Unfortunately, with older vehicles that have these basic types of computers and error codes, the best way that I know of (without a time machine or access to what Toyota techs had back when these were still new) is troubleshooting by process of elimination, AKA throwing parts at it until you fix it. At face value that sounds pretty crappy but there are always parts that are more likely to fail than others, and the FSM will always point you in the right direction. Here is the FSM information for a code 5 error on a 84-85 van:

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So, if it's not your o2 sensor, the next places to look would be the circuit and, finally, the ECU. Again, these o2 sensors are wear-and-replace-parts, and are subjected to extreme heat and generally terrible conditions. You only realistically get so many years out of these sensors before they give up the ghost and it's time for a new one. A chain breaks at its weakest link, and these typically are it when you get a code like this. Of course, if this turns out not to be the case, then the o2 sensor circuit and the ECU would be your next logical steps.

And I found the denso sensor on rockauto for $22.79 + shipping. You can easily google a 5% off coupon for rockauto so I don't imagine it costing much more than $25 shipped to your door.