96 Camry 4cyl. 111,000mi. CATALYTIC CONVERTER(manifold) failing

Is the check engine light on? The computers have a very good diagnostic routine for the cat converter that sets a check engine light and trouble code when there's a problem. See this post on cat converters
Diagnose catalytic converter Ricks Free Auto Repair Advice

Catalyst System Efficiency Below Threshold (Bank 1)

You have two oxygen sensors. one is before your catalytic converter and one is after. The sensors should read a difference indicating your catalytic converter is working. When they don't read a difference is when you get this code. There are several possible causes, but most often its a damaged catalytic converter. To know for sure you will need a diagnosis.

before you go changing out the catalytic converter realize that the engine light comes on with only a few percent of the catalytic converter plugged - so before you change it, clan out the engine combustion chamber. i did this on my ford Taurus and with a $5 can of carb cleaner, it solved the problem

YOU HAVE 2 OXYGEN SENSOR UPSTREAM OXYGEN LOCATED ON THE EXHAUST MANIFOLD. THE DOWN STREAM OXYGEN SENSOR LOCATED BEHIND THE CATALYTIC CONVERTER.YOU CAR HAS ONLY 2 OXYGEN SENSORS.

v-6 has two O2 sensors, one located in the exhaust manifold and the other near the catalytic converter

O2 sensors are a possible cause. It has one converter, but 2 oxygen sensors...1 upstream, and 1 downstream of the converter. Which means one is very easy to get to (under the hood, on the manifold), and one is not. Usually the check engine light will come on if O2 sensors are faulty.

I have no diagram,but there is what they call upstream,and down stream locations.Upstream,is above the cat-converter,the down stream is below the cat-converter.This would be per-side.You have a left,and a right exhaust manifold.The exhaust pipes run from each manifold(left and right)to the catalytic converters.So you would have to have on the left side a up stream o2 sensor,above the catalytic converter.and a down stream o2 sensor,below the catalytic converter.this would also be on the right side also,this would be the total of four(4) o2 sensors.I hope this is helpful.

Not just a faulty catalytic converter will trigger this code. Many other issues can trigger this code.

• Leaded fuel was used where unleaded was called for
• An oxygen sensor is not reading (functioning) properly
• The engine coolant temperature sensor is not working properly
• Damaged or leaking exhaust manifold / catalytic converter / exhaust pipe
• Delayed spark timing
• The oxygen sensors in front and behind the converter are reporting too similar of readings

Possible solutions:

• Check for exhaust leaks at the manifold, pipes, catalytic converter.
• Replace Coolant Temperature Sensor
  
• Use a scope to diagnose the oxygen sensor operation (Tip: The oxygen sensor in front of the catalytic converter normally has a fluctuating waveform. The waveform of the sensor behind the converter should be more steady).
• Inspect the downstream heated oxygen sensor (HO2)
• Replace the catalytic converter
• Have timing adjusted
  

The code PO420 you have is Catalyst System efficiency is below threshold bank one. Usually replacing the O2 sensor that's the front most in the exhaust system will cure your problem,

Your Check Engine light is on and you find a P0420 code for a catalytic converter fault. Does that really mean your converter has reached the end of the road and needs to be replaced? A new converter can cost $600 to $1000 or more if a new car dealer or repair shop does the job, or maybe half that amount if you do it yourself. It's an expensive fix that may or may not be necessary. The problem, in most cases, is an emissions issue, not a performance issue that affects the way the engine runs.

A P0420 diagnostic trouble code is a "generic" fault code that is set when the Onboard Diagnostic II (OBD II) system sees a drop in converter efficiency. The OBD II system monitors catalyst efficiency by comparing the switching activity of the upstream and downstream oxygen sensors in the exhaust. The upstream O2 sensor in theexhaust manifold reflects the condition of the exhaust gases as they exit the engine. The downstream O2 sensor in or behind the catalytic converter reflects the condition of the exhaust after it passes through he converter.

The catalytic converter is like an after-burner. It oxidizes (burns) any residual fuel vapors (unburned hydrocarbons or HC) in the exhaust. It also burns any carbon monoxide (CO) in the exhaust. The exhaust must meet federal emission standards, and if a problem exists that causes emissions to exceed the federal limits by 150%, the OBD II system is supposed to catch the fault, set a code and turn on the Check Engine light.

Thanks

If you are going to change the convertors I would just replace the first one closest to the engine. But when a computer code says "bank" they usually are refuring to the Oxygen sensor that is there , they are classified as bank 1 and bank 2. 1 being closest to the engine and 2 being farther down the exhaust. If its a computer code that told you bank 1 you might need to look at your oxygen sensor instead of your catalitic converter.