Catalyst efficiency below threshold bank 1

I have a '98 K1500 5.7L that is reading the code for catalyst efficiency below threshold on bank 1. I have replaced both catalytic converters and the codes cleared for about two months now one of them is back on. The code came on after a long uphill climb and I am lost as to where to start looking for solutions. Any help would be greatly appreciated. Thanks Termite

I have a '98 K1500 5.7L that is reading the code for catalyst efficiency below threshold on bank 1. I have replaced both catalytic converters and the codes cleared for about two months now one of them is back on. The code came on after a long uphill climb and I am lost as to where to start looking for solutions. Any help would be greatly appreciated. Thanks Termite
What was code no?
Did you use good OEM type cats or the el cheapo?
A lot of cat codes are caused by bad 02 sensor or engine misfires and or rich exhaust.
How is engine tune up?
Any other codes?

Any exhaust Leaks?

MT- The code no is P0420. I believe the cats are magnaflow brand, so i'm not sure of the quality. I have replaced all four of the 02 sensors around three years ago now, the truck has had a slight misfire that I have not been able to resolve but its not noticable enough to throw a P0300 code. The last tune up has been a while ago (not sure of when offhand) but I have checked the plugs and they look good. There are no other codes reading out.
TBlake- there is an exhaust leak on the manifold to Y-pipe flange on bank 2 that I can not seem to get to seal with replacing the gasket.

Miss fires and engine running rich is the leading cause of cat codes.
And as Tblake said exhaust leaks will throw off 02nsensors.
Also el cheapo cats.

DTC P0420, P0421, and P0430 & P0431: Check Possible Cause Of Misfire DTC P0420 and P0421 indicate bank one catalyst system efficiency is minimum requirement. DTC P0430 and P0430 indicate bank 2-catalyst system efficiency is minimum requirement. Possible causes are as follows: Use of leaded fuel. Oil contamination. Cylinder misfire. Fuel pressure too high. HO2S sensor improperly connected. Damaged exhaust system component. Faulty ECT sensor. Faulty HO2S. Ensure ignition timing is correct. Retrieve all Continuous Memory DTCs. If misfire code is not present, go to next step. If misfire code is present, isolate cylinder and repair as necessary. Check HO2S Monitor DTCs If DTCs P0136, P0138, P0140, P0141, P0156, P0158, P0160, or P0161 were present in step 1), service as necessary before continuing. If none of these codes are present in step 1), go to next step. Check ECT Sensor DTCs If DTCs P0117, P0118, P0125 or P1117 were present in step 1), service as necessary before continuing. If none of these codes are present in step 1), go to next step. If any codes except P0420, P0421, P0430 and/or P0430 were present in step 1), service as necessary before continuing. If no codes except P0420 and/or P0430 were present in step 1), go to next step. Check Rear HO2S Wiring Harness Turn ignition off. Ensure HO2S wiring harness is correctly routed and connectors are tight. Repair or replace as necessary. If wiring harness and connectors are okay, go to next step. Check Fuel Pressure Turn ignition off. Release fuel pressure. Install fuel pressure gauge. Start engine and allow to idle. Note fuel pressure gauge reading. Increase engine speed to 2500 RPM and maintain for one minute. For fuel pressure specifications, see FUEL PRESSURE SPECIFICATIONS article. If fuel pressure is as specified, go to next step. If fuel pressure is not as specified, go to CIRCUIT TEST HC. Check For Exhaust System Leaks If exhaust system leaks, it may cause catalyst monitor efficiency test to fail. Inspect exhaust system for cracks, loose connections or punctures. Repair or replace as necessary. If exhaust system is okay, go to next step. Check For Exhaust System Restrictions Inspect exhaust system for collapsed areas, dents or excessive bending. Repair or replace as necessary. If exhaust system is okay, go to next step. Check Manifold Vacuum Install tachometer. Connect vacuum gauge to intake manifold vacuum source. Start engine and raise engine speed to 2000 RPM. Manifold vacuum should rise to more than 16 in. Hg. If manifold vacuum is okay, go to next step. If manifold vacuum is low, go to step 11). Leave tachometer and vacuum gauge connected. Start engine and raise engine speed to 2000 RPM. On a non- restricted system, manifold vacuum should quickly rise to normal range as increased RPM is maintained. On a restricted system, manifold vacuum will slowly rise to normal range as increased RPM is maintained. If manifold vacuum is okay, no indication of exhaust leak or restriction has been detected and testing is complete. If manifold vacuum is low or slow to respond, go to next step. Leave tachometer and vacuum gauge connected. Remove exhaust pipe from exhaust manifold. Start engine and raise engine speed to 2000 RPM. If manifold vacuum is now okay, fault is downstream from exhaust manifold. Reconnect exhaust pipe to exhaust manifold and go to next step. If manifold vacuum is still low or slow to respond, fault is in exhaust manifold or intake manifold gasket. Repair or replace as necessary and repeat QUICK TEST. Leave tachometer and vacuum gauge connected. Disconnect muffler/tailpipe assembly from rear of catalytic converter. Start engine and raise engine speed to 2000 RPM. If manifold vacuum is now okay, fault is in muffler/tailpipe assembly. Repair or replace as necessary and test-drive vehicle to verify elimination of symptom. If manifold vacuum is still not okay, fault is in catalytic converter. Repair or replace as necessary. Check tailpipe/muffler assembly for debris from catalytic converter. Test drive vehicle to verify elimination of symptom.

Thanks for the info. It'll take me a while to check evrything but should be able to within the next month and will keep you posted on what I come up with. Thanks- Termite

Thanks for the info. It'll take me a while to check evrything but should be able to within the next month and will keep you posted on what I come up with. Thanks- Termite

You are welcome and good luck.
Let us know how goes.

Thanks for the info. It'll take me a while to check evrything but should be able to within the next month and will keep you posted on what I come up with. Thanks- Termite


with the cat converter dtc failures you must first repair these exhaust manifold leaks..then the miss fire issues ...check the ignition system for failures ...including the distributor / bearings