Scan tools give much more information than trouble codes. In addition to retrieving and clearing codes, they let you watch sensor outputs and computer values in real time. For example, you can monitor transmission temperature, coolant temperature, throttle position, and oxygen sensor voltage, among others. My Actron 9110 tool (not for OBDII) allows me to look at about 50 parameters. Many here have Actron 9145 scan tools. These cost about $200 - 300. The GM dealer'* scan tool, the Tech II, communicates with the PCM and allows you to run tests on components and systems. Nice but who can afford $2000+?

Something similar happened to me. I was using 93 octane gas, until recently, when the price shot up again. I filled the tank w/ 87 octane @ $2.47 / gall. One late night / early morning, I was racing a buddy'* Prelude on the highway. As the tranny shifted from 2nd to 3rd (approx. 90 mph), I noticed a jerk in acceleration. Then, the check engine light blinked several times then remained on. I noticed a rough idle and a loss of power from the midrange up. The next day, I had AutoZone (local parts store) read the code for free. I forgot the actual code, but it was "Multiple Misfires." We cleared the code and check engine light hasn't come back yet.

It had been almost 2 years since last plug / wire change over. I bought iridum plugs, ordered 8.5 mm MSD and MSD coils (center terminals are rusting). I pulled the closest plug to the coil and discovered that the side electrode had either melted or broken off, thus probably the cause of the mis-fires. Also, when pulling the wire, the terminal on the wire stuck to the plug. I had to buy a $5.00 replacement wire. I only replaced the front three plugs, for fear of tearing another wire out with the terminal missing (and not having the new wires handy). The rough idling is gone, however, the replacement wire doesn't exactly fit properly on the OEM coil. This is causing poor firing and I cannot give more than 50% throttle without what sounds like "bucking."

My new wires and coils have arrived today and I will be installing them tonight!!
Hopefully, this will bring my Bonnie back to normal levels.

Now you see why you are required to run 91 octane minimum. If you're lucky, your pistons and valves survived.

Never run below 91 octane in an L67. Run 92 or 93 if you can get it after you change to a smaller pulley.

P1406 has to do with your EGR Valve Pintle Position according to my Alldata.