P0606 - limp mode

[06YarisRS]

I've chosen to post this here, although it would also be appropriate for the Forced Induction forum. Had a scare last night. I'd noticed lately under max boost, I was getting some erratic shifting accompanied by a flashing MIL. Last night I boosted, the MIL came on and the car went into limp mode. I'd also noticed lately that when the MIL blinked that I had a sharp drop in O2 sensor temperature. The bank 1 sensor 1 and bank 1 sensor 2 would plummet from ~1400F and ~1100F to ~280F and ~260F respectively. I scanned the computer when the car was in limp mode and it returned a P0606 (Failed PCM/ECM).

The drop in O2 heater circuit temps is what twigged me to the possibility of a misfire associated with boost. What would cause 2 sensors in close proximity to be adversely affected at the precise time? I figure the unburned fuel and water (from my injection system) was not being combusted, forced out the exhaust valve and rapidly cooling the wideband and downstream sensors. This resultant voltage change would be interpreted by the ECU as a bad thing and limp mode would be triggered. I figured I might have plug problems.

I removed the plugs and noted that the gapping was inconsistent across all four. A couple were out by less than ~0.06 on the wide side. Instead of regapping these plugs (these are a step colder plug that I'd put in about a month ago), I reinstalled the plugs that I'd had in before (probably less than a couple 1000 kms on them). Under the pressure of boost, plug gapping is critical. Slightly too wide a gap under the pressure of boost will result in too much resistance and a failed spark. On a side note, other than the slight gapping issue, the plugs looked perfect (no carbon deposits, no browning on the ceramic, no oil). Perfect, yet surprising as the car runs on the rich side, especially in boost. Then again, my water injection system does 'steam clean' the combustion chamber. I'm going to drop my endoscope in there soon and have a look at the cylinder walls and piston tops for fun.

I've done 8 - 9 full boost pulls from ~60km/h up and not the slightest hesitation. Fingers crossed that it stays this way! The question becomes, why did this just start to happen? After I found a small vacuum leak and repaired it, my boost increased probably 1/2 psi. I may have just reached the threshold where the pressure, combined with the gap issue resulted in failed spark. Another consideration is the volume of water I'm injecting. It may be a smidgen too much, so I may dial that back a few cc's. The way my system is set up, it might require the occasional tweak to regulate the volume of injection.