Thanks Cam. I think this will be really interesting (at least for me ). And take your time, we're patient (...after all, we've been waiting almost a year for you to drill your manifold for intake port temps -- i ain't buyin that excuse about ruining it, you've blown up lots more expensive stuff than that :laser: don't hit me, don't hit me, I'm still drinking from last night when the doc said I could go ahead and maybe have a small glass of wine). Here's the codes that were listed: DTC P0138 HO2S Circuit High Voltage Bank 1 Sensor 2 DTC P0137 HO2S Circuit Low Voltage Bank 1 Sensor 2 DTC P0301 Cylinder 1 Misfire Detected DTC P2096 or P2098 Post Catalyst Fuel Trim System Low Limit And I'm figuring the missing one is one of these (I think someone said that): DTC P0108 Manifold Absolute Pressure (MAP) Sensor Circuit High Voltage DTC P1108 BARO to MAP Sensor Comparison Too High 0138 - due to your tune for the H/C - the code you usually get? old code before the problem. 0137 - this would be the opposite... too lean compared to the front sensor? or just pegged lean. 0301 - ok... how come? plug got fouled? how? 2098 - fine. That jives with the 0137 0108/1108 - alright. So maybe it's pumping/pulsing compressed air back INTO the intake manifold? that would point towards the intake valve and not the exhaust valve. I'm changing my bet to the intake valve. I'm stumped on how we went lean on the rear 02.