Replacing fuel injectors. the no.3&4 spark plugs get fried after a couple of months. the computer reads mixture is to lean.

I'm not sure what you mean by testing spark plugs. The diagnostic code defined as "system too rich, bank 2" indicates that the reading reported by the bank 2 cylinder bank (containing cylinders 2, 4, 6, and 8) is consistently reporting a rich exhaust mixture rather than a reading toggling between lean and rich. This might mean you have a fuel injector that is not providing the correct amount of fuel, a spark plug that is not properly burning the air/fuel mixture, an oxygen sensor issue, a wiring issue to the sensor, etc.

-Rod

does it start every time
check list
running hot
too much revs and load
fuel mixture too rich /lean
coil problem
burnt valve
burning oil

It just can't do that. No way to shut down one side of an engine.
Misfires are caused by lack of fuel, spark, or compression. One of the three is missing. One whole side-that rules out spark. Coil pack design just won't affect one side of the engine. That leaves fuel or compression. Some multi-port injection systems are so-called "bank fired" -that is, one side or bank of the engine will pulse the injectors at the same time, first one bank, then the other. But in the early '90's, some cars came out with sequential fuel injection, which pulses the injectors individually, following the firing order of the cylinders, more precise fuel metering that way. I would have thought that an Avalon would be sequential injection, but you could check. Anyway, you need to check if the injectors are working on 2, 4, & 6. The power wire to all injector systems is a single wire that is spliced to each injector. Conceivably, the splice to that bank or side may have broken. With key on, every injector should have voltage at the injector connector. Only two wires to each injector- a power wire and a ground wire. If no power on 2, 4, or 6-that is probably it. (On bank fired injection, all three ground wires on each side splice together before the ground wire goes into the computer. With sequential, each injector ground wire goes to the computer separately.)
Beyond fuel, it is possible that the fuel/air mixture is being affected on only one side. If you have a noisy exhaust leak on that side, it could lean out the mixture.
Finally, low compression will cause a misfire. Possibly, a camshaft on that side is not in time. Only way to check is to do an engine compression test.

How the computer determines a misfire is through a lagging effect in the crankshaft turning-when one cylinder is not producing power, the crankshaft doesn't turn as fast, and the computer senses it through the crankshaft sensor.
Misfires are usually a fuel problem, spark problem, or a low compression problem. Compare the spark at #4 with the other cylinders-all should have a blue, snapping spark. If good spark, check the spark plug -it might be defective. Compare the spark plug in #4 with the others. If it has a black sooty appearance, it is running rich-injector may be leaking. If the lower insulator on #4 is whiter than the other plugs, it is running lean, and the injector may be partially clogged, or not pulsing well.
If gas and spark is good, do an engine compression test. If #4 has low compression, it could be a valve problem, or worn piston rings, or a head gasket failure.

PO 171 Bank 1 too lean
PO 174 Bank 2 too lean
Why ?
The front O2 Sensors need to be replaced at
100,000 miles or sooner, if they have actually failed
They are reading too lean because the system is too
rich & the computer is driving the fuel trim or injector
pulse lean, thus the codes or failed sensors, yes both
Have to check fuel trim with a scan tool
Have to check fuel pressure
Have to check there are no leaking injectors
Have to check for Vacuum Leaks from the intake
all the way to the exhaust converter
Your working a rich condition keeping in mind the system
is driven lean
You look at fuel trim short & long,sort of a past history
of events
May want to look at converter temps also
The converters won't last long if you have or has
a rich condition from fuel or cylinder misfires
Now the spark plugs can't be over 3 years old
& no mifires,get that out of the way before working
the O2 issues

Could be a number of issues. Fuel pressure or regulator issue, but...
If you have a coolant sensor problem, as in reading wrong, and it tells your computer it is colder than it actually is,,,you will get more fuel delivery.

P0301 - Cylinder 1 Misfire Detected.

Causes:
- Improper spark plug at cylinder #1
- Insufficient compression at cylinder #1
- Incorrect fuel pressure
- The injector circuit is open or shorted at cylinder #1
- Fuel injector #1 may be faulty
- Intake air leak

Solutions:
- Perform Injector Service
- Replaced Spark Plug #1
- Replaced Injector #1
- Replaced Ignition Coil #1

P0303 - Cylinder 3 Misfire Detected.
Follow info from P0301.

P0316 - Misfire Occurred In The First 1,000 Engine Revolutions.

Causes:
- Damaged CKP sensor.
- Ignition System.
- Fuel Injectors
- Running out of fuel
- Fuel Quality
- Base engine
- Damaged Powertrain Control Module

Solutions:
Usually the P0316 is combine with a P03XX codes. The P03XX code indicates in which cylinder the misfire occurred.

EXHAUST LEAK WILL CAUSE LEAN FUEL MIXTURE.WEAK FAULTY FUEL PUMP WILL CAUSE LEAN FUEL MIXTURE.CHECK FUEL PRESSURE SHOULD BE 44 - 50 PSI.

Fuel Filter and Pump-won't cause a lean condition,unless you don't have sufficient volume.
Air leaking in around injector O-Rings
Any kind of air leak,including intake manifold,loose O2 Sensors,Etc,Etc

6 ASE people should be able to figure it out!!!

Look at all Data on an Oscilloscope.
The fact that the computer reads lean--IS IT a lean condition.
Check tailpipe emissions to confirm,lean

There are three kinds of gasoline engine misfire scenarios, first there is the "under load" misfire and there is the "at engine idle" misfire, and finally there is misfire continuously. All engine misfires exist because one of three things has occurred. First, a cylinder has lost compression, a cylinder needs a certain amount of compression to operate correctly. Second, the ignition system has failed or is failing intermittently, spark is needed at the time of compression to ignite the fuel air mixture. Third, the fuel air mixture is incorrect, proper mixture is needed for the ignition system to ignite fuel properly. If any of these conditions occur in the engine, the engine will misfire.
1. Low or no compression can be caused by

a. burned or leaking intake or exhaust valves
b. worn or broken piston or piston rings
c. worn out camshaft
d. wrong weight motor oil was installed holding the cam followers from adjusting
e. broken valve spring
f. failed head gasket.
2. Ignition system has failed or is failing
a. spark plug has fouled or is worn out
b. ignition coil
has failed
c. spark plug wires have shorted
e. engine control module coil driver has failed
3. Fuel/Air Mixture is incorrect
a. vacuum leak at the intake manifold
b. fuel injector has failed
c. EGR valve is stuck open
d. mass air flow sensor has failed
e. oxygen sensor has failed
f. air intake boot is cracked

common problems:
1. fuel injector has failed or is failing
2. spark plug wire has shorted
3. spark plug is worn out or is cracked
4. ignition coil has failed is failing

Testing a coil on the car is pretty easy. No special tools are required. Just remember to be careful, the amount of electricity generated by your ignition system can be dangerous. If your coil is already off the car, or if you would like a more specific data-driven test, you can bench test your coil. To set up the test, remove one spark plug wire from its plug, then remove the spark plug using a spark plug socket. Next put the spark plug back into the spark plug wire. Be careful not to let anything drop into the empty spark plug hole -- very bad.