils by unplugging them, checked spark plugs and PCV. Additionally I checked for codes and monitored O2, TP, ECT sensors, VTEC and other various sensors.
Four days ago going up a mountian pass with my family which I have done many times before the van wouldn't accelerate up the hill. I could hold speed by flooring it but the van wouldn't accelerate and the van wouldn't go over 4500 RPM. No misfires, no codes, no stumbling but the van just didn't seem to have enough power. Agian I checked codes, and monitored live data. I used the scan tool to perform a test of the idle air control system, EGR, VTEC, and EVAP system. Agian everything tested find.
Yesterday was about 200 miles from home on the other side of the pass. I accelerated hard to merge onto the interstate and at about 30 MPH the car started missfiring and stumbling and wouldn't accelerate. The check engine light begean flashing and turned off once I stoped trying to accelerate. I was able to gently get the van up to freeway speed without any other issues. I checked codes and found the p1399 code was pending and that missifires where detected on cylinder 1,2,4,6. I cleared the codes and continued to our next stop. From this point the p1399 code began to set anytime my rpm was above 3500 and the van would agian began to missfire and stumble, and not accelerate. The van would now not go over 70 on flat ground and any hill encountered I would have to decrease my speed to 50mph to keep the check engine light from flashing and prevent driveability issues.
While driving home I discovered that I couldn't accerate above 4500 RPM and anything about 3700 RPM would result in the van actually slowing down like it wasn't getting enough fuel but I knew this was not the case based on a normal fuel trim. I decided based on this symptom that a plugged catylitic converter could be my issue even though that particular code was not set. Once I arrived at my friends house I decided to pull out the upstream O2 sensor and clear the codes. Once completed I took the car out and discovered that I could now accelerate pass 70mph, and could reach 6000 rpm without any driveablity issues. I checked codes agian and the p1399 code was no longer pending.
This seemed to confirm my diagnosis and once I reached my home I ordered a new Walker 16457 Ultraex Direct Fit Catalytic Converter, and upstream and downstream NGK/NTK O2 sensors. Today I pulled my old Cat off and discovered that the back half of the converter substrate had broken loose. I knocked out the substrate and reinstalled the converter and o2 sensor and agian verified that the van ran without any drivability issues confirming my diagnosis that a plugged converter was causing my p1399 code. This also fixed my rattling noise in my exhaust system that has been present for the last year.
I will report back after I get my new parts installed and let everyone know how the o2 sensors work out. I know that most people point towards plugs, coils, egr or valve adjustment for fixing this code but I wanted to let people now about my experience.