This means that the fuel mixture is out of range, that it cannot adjust
the mixture properly.  Your goal is first to figure out if it's gone too
lean or too rich, and you can usually tell that from the smell of the
exhaust... metallic and sharp tends to be lean, fuel-smelling tends to
indicate too rich.  The problem is that you have to catch it when it's
actually acting up.

Mixture problems can be caused by vacuum leaks, injector problems,
bad fuel pressure regulators,  bad air mass sensors, clogged air
filters, etc.   And some of those problems are intermittent.

The thing is that if there is a problem even for a short amount of time
(say a dead spot in the pot on the air mass sensor), it will register a
code.... but you will never be able to catch it while the problem is actually
happening since it only happens for a fraction of a second.

The key for intermittent problems is to use a scanner that actually reads
all the parameters in the ECU and to drive around and watch them.  Seeing
the actual values going in and out of the ECU is about the only way to tell
what is really going on.

I'd do a check for vacuum leaks before anything else, just because it'll
only take five minutes with a propane torch and save you some time if it
turns out to be a leak.  
--scott

You have P0(zero)170.

Bank 1 is the bank containing the #1 Cylinder, so you only need look for
vacuum lines on that side of the motor. The #1 Cylinder is the one that is
forward-most on the engine. As you look straight down on the motor, you can
see that one side is forward of the other, this is Bank 1.

I would suggest that IF you have a vac leak -- which would throw the fuel
trim out on one side -- then that leak would have to be on the intake
manifold in a location that does not blend air for both sides of the engine.
I would tend to discount a vac source that at a port where the intake air
can be diverted to the other bank. Find bank 1 and trace the vacuum hoses
connected to and inspect them for cracks.

There is a pressure regulator on the fuel rail that can fail, and this can
cause the fuel delivery to suffer on that side of the engine, and this could
show up as a fuel trim problem for Bank 1 but not Bank 2.

Fuel Mixture is sensed by the O2 Sensor. There is a specific code for this
sensor gooing bad, but it's possible for the sensor to detect an out of
range condition that remains out of range even after the computer tries to
correct it.