2002 Z71, 5.3 had code P0135, now P0420

Just bought this truck with 140K on it. Had code P0135 that said bad Bank 1, heated O2 sensor 1. The fwd O2 sensor was original so I replaced it and cleared the code. The light came back the next day but now it's P0240, Cat Efficiency low bank 1. Possible causes:Cat Defective, Engine misfire, large vacuum leak, or engine oil leak into exhaust valve guide. Is the code change a clue??? Can I swap sensors from the 2 bank to see if the code moves over? I've seen the paragraph from MT but was hoping the change from P0135 to P0240 might narrow the search.

Nearly every time I have run into the Cat efficency code, it has required a new cat.

is it a plugged up condition that I can drill a hole in it to verify?

The converter has lost the ability to store enough oxygen to do its job, the pcm looks at the pre and post cat o2 sensors to determain this. I suppose drilling a hole in the convereter would allow more oxygen in, but not sure it would be an effective test.

Sooo... what if I eliminate both CATS? I know it happens and there is some way of keeping the SES light out. On my S10 I put spark plug extensions under one of the O2 sensors and the light has stayed off. You're not obligated to answer if it might incriminate your status here. I know MT's answer. I live in a state with no annual inspection. If I sell it in this state, there's still no inspection. Just sayin...

Drive it for a while and let the PCM learn the new 02 sensor. Then clear the code and see what happens. Yes you can swap sensors, but they have to stay either pre cat or post cat.

Thanks for the input guys. It's getting 16 MPG in town so I can live with it for a couple weeks. I will post back with results. Since we just got the truck, I'll be changing all fluids and filters. Any recommendations for diff and transfer case oils.

Thanks for the input guys. It's getting 16 MPG in town so I can live with it for a couple weeks. I will post back with results. Since we just got the truck, I'll be changing all fluids and filters. Any recommendations for diff and transfer case oils.


being this old the O2 sensors are recommended to be replaced when over 100,ooomi.

your vehicle all the sensors are the same . all 4 of them . just look and see.

swaping the sensor and having the code change this would mean the sensor is n/g.

I would replace the up stream sensors both . these control fuel mixtures.


the down stream sensors just report cat issues.

I would also check fuel pressure/test proceedure ,and check the fuel regulator for diaphram damage.

how to check cat operation???

start the cold engine , crawl under the vehicle. place one hand on each converter. they should both get hot at the same time /amount.

if not then the colder cat is n/g. cold cat is a dead cat.

being coated with carbon does it. bad injector /valves/spark plug/etc.

if the cat is bad try to find out whats broken.

Thanks jCat. I need to get a book for this vehicle to figure out where the regulator is and how to do the fuel pressure checks. I'll be checking plugs and injectors first by pulling connectors off one at a time. It seems to be running very smooth and getting 16 MPH ain't bad but easy checks to make so I'll do these first.

Thanks jCat. I need to get a book for this vehicle to figure out where the regulator is and how to do the fuel pressure checks. I'll be checking plugs and injectors first by pulling connectors off one at a time. It seems to be running very smooth and getting 16 MPH ain't bad but easy checks to make so I'll do these first.


since it is running good , first do the fuel pressure test . then replace the up stream O2 sensors.

see what happens. see how the code changes. start swaping the down stream sensors.

do not throw away the old O2 sensors. you can use these to do more testing. check if they are good. if any are bad after testing then dump them.

the fuel test port is on the passengerside fuel rail. the fuel regulator is on the driverside. this has a vacuum line to it very small dia..

with no vacuum to the regulator the pressure will rise to 62psi..

53-62psi is the fuel pressure spec. key on engine off 60-62psi, should hold not drop off. if it drops off the injectors or pump or leak is in the fuel system. make sure the regulator vac hose has no fuel smell or fuel . if so replace it.

engine running must be over 53psi . should be 55-58psi if good. if lower then pump /filter may be the reason weak/dirty etc..

Thanks j cAT. I've replaced the upstream sensor already on the side that codes' bad. That's when the code changed to the downstream sensor, same side. I'll swap the down stream sensors to see what happens next. Then go for fuel pressure checks. I'll check the regulator next chance I get. It's my sons truck and he's at college but close by.

Thanks j cAT. I've replaced the upstream sensor already on the side that codes' bad. That's when the code changed to the downstream sensor, same side. I'll swap the down stream sensors to see what happens next. Then go for fuel pressure checks. I'll check the regulator next chance I get. It's my sons truck and he's at college but close by.


the 420 code is the driver side downstream O2 sensor. the PCM/computer compares the upstream /downstream O2 sensors as they swing in voltage. if they do not agree to a point that the PCM feels is proper as it was programed then the 420 code is generated.

could be the downstream O2 is off or the new one upstream O2 sensor is off.

make sure the wires and connectors are proper . no splicing is allowed.

they must be the correct sensor for this vehicle. the driver side sensors do get into trouble with the exhaust because of the tight location the wires run. make sure the wires are secured as built.