codes says: Catalyst efficiency below threshold (bank 1). What causes this code and how do I fix it????

Thanks Kyle

you have a bad o2 sensor or your cat is getting plugged

I have replaced all the o2's how do I unplug the cat?

Most likely a bad O2 sensor. Do yourself a favor and buy all new ones from the dealer. It will hurt, but it will save you down the road. The BOSCH ones are junk and fail OFTEN, I've also seen them read incorrect alot and burn up cat's from it.

Guys, this is incorrect. It's a common misconception though. Catalyst efficiency is set when the O2 before the cat is active AND the O2 after the cat is active. This means the cat isn't doing it's job. If the front O2 goes weak the code is set for that and the rear is ignored until fixed. If the rear O2 is weak the PCM thinks the cat is doing it's job and nothing happens until the rear O2 is dead. Then a code sets for that. Therefore a weak O2 cannot cause a catalyst efficiency code.

This usually means the catalyst is gone. Sometimes a thourough tune up with the whole nine yards will buy you some more time but it usually means kitty time. Find out if there's a base tune problem that burned up the kitty. Fuel leaks or misfires are the most common on that engine.

Vernon

Therefore a weak O2 cannot cause a catalyst efficiency code.

In theory this is correct, but with GM's I have seen this code happen on the 96-98 5.0L's and 5.7L's and it was an O2 in them in almost every instance. GM's OBD isn't the best in my opinion on the L31 style PCM as I have seen them throw false codes too many times to count. I've seen a mis-fire code come up as a failed O2 and failed O2 come up as random mis-fire. Just what I have seen from working on quite a few of them, so I hardly ever trust em when they throw a code.

I'm with vernon here. 99.99999% of the time, with that code it's kitty time. The diagnostics are short and simple:

Did you perform the Powertrain On-Board Diagnostic (OBD) System Check?
--
Go to Step 2
Go to Powertrain On Board Diagnostic (OBD) System Check

2
Are any other DTCs set?
--
Go to the applicable DTC table
Go to Step 3

3
Check the exhaust system for the following conditions:
• Leaks

• Loose or missing hardware

Repair as necessary. Refer to Exhaust System Inspection in Engine Exhaust.
Did you find a problem?
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Go to Step 5
Go to Step 4

4
Verify that the correct original equipment three-way catalytic converter is installed.
Check the converter for the following:
• Dents

• Severe discoloration

• Holes

Ensure that the oxygen sensors are properly installed and that the wiring connections are properly retained and not damaged.
Repair as necessary. Refer to Exhaust System Inspection in Engine Exhaust.
Did you find a problem?
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Go to Step 5
Go to Step 6

5
Using the scan tool, clear the DTCs.
Start the engine.
Allow the engine to idle until the engine reaches normal operating temperature.
Select DTC and the Specific DTC function.
Enter the DTC number which was set.
Operate the vehicle, with the Conditions for Setting this DTC, until the scan tool indicates the diagnostic Ran.
Does the scan tool indicate the diagnostic Passed?
--

Go to Step 6

6
Replace the catalytic converter. Refer to Catalytic Converter Replacement in Engine Exhaust.

Is the action complete?