I was brought to this particular post by Googling for the following string:

PMU FORCED SHUTDOWN, CAUSE = -122

Short answer: This particular cause code indicates the power supply beyond the G5 (the UPS or wall).

Long answer: I need to give some background (doesn't everybody?)...

I have been a Mac support professional for the past three years, and I worked in prepress for the six years before where I maintained the Macs in the office. I have held a position supporting Macs at a "national resource" applied physics laboratory for the last two years. I earned my ACDT and ACPT certification last fall and help to support about 600 Scientist/Engineer Mac users. I help really smart people (a lot of rocket scientists) all day with their Macs.

A few months ago, I was able to purchase a new DP 2.0 GHz G5 tower for home. It ran Panther just fine for several weeks. Then Tiger was released and I upgraded. All of the sudden, I would return to the computer after a day at work, or otherwise stepping away for a while, and it would be powered down. I poured over the logs and noticed the above entry during the subsequent boot sequence. My first resource was Google and didn't find a whole lot. Nothing definitive about specific cause codes anyway. I read something about someone having a similar problem fixed when an Apple Store replaced the power supply. I thought, OK- they could be related, but continued my search over the next week or two, carefully noting times and log entries for these shutdowns in iCal. There's really not too much out there on the web about this. I tried the basics- the hardware test CD and the more advanced Apple System Diagnostics CD- they turned up nothing on overnight loops. I zapped the PRAM and reset the PMU. I moved up the scale of drastic measure and reinstalled Tiger (I was having a few other small OS issues too)- nope same problem. I wiped the drive and did a fresh install- same problem. I came to the conclusion that the Tiger upgrade was a coincidence.

I decided to take the computer to my local Apple Store. I was hesitant because I knew I could do the hardware work myself, but I figured I do this all day at work, why not let someone else take care of me for a change? Well, I prepared my Mac with a new temporary admin account and password, printed out my iCal with crash dates, and some select system.log snippets with the "PMU FORCED SHUTDOWN, CAUSE = -122" line highlighted, and headed off to the Apple Store to let them know who I am and what I found. This was early on a Monday afternoon. They assured me they would triage it later that day and I noticed the sign in the store stating 3 day turnarounds on repairs. I waited and checked the online status of my machine. By Wednesday, the online status was still "waiting for triage." I was a bit upset. I stopped by the store on my way home from work to see what was up. The staff assured me that the problem had been narrowed down to the PMU on the logic board, and a replacement logic board was already sitting in a box next to my mac in the back. They would install it and test it the next day. I called the next day and the tech assured me that the repair was done and they would need to test it until tomorrow- Friday at this point. I was a bit frustrated, but agreed to wait. I picked up my G5 Friday after work, and the paperwork showed that they only replaced the battery. Whatever- I was a bit peeved, but I returned home and plugged my G5 in. It ran fine that evening. Saturday morning it shutdown on me again! Didn't take too long. Same system.log entry.

I wasn't upset at the Apple Store for only replacing the battery. In my certification, I was taught to start with the small (read:cheap) stuff- it just makes sense (get it? c'mon, chuckle). I was upset with the Apple Store for taking FIVE DAYS to do so. I keep these batteries in my office desk drawer for the very reason that they are OFTEN needed. I was upset that someone there told me the logic board was to be replaced and it was not. I was upset at the Apple Store for not keeping their online status updated.

Back to the real reason why you've read this far...

I was done with the Apple Store and decided to deal with it myself. I ordered a replacement logic board and power supply from Apple. The parts promptly arrived and I eagerly installed them. The darn G5 shut down the following morning- again with the same system.log entry. OK. Stop and think. Well, I had enough time to think while I pulled the replacement logic board and power supply back out of my G5 and reinstalled the original components.

I have my G5 plugged into an APC 500 UPS. It's a decent one with a USB connection that OS X can use for communication. I disconnected the USB connection- same shutdown problem the following day. My UPS had been recently chirping (indicating a failing backup battery), so I plugged the G5 into the surge-only protection side. Same problem. I was almost out of ideas. So I decided to try a very drastic measure.

I saved any open documents...

quit iTunes and iPhoto to protect my libraries...

and jerked the power cable from the back of the computer.

Wait a few seconds, powered it back up, and checked the system.log. WHAMMO!!! The very same log entry!

Now, to lay people (myself anyway), when I read the words "PMU FORCED SHUTDOWN," my brain interprets that as the PMU forced the shutdown. Nope. That led me on a wild goose chase through lots of potentially expensive hardware tests (warranties rock!).

I have ordered a replacement battery for my aging UPS in the hopes that it will help condition my home power, and the problem will go away. I won't be satisfied that the problem is resolved until I have 3 weeks of uptime or so, but I'll keep you posted.

Thanks for reading this far, I'm not sure I would have...

Did that fix the problem?

I'm having a similar problem, only twice so far, and only when I open a new application. I hear a click and the machine is dead, and I see the same CAUSE = -122 message. This is the most useful page Google has turned up so far. I don't use a UPS though . . .