This is reposted from another user, but I have followed these instructions and have done the repair for under $200. This is the only part you need to replace:part 33146-SJA-003. I purchased through the DCH website.
"Recently, my beloved '05 RL began displaying that annoying AFS Failure message with the equally annoying flashing light. At first, the system could be reset by turning the car off then back on again; however, ultimately, it was continuous. I read numerous posts here on the forum and simply wanted to share my experience with others to help them resolve this matter quickly and efficiently.
There are a few posts discussing the actual computer/in-cabin module as failing. This to me didn't seem to add up (in my situation). A few had mentioned that the AFS stroke sensor attached to the rear driver's side shock was the culprit. When I took my wheel off, I could see that this part was horribly rusted (I'm in MN so we see a fair amount of salt). I figured this had to be the culprit. I ordered the part 33146-SJA-003 from oemacuraparts com for a bit over $170.
To replace the part, I will outline the basics in case someone wants to consider tackling the task on their own.
1) Remove driver's side rear wheel
2) Use a 10mm socket to remove the two upper bolts attached to the sensor.
3) Use the 10mm socket to remove the lower bolt attached to the shock.
4) There is no need to adjust the stroke sensor that is sent to you. It appears that it is adjusted from the factory, for I simply replaced it and it worked just fine.
5) Disconnect the cable. There is a small clip attached to the bottom of the sensor securing the cable that is easiest to remove after you have removed the two bolts & single nut. I had a bit of difficulty removing the connector so be patient.
6) After you remove the connector, look inside the connector (the part still attached to the car). You will need to scrape away the corrosion from the terminals. I used a tiny jewelers screwdriver to scrape the terminals.
7) I reconnected the cable and secured the wire to the bottom of the sensor
8) Re-used the two bolts & nut to re-install the sensor.
9) Re-install the tire.
10) Start the car and enjoy the absence of the annoying warning light.
I did disassemble the sensor to see if it really had failed and if so what had failed. It appears that the housing on the original sensor wasn't completely waterproof. Some of the inner components were nearly completely corroded. In fact, when I re-assembled the sensor and noted that it truly wasn't sealed well. I presume the gaskets had worn out over time and ultimately let water into the sensor's inner workings."
I hope this information helps others.