In the pursuit of returning my NA to it’s former glory I’ve been replacing parts that haven’t ever been replaced since it was built 20 years ago. The O2 was one of these parts. It was still using the single wire system from that factory. Which is an effective system, it gets the job done, and works most efficiently for long drives (at least more than the 7 minute drive to work). For those of you who don’t know what a 4 wire O2 sensor is, as I did not know either, it is similar to a 1 wire sensor in that it tells the ECU how much oxygen hasn’t been burnt by the engine. The difference is the 3 extra wires, two of the wires are for the heater in the plug which heat the sensor to it’s optimal operating temp in 20-30 seconds which is important for gas mileage on short drives and situations in which the sensor would fluctuate in temperature. The last extra wire is a dedicated ground for the signal, which supposedly is better for, as it doesn’t introduce extra resistance to the reading which can happen with 1 wire systems where the header is acting as the ground itself.
In both those guides they don’t really stress how important it is not to use solder on the signal and signal ground. Doing so drastically changes the resistance and therefore the reading the ECU sees. Using solder on the heater wire (both white wires on the Bosch 13275) is ideal. And use crimps on the gray (signal ground) and black wires (signal).