Treat the gauge and blower issues separately. With the key on and engine off, disconnect the oil-pressure and temperature sender wires and ground them to a good ground; the gauges should swing full scale and drop when ungrounded. For fan speeds that work only on high, replace the blower motor resistor in the heater box, under the dash or under the hood. The speed sensor was already replaced and is not the remaining cause.
A 1999 Ford Ranger owner reported that the temperature and oil-pressure gauges jump to center and stay there after startup, and that the heater fan works only on the highest setting. One reply noted these are separate problems and suggested a speed sensor if the speedometer were erratic, plus a blower motor resistor for the fan. The owner said the speed sensor was already replaced and the other gauges still misread. Another reply advised grounding the disconnected oil and temperature sender wires with the key on and engine off to see whether the gauges swing full scale, and locating the blower resistor in the heater box under the dash or hood.
