One problem related to o2 sensor has been reported for the 2006 Toyota 4Runner. The most recently reported issues are listed below. Please also check out the statistics and reliability analysis of the 2006 Toyota 4Runner based on all problems reported for the 2006 4Runner.
Cel vscoff vsc trac. Had it checked, was told #6 cylinder plug or coil, replaced coil and all spark plugs . Cel light went off the next day but came back on following day. Noticed a sound of water moving in a pipe or what sounded like in a pipe behind dash for a second or so. Was idling and running rough while driving, engine was shaking. Went on some web sites read some comments from other owners ,bad head gasket. 4runner has been well taken care of never abused only 8 years old, normal miles I think, not very happy with this. Bad design of cylinder head. I own a 94 4by 4 pick up bought new 250,000 miles lot a original parts still on truck runs very well except for bad rot on chassis happy with truck was very good to me as was I to it. I also have a 01 tundra sr5 bought 2 years ago with 132,000 runs well, 02 sensor replaced pass. Front went well but now have to replace pass. Rear impossible since rot will not permit me to have to replace cat converter, noticed lots of rot on frame as well as other areas under truck. Have loved Toyotas till now only had good things to say about them, starting to have second thoughts.