Help with code 172

94 GT 5.0

New Member
Jun 16, 2009
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New member here with a couple of questions. I have a completely stock 94 GT, runs perfect, no missing, stalling, etc. It has 75,000 miles on it if that helps. I have CEL coming on after about 3 or 4 miles of driving. I tested for codes and came up with 172 which is 02 sensor showing lean bank 1. I went ahead and bought sensor but I'm still getting code. I also bought the MAF spray and cleaned it as best I could. I was talking to the mechanic at work and he said low fuel pressure and/or a vacuum leak could cause this as well. I changed the fuel filter hoping that was it, but code still comes back. Could fuel pressure be culprit, I guess what I'm asking is, if it was pressure it seems like I would get a code for the other side as well, so I was thinking it had to be something else, or is it possible to have low fuel pressure only on 1 side?? I haven't started to check for vacuum leaks yet, is there a problem area I should try first regarding the vacuum? Any other advice for what could cause this, thanks.
 
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Sounds like your mechanic is on the right track. The fuel pressure is regulated by manifold vacuum, so a decent leak will deliver the wrong vacuum. However, that would normally give you lean/rich on both sides, not just one. It's worth a check anyway.You can hook up a fuel pressure gauge to the schraeder valve on the fuel rail near the oil fill cap. You should have a constant ~33PSI at idle. The fuel pressure regulator is on the back corner of the fuel rail with a vacuum line leading to it. While at idle, pull off the vacuum hose (plug it with your finger). The vacuum should be ~38PSI.

If I remember right, low values usually mean a bad fuel pressure regulator. Fluctuating values usually mean a bad fuel pump.

The fuel rail in theory should have even fuel pressure on both sides. Since our fuel system is a "return" fuel system (unused fuel is routed back to the tank), I would think that a blockage or restriction in the fuel rail would affect both sides. You could possibly have clogged fuel injector(s) on the driver's side, meaning that the fuel from the rail isn't getting into the cylinders.

You also could have an exhaust leak on the driver's side. If the exhaust is vented before it gets to the O2 sensor, the computer will read lean. Might be worth checking if your exhaust manifold bolts are tight.
 
At the EEC, the passenger side O2 signal wire is EEC pin 44 (grey/lt blue).

Be sure the passenger side O2 wiring is not mooshed into the frame or anything.
 
GOT IT !! I was tracing the wires out gettting in order to test for signal, well, where the 02 sensor merged with main harness (near front crank) the wires were broke in 1/2. Re-spliced and good to go, no code. Thanks to all.