P0420 – Bad CAT and other issues?

Warning, long explanation ahead, get ready to read a little. Sorry but wanted to give as much info as possible here. Thank you for your patience.

A little bit of background/context before I get to my overall questions…Have a 2003 Toyota Celica with the 1.8L VVTi and 225,000 miles on it. The engine light went on and the scanner provided a P0420 (Catalyst Efficiency Below Threshold). I instantly checked the CAT using the upstream and downstream O2’s. The O2 wave forms were identical at idle and at 2,000 RPM’s for both sensors (both fluctuating from approx. .2-.9V) so I instantly thought the CAT was bad. I then checked the temperatures before and after the CAT which showed approx. 250 degrees pre CAT and approx. 230 degrees post CAT. Seems too cold so I’m a believer that the CAT is bad.

Now this particular vehicle has an aftermarket universal CAT welded into the original exhaust. This is the second one, the first one lasted 2 years and now this one has been installed for two years and seems bad as well. Right as I was beginning to look at the car and do the testing it had trouble starting and staying running unless I gassed it up to 1500-2000 RPM’s. After getting it to start I revved the engine high and drove it pretty hard to see how it would perform. It did fine and when I parked it eventually settled down to an idle of 600-800 RPM’s and stopped stalling. This got me thinking…now onto my questions.

Based on the poor drivability issue it briefly had, is the CAT dying every two years because it’s a cheap universal one or is there another issue here. Example: due to the age of the engine could it be burning oil or not burning fuel right and that is contaminating the CAT? I wanted to see if anyone had any suggestions on additional checks I should do before putting a new CAT on (which I would put a full new pipe with integrated CAT designed for the car not a universal one) in order to verify any other issues so the new CAT doesn’t have similar long term issues like its predecessors. Since doing all this the car has been fine. I also cleared the code a week ago (and it has been driven every day since) to see if it would come back and it has not. Thoughts on what’s going on?

69Camaro wrote: Warning, long explanation ahead, get ready to read a little. Sorry but wanted to give as much info as possible here. Thank you for your patience.

A little bit of background/context before I get to my overall questions…Have a 2003 Toyota Celica with the 1.8L VVTi and 225,000 miles on it. The engine light went on and the scanner provided a P0420 (Catalyst Efficiency Below Threshold). I instantly checked the CAT using the upstream and downstream O2’s. The O2 wave forms were identical at idle and at 2,000 RPM’s for both sensors (both fluctuating from approx. .2-.9V) so I instantly thought the CAT was bad. I then checked the temperatures before and after the CAT which showed approx. 250 degrees pre CAT and approx. 230 degrees post CAT. Seems too cold so I’m a believer that the CAT is bad.

Now this particular vehicle has an aftermarket universal CAT welded into the original exhaust. This is the second one, the first one lasted 2 years and now this one has been installed for two years and seems bad as well. Right as I was beginning to look at the car and do the testing it had trouble starting and staying running unless I gassed it up to 1500-2000 RPM’s. After getting it to start I revved the engine high and drove it pretty hard to see how it would perform. It did fine and when I parked it eventually settled down to an idle of 600-800 RPM’s and stopped stalling. This got me thinking…now onto my questions.

Based on the poor drivability issue it briefly had, is the CAT dying every two years because it’s a cheap universal one or is there another issue here. Example: due to the age of the engine could it be burning oil or not burning fuel right and that is contaminating the CAT? I wanted to see if anyone had any suggestions on additional checks I should do before putting a new CAT on (which I would put a full new pipe with integrated CAT designed for the car not a universal one) in order to verify any other issues so the new CAT doesn’t have similar long term issues like its predecessors. Since doing all this the car has been fine. I also cleared the code a week ago (and it has been driven every day since) to see if it would come back and it has not. Thoughts on what’s going on?

I was actually thinking about an oil consumption problem as I was reading about your P0420, then you mentioned it further down. :lol:

This would not be the first Toyota with an oil burning problem, though not as bad as the newer 2.4L's. If you're a Premium member, there's a fantastic case study on the same 1.8L engine with a severe oil consumption issue:



Pulling the plugs would be the easiest way to see if there's a problem. It'd also be worth looking at short and long term fuel trims under varying loads to verify that the ECM is in good fuel control.

The code setting once and not returning can be characteristic of a catalyst that on the borderline of failure. It may barely fail one trip, but barely pass the next, therefore you don't get a code.

If you have a capable scanner, you can use Mode $06 data to check for this condition. As an example, I had an '07 Jeep Patriot that would intermittently set a P0420. This is the Mode $06 result I found for the catalyst monitor:



Then, after I cleared the codes and reran the catalyst monitor, this is what I got:



Failed one trip, passed the next. :lol:

Thank you Tyler. Yeah, I’ve seen my fair share of the 2.4L Oil Consumption issues. I’ll check out the case study when I become a premium member (plan to sign up when I have more time so I can actually focus). I pulled the plugs and only one had a small amount of oil on it and it looked like that was due to an improper seal in the tube, so when the valve cover gasket started to leak it made its way in the cylinder. Not sure that would cause all this, but who knows.

I have a Snap-On Modis Ultra. I’m unfamiliar with MODE $06, any resources to get an education on what it is and how to use it?

69Camaro wrote: I have a Snap-On Modis Ultra. I’m unfamiliar with MODE $06, any resources to get an education on what it is and how to use it?

Nice, I have the same Modis! Treating you well so far?

Getting educated on Mode $06 is challenging. Most of what I know I've gathered from scanning known good vehicles, plus whatever I can get from trade magazine articles and iATN. There are training courses out there, but I've not taken one yet. I've had my eye on this class from AVI On Demand - AVI material is usually pretty good, and Jim Wilson is a smart cookie.

About your Celica, specifically, I'd need to check Mitchell to know which Test ID the catalyst monitor is. Snap-On scanners will usually tell you which monitor is which on domestic makes, but rarely on asian.