My 1997 Explorer has the check engine on, o/d light blinking since transmission replacement. I had noticed a lot of these problems but they only occurred when I replaced mine. It was a 97' worked great but after many years it needed to be replaced("made an ugly roaring noise when I started it.") The mechanic said they only had one (transmission) that was an upgrade I forgot the year but I'm thinking 02' he said it would work so I agreed. I am thinking this is an electrical problem and I think it's the problem to all the other problems I reported. I still love my SUV it has a lot of good memories.

My first indication that something was wrong with my vehicle was when I started it in cold weather and the check engine light came on. After starting, my engine would idle extremely low, but it would occasionally rev to about 1800-2000 rpm on its own. As it came back down, it would usually idle around 400 rpm, which was too low for it to continue running in cold weather, and it would shut off. I learned that the previous owner (my father) had the same problem. He said a repair shop replaced the ECU (engine computer or PCM), but the problem went away for only a short time. Also, the vehicle seemed to run fine after warming up for about 5 minutes.

There are usually no problems in warm weather (temperatures > 80 degrees Fahrenheit), and the check engine light doesn't come on if it's reset as long as the weather stays warm.

I had Autozone plug in a diagnostic tool, and the associate that plugged the tool in told me my oxygen (O2) sensors were bad. I replaced the oxygen sensors myself, but the check engine light did not go off. I know that it's possible to reset the check engine light, but I became convinced that it should go off if the problem that caused it went away. Besides, the weather was also warm, so I knew the check engine light probably wouldn't come back on anyway.

After waiting a while and expecting the check engine light to reset if the O2 sensors were responsible for the problem, I took the vehicle to another autozone and asked for the problem codes stored in the computer. The codes their scanner retrieved were P0171 and P0174, which translate to a lean fuel trim on bank one and bank two, respectively (each side of the V in a V6 engine is a bank). They printed it out for me, and the printout also included an explanation of the codes and probable causes.

It said if bank one and bank two both triggered the same kind of code, to suspect a fuel condition problem or a problem with the MAF (mass air flow sensor).

I looked up information since I am a DIY-novice, and found that the MAF commonly causes problems when it gets dirty. I took it out, cleaned it thoroughly with electrical parts cleaner, and allowed it to dry overnight, since it won't function properly if wet. My check engine light still did not go out, and I noticed that the engine was knocking. I'd learned that an engine that runs lean can cause permanent engine damage, so I started worrying.

I happened to find a coworker that keeps a diagnostic tool in his vehicle. His is also capable of checking sensor data. After work, he let me use it, and it showed that the computer still had the same codes stored. I cranked my vehicle up and checked the sensor data. The first thing I saw was that the MAF seemed to be functioning properly and fluctuated when the gas pedal was depressed.

My coworker and I then checked the other sensor data and saw that the fuel trim was at -100%. This means that the ECU was apparently adjusting the fuel supply to the engine to the minimum possible, which explains why the ECU reported the engine was running lean.

The next thing we saw was that all four of the oxygen sensors were reading 0 volts. This causes the ECU to think that there is no oxygen in the exhaust, which it would correct by reducing the fuel supplied to the vehicle.

I learned later that the power supply for the oxygen sensor heaters is on the same fuse. If the oxygen sensors do not receive power for their heaters, they will not function properly. They may function somewhat if the engine has warmed up, though. Originally, oxygen sensors did not have heaters, and were mounted on the exhaust manifold so they would warm up quickly. I could not find a fuse in the owner's manual that was specifically for the oxygen sensor heaters (HO2S). I checked a few fuses, but all of them were good.

I'm thinking it might be a fuse I haven't checked, or it could be the connection between the O2 sensors and the ECU.

I'm thinking I might need a service manual to find where the fuse is and to find where the wires from the O2 sensors hook up to the ECU. I've been wanting to sell the vehicle and buy a new fuel-efficient vehicle, though. I'm tempted to just cut my losses and trade it in, because I know a dealership would have good testing equipment and access to manuals and wiring schematics so it might not be a very big deal to them to repair the problem.