Ticking is usually valves with excessive clearance or injectors. The injectors' tick is quieter, and should be the same on all of them. Get a mechanic's stethoscope (I got mine cheap at autozone) and hold against the valve cover in different places. See if one area is louder than others. You can do the same with the injectors - holding the little rod against them. You should hear the same volume clicking from each of them.
Code 25 means that the signal the computer is getting from the oxygen sensor is below .45 volts for at least a minute and a half. That's a "lean" signal, and when the computer receives it, it will increase the amount of gas being injected to richen the mixture. Normally, this cycle takes less than a second, so if the computer receives the lean signal for 90 seconds, it means one of three things:
1. The mixture actually IS lean, and therefore there's a reason for it - possibly clogged injectors, low fuel pressure, a problem with the firing of one or more injectors, or an exhaust leak. Solution is to find and fix the problem.
2. The O2 sensor has been gummed up and is no longer capable of correctly sensing the oxygen content in the exhaust. Solution is to replace the O2 sensor with a direct-fit Denso (ideally) or NGK (NTK) - if $$ are tight.
3. The O2 sensor is working properly, but the voltage it is trying to send is not making it back to the computer because of corroded wires or connectors. Solution is to fix wiring.
If the O2 sensor has over 90k miles, or if there has been crud going down the tailpipe from a seafoam treatment or other stuff, then you need to replace the O2 sensor.
If you think the O2 sensor should be fine, run two tanks of Red Line SI-1 Complete Fuel System Cleaner through - that's about the only injector cleaner that still has a good amount of polyether amine (PEA) - the really effective injector cleaning chemical. Add a bottle to 2/3 to 3/4 tank of gas, run down to 1/8 tank, add the second bottle and fill again to 2/3 to 3/4 tank of gas, and again run down to 1/8 tank.
If neither a new sensor nor injector cleaning helps, check the resistance of the wires between the sensor and the computer. See the ENGINE - MFI SYSTEM - TROUBLESHOOTING and the ENGINE - MFI SYSTEM - OXYGEN SENSOR sections of the factory manual for help. If you don't have a manual, google 1993 toyota pickup service manual.
