I purchased a 1996 Volvo 960 with a very clean body, a clean interior, lots of new parts, and no apparent leaks or smoking for $1300. So, what was the catch? A check engine light, some black soot when you first start it, and the aroma of unburnt fuel near the exhaust. I took a gamble and purchased the car thinking I would dump about $500 into it, and if it still had issues I would sell it.
After reading the OBD codes, it had codes for a bad mass airflow sensor, and both O2 sensors. So, it is time to get to work on the car to get it working right! I start with the easy stuff: new spark plugs, oil/filter change, new air filter, and I cleaned the mass airflow sensor with spray I purchased from the local auto parts store. I purchased new Bosch O2 sensors (both front and back), pull the car up on ramps, and disconnect the negative terminal to the battery. The front O2 sensor is a little hard to get off, but with some "persuasion" I was able to get it off. The back one is easier to get to, and is a pretty easy process. Now, I run the cables up to the front using the same path as the old cables, and when I get to the connectors the fun begins. I noticed the front O2 sensor connector is wrapped in electrical tape, and the back O2 sensor connector has been cut and replaced with an obviously different connector (the wire coloring was different than what came from the harness). After pulling the electrical tape off one of the connectors, I discover whoever did this just twisted the wires together and taped it. I also noticed the wiring colors just didn't seem right, but hey maybe that is the way it is. I break out the soldering iron, some solder and solder the connections. I tape it up, and connect the new O2 sensors. I connected the battery back, backed it off the ramps, and so far so good. There is still black soot and the odor of unburnt fuel by the tail pipe. I drive it to work the next day, and about 40 miles or so the check engine light comes back on. I take to my buddy's place that has a reader (he is a mechanic), and it is throwing a rear O2 sensor error. Okay, so first thing that comes to mind is that this is a brand new sensor, and that wiring is bugging me. So, I try to google the wiring schematic and I come up empty. Not sure if it was my search or if it doesn't exist. My buddy found it on his shops program! So after printing it out and pulling the car back up on ramps low and behold the wiring is not right! After correcting the wiring, I have driven it several hundred miles without the check engine light (as well as passing the emissions test that allows me to get the tag for the car)! The black soot is no longer an issue as well as the smell of unburnt fuel. My theory is now proper communications are right the fuel is being properly burnt? So, here is how it goes in case you find yourself in the same position:
Front O2 Sensor:
- Pin 1 = BL = Ground
- Pin 2 = Y/SB = Control Signal front headed oxygen sensor (HO2S) sensor preheating
- Pin 3 = Gr/OR = Signal -
- Pin 4 = OR = Signal +
Rear 02 Sensor:
- Pin 1 = BL = Ground
- Pin 2 = P/SB = Control Signal rear headed oxygen sensor (HO2S) sensor preheating
- Pin 3 = Y/VO = Signal -
- Pin 4 = Y/GR = Signal +
Link to Schematic
