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Hi Steve ..it refers to the location of the oxygen (o2) sensors relative to the engine. The upstream sensor is the one closest to the engine. The downstream sensor is furthest from the engine. Think of it as the way a stream of water, or in this case, a stream of exhaust flows.... From the engine To the tailpipe. Changing them is no problem, getting to them sometimes can be a bear,, if you can reach them,, its simply remove the old one with a couple of bolts, and unplug it,, and replace
MANY THINGS CAN CAUSE THE LIGHT TO COME ON THAT LIGHT INDECATES A MALFUNCTION IN THE SYSTEM ITS HARD TO SAY WHAT CAUSED IT WITH OUT HAVEING IT CHECKED GO TO AUTOZONE THEY PROVIDE A FREE SERVICE CHECK ( OBD 2 96 & LATTER) GET IT CHECKED OUT THEN I'LL TELL YOU HOW TO FIX IT
You'll need to give us the specific code, in the format P0xxx. Just saying "O2 sensor" doesn't mean anything. It could be the heater circuit, fuse, o2 sensor itself, etc. Changing an O2 sensor isn't too hard but you may not need to replace it/them.
Code PO131 is O2 sensor circuit low voltage , bank 1, sensor 1. That sensor I would say is the one in the exhaust manifold. I would check voltage to the sensor from the computer. If the voltage is not 5 volts, then it could be a computer problem or a wiring problem. I have seen factory wiring and connectors give a voltage problem. Check for a damaged or burnt, pinched wire. You need to know for sure that the voltage is correct to the sensor. Intermittent problems are hard to trace , but patience and knowledge helps.
o2 sensor is usually located on the manifold of most 4 cylinder engines they are normally a one wire connection. you will need the following:
New oxygen sensor....these can be really expensive.
a open end wrench that fits the O2 sensor, or an O2 sensor socket and a ratchet to loosen and tighten the O2 sensor
Start by driving the car to get the car warm o2 sensors can be very hard to get out if they are cold.
if you think you might burn yourself on something hot put thick gloves on, you will have to disconnect the on wire on the o2 sensor, unfortunately you cant wear gloves while doing this, so be extremely careful as the exhaust will be very hot.
After wire disconnected loosen o2 sensor by going counter clock wise using either a o2 sensor socket or a appropriate wrench
while wearing gloves, after the sensor is loose enough unscrew by hand, remember it is still very hot.
you can let engine cool down now if you want.
put new o2 sensor in tightening it firmly and replug the electrical connector back up.
if your check engine light does not go out take to autozone and have them reset your computer, or you can disconnect your battery cables for 20+ minutes and this resets the computer also.