96 Bonneville SSEi, 212,000 miles. Factory stock, did a heater core install and soon after noticed intermittent electrical issues with the radio display (display shuts off, radio shorts out then restarts, automatic climate control indicates it'* working but nothing from the blower motor) but the car otherwise ran fine. Then it lost an alternator, replaced that with no issue. Soon after that, the transmission began to slip in the lower gears. Not all the time, but it was more noticeable when taking off on hills. Fuel consumption went through the roof (average 11.5 mpg). Thought I might have a bad O2 sensor and maybe a clogged transmission filter. Changed out O2 and dropped the pan to replace transmission filter and fluid (first mistake). So far, only codes were a P0301 and a P0171. Swapped out bad coil and cleaned throttle body. Removed throttle body screen in front of the MAF (old trick from my Firebird days). No more codes, or so I thought, intermittent P0171. Anyway the transmission slipping gets worse, now it will not move unitll you rev it over 1500-2000 rpm, and it will hold first and second gears until well past 4000 or u force a shift by lifting the throttle. Does not matter if I force a manual 1st or 2nd gear start or leave the gear selector in OD. Checked for codes with my Actron code reader, no codes other than the intermittent P0171. I'm pretty much resigned to replacing the tranny, but.. Every now and then, the clouds part and the dang thing drives like nothings wrong. I disconnected the battery and the ECM/BCM connectors so the computer would defaut to factory and hopefully "unlearn" whatever is causing the transmission to bind up. No luck, and no new codes either. Anybody have any idea what may be causing this?

I believe your transmission has a vacuum line. Should be drivers side front I believe. Check that. If that line is cracked it will effect the transmission, and can also give you the p0171 code.

I'll check that out on my lunch break today. Had to be a taxi driver this morning, same behavior until about 30 minutes into the trip, then everything "magically" cleared up and it drove like normal. I think i will check the chassis grounds. I checked the ground bus near to the driver'* seat and it was fine, no corrosion and i don't believe i left any grounds loose during the heater core install but sometimes you never know. Still no new codes so i'm pretty much stumped. This one is my DD until my 2006 HHR sells to finance my purchase of a small to mid-size truck. I hope I don't have to switch back to the HHR, not that theres anything wrong with it, i love that car but I need a truck right now and this Boneville is worth squat, especially with these issues.

Success! Sorta.. Checked the vacuum line to the modulator, sure enough, it was fragged. I figured i'd pick up a new one since the Bonneville was born with the one on the transmission and it was cheap enough (16.50 with sales tax). The other problems have actually taken care of themselves for now, so I replaced the modulator and crossed my fingers. Went around the block and same problem. Only this time, no slipping/crazy shifting, just no power. Get in the driveway and pop the hood, wonder why my underhood temps are so hi and I notice my exhaust manifold glowing RED HOT. Look under the car and the Catalytic Converter RED HOT too. Looks like i've got a plugged cat. At least I know the problem now...

Yep, that is a clogged cat. And that back pressure will rob you of power. Good to hear you found the shifting problem.

I never would have thought to check for a modulator. Havent seen one of them since my uncle sold his 73 Chevy pickup years ago. I thought the E in 4L60-E meant electronic shift control. Thanks for the tip!