1997 GMC S-15 Jimmy 4.3L with OBD-II has p0420 code. Have replaced all 3 Oxygen sensors, MAF, MAP, Sparkplugs, Cap/Rotor, fuel filter, thermostat, and both temp sensors. Cat Convertor replaced before 5000 mile roadtrip and SES light came on with p0420 code After trip. Parts listed above replaced Since light came on.

P0420 OBD-II Trouble Code: Catalyst System Efficiency Below Threshold (Bank 1) P0420 code means that either the upstream or downstream oxygen sensor in the catalytic converter is not working. In short your catalyst is worn out.

following answer courtesy of OBD-CODES.com OBD Codes Your OBD-II Trouble Codes Repair Site Home Trouble Codes FAQs Forums Store P0171 OBD-II Trouble Code Technical Description System Too Lean (Bank 1) What does that mean? Basically this means that an oxygen sensor in bank 1 has detected a lean condition (too much oxygen in the exhaust). On V6/V8/V10 engines, Bank 1 is the side of the engine that has cylinder #1. The P0171 is one of the more common trouble codes. This code is triggered by the first downstream (front) O2 sensor. The sensor provides a reading of the air:fuel ratio leaving the engine's cylinders, and the vehicles powertrain/engine control module (PCM/ECM) uses that reading and adjusts to keep the engine running at that optimum ratio of 14.7:1. If something is not right and the PCM cannot maintain the 14.7:1 ratio, but rather there is too much air, it triggers this code. You'll want to also read our article on short and long term fuel trims to help understand the operation of the engine. Note: This DTC is very similar to P0174, and in fact your vehicle may show both codes at the same time. This diagnostic trouble code (DTC) is a generic OBD-II powertrain code. It is considered generic because it applies to all makes and models of vehicles (1996-newer), although specific repair steps may vary depending on the model. Symptoms You will more than likely not notice any drivability problems, although there may be symptoms such as: a lack of power detonation (spark knock) rough idle hesitation/surge on acceleration. Causes A code P0171 may mean that one or more of the following has happened: The MAF (Mass Air Flow) Sensor is dirty or faulty Note: The use of "oiled" air filters may cause the MAF to become dirty if the filter is over-oiled. There is also an issue with some vehicles where the MAF sensors leak the silicone potting material used to protect the circuitry. There could be a vacuum leak downstream of the MAF sensor Possible cracked vacuum or PCV line/connection Faulty or stuck open PCV valve Failed or faulty oxygen sensor (bank 1, sensor 1) Sticking/plugged or failed fuel injector Low fuel pressure (possible plugged/dirty fuel filter!) Exhaust leak between engine and first oxygen sensor Possible Solutions A lot of times, cleaning the MAF sensor and finding/fixing vacuum leaks fix the problem. If you're on a tight budget, start there, but that may not be the fix for certain. So, possible solutions include: Clean the MAF sensor. Consult your service manual for it's location if you need help. I find it's best to take it off and spray it with electronics cleaner or brake cleaner. Make sure you are careful not to damage the MAF sensor, and make sure it's dry before reinstalling Inspect all vacuum and PCV hoses, replace/repair as required Inspect all hoses and connections in the air intake system Inspect and/or test the intake manifold gaskets for leakage Check for a dirty fuel filter and proper fuel pressure Ideally you'll want to monitor short and long term fuel trims using an advanced scan tool If you have access, you may want to run a smoke test

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EOBD II Error Code: 25
Description:
System too Lean, Bank 1
Possible Causes:
- Intake/exhaust leak.
- Secondary air injection (AIR) system.
- Manifold absolute pressure (MAP) sensor.
- Mass air flow (MAF) sensor.
- Fuel pressure/pump.
- Injector(s).
- Evaporation emission (EVAP) canister purge valve.
- Heated oxygen sensor (HO2S).
- Contaminated fuel.

EOBD II Error Code: 26
Fault Location:
System too Rich, Bank 1
Description:
- Evaporation emission (EVAP) canister purge valve .
- Fuel pressure.
- Injector(s).
- Heated oxygen sensor (HO2S).
- Contaminated fuel.
Hope that works.

you've mentioned you've replaced the fuel pump, but the question is is the fuel pump getting any current from the battery or fuse box?

replace fuel filter, air filter, spark plugs, wires, distributor cap /rotor, if you have one. new oil with your filter ? just thought I'd ask, have a auto parts store pull code to see why your service engine soon light is on. this will tell you what sensor needs replaced, I'll bet its the oxygen sensor !

CODE PO102 IS MAF SENSOR CIRCUIT LOW FREQUENCY CONDITIONS / CODE PO420 IS CATALYST SYSTEM LOW EFFICIENCY BANK 1 CONDITIONS. BAD MASS AIR FLOW SENSOR WILL AFFECT CONTAMINATE DAMAGE CATALYTIC CONVERTER DUE TO RICH EXHAUST EMISSIONS.

My 96 started out that way, then got worse and worse. Got all kinds of trouble codes,like multiple misfires, maf sensor bad, map sensor, O2senssor rich. Turned out to be the distributor cap and rotor. Rotor had carbon track on the backside. Other than that, they looked perfect. New ones solved the problem and all the trouble codes went away. All that misfiring was causing all the sensors to throw false codes. I figured out I needed to look for something not so obvious when the $100 maf didn;t change a thing.

chek filters for fuel on frame and in tank

are you running regular gas or not you should be running special unleaded fuel

misfiring can cause a P0420 code.
I would recommend a set of honda plug wires and plugs if the P0420 code returns.
I have used bolt in aftermarket bolt in catalytic converters but don't remember the brand name they fit ok and i have had 1 or 2 return with a P0420 code out of about 20. I have never replaced the oxygen sensor on a civic like yours.
I always fix the misfire problems, reset the computer, and have the customer drive it and see if the code p0420 returns. 50% don't have the P420 code return. drive it and see what happens and go from there.