Just got the dreaded service engine soon light, I took it to Auto Zone and had them pull the codes, it was P0420 coming up twice, “Catalyst System Low Efficiency (Bank 1).”

Is this just a bad oxygen sensor? If so, which one? Where can I find it? I am operating under the assumption that it is the downstream sensor, am I right? I found videos showing where to find the upstreams but nothing on the downstream. Or is it something else?

How serious is this? Do I risk damaging anything else running it whileI wait for the part (assuming it is the sensor)? Is it easy to fix or am I going to need a mechanic?

 

A bit of an update, I just did some soldering (removing a splice in the wiring harness that goes to the blower motor control unit), and disconnected the battery while I worked, but when I hooked it back up when I was done the code was gone. Did disconnecting the battery cause the code to clear? Will it come back if it’s still a problem?

 

I dont believe itll hurt anything, as if the code goes away, it tends not to be a serious one. But I would keep an eye out. I'm not too sure on the rest of the info..

 

Disconnecting the battery will erase the code, but it will probably come back soon when the cat monitor (OBD2 self-test) runs again. It's probably NOT the O2 sensor, but you can tell for sure by hooking up a scanner and watching the O2 sensor activity, particularly on the downstream sensor.

 

From my understanding not with OBD II - disconnecting battery will erase the radio presets but that's about it - any hard set code will be there until cleared with a scanner - OBD I does not have a solid memory and clears with battery disconnect

 

It erases them on mine, which is a 2000.

 

What do you think it is if not the sensor? The catalytic converter itself, I hope not. How much would having that replaced on a 1998 run me with labor? Is it something that I (someone with very limited mechanical ability or experience, and few tools, and no welder, or knowledge how to use one) could reasonably do myself with jack stands and basic hand tools?

Not sure if it’s relevant, but in the few days before this I replaced the PCV Valve, changed the air filter (the new one is a Fram Tough Guard), and replaced the gas cap (the tether was broken). Also shortly before the light came on I got fuel and since leaving the gas station I feel like my acceleration is WAY down, I was wondering if I was imagining it, then I saw the service engine soon light.

 

It could be the catastrophic converter, but it's strange that your troubles all started right after you did all that stuff. And it's weird that your power would be down, too. Does it smell funny or anything, like you might have gotten bad gas (driver dropped a load of diesel in the gas station's unleaded tank or something)?

If it does need a catatonic converter, they're not super expensive. I think I paid about $125 for mine at Summit and welded it myself, but any muffler shop should be able to do it for you pretty cheap.

 

No strange smells that I have noticed, just a loss of power. Though on my way home today from a pizza run I thought I may have briefly smelled gasoline, though my window was open and it started right after I passed another car so I can’t say for sure if it was me and my driveway was only a few hundred feet away, so I don’t know if it would have gone away or not, nothing before that though. The truck did seem like it may have taken an extra second or two to start when I left too, but it does that sometimes (it was the first I had started it today so it was a cold start) so I don’t know that it was related. The loss of power was still apparent though.

Another odd probably unrelated thing is that I had the computer thing on the roof set to show real time mileage (normally I have it set to range) and it was giving me weirdly high numbers, according to the computer I was getting about 25 MPG, city. Yet my average says 15.6.

My fuel is low, maybe an eighth of a tank left, if it was a bad fuel issue do you think refilling at a different gas station might help?

 

If it's bad fuel, that would definitely help. But bad gas is pretty rare, in my experience.