My check engine light recently came on in my 2007 Honda Accord Coupe. Here are the error codes the guys at AutoZone were able to get for me from the car's computer:

• P0420 - Catalyst Efficiency Below Threshold (Bank 1)
• P0128 - THERMOSTAT RANGE/PERFORMANCE
• P0420 - Catalyst Efficiency Below Threshold (Bank 1)
• P0128 - THERMOSTAT RANGE/PERFORMANCE

Both codes were thrown twice. My car has around 127,000 miles on it. I know that this may be indicating that I'll have to replace my catalytic converter but I know that can be pretty expensive. Some other resources I found online recommend trying to replace the O2 sensor because it's potential the "P0420" is being thrown because the sensor is faulty. Do you believe that is worth the trouble? Most of these online resources have said doing this usually does not fix the problem.

I am also curious about the "P0128" because I've been keeping my eye on the engine thermostat on the dashboard and the needle is consistently in the same spot once the engine has warmed up. Is it possible that these two error codes are in relation to each other or is it more likely that they are two different issues? Has anyone ever gotten both of these codes at the same time before?

Also, I am 25 and this is my first car I've ever owned so I apologize if I am coming across like a noob in my post.

 

Is your accord a 4-cylinder or V6 engine?

Have you had any work done on the car recently, or have you had any kind of major or minor accident not long before the check engine light turned on?

P0128 in the Honda Shop manual means cooling system malfunction. I'd recommend addressing P0128 first, but the troubleshooting list is fairly long. Do the radiator fans constantly run when the engine is running (make sure the a/c is turned off when doing this test).

 

I apologize for my late response. It has been a crazy summer so far.

The car has a 4-cylinder engine.

The only work I have done on it worth noting is I replaced the PCV valve shortly after I first bought the car which was back in November of 2017. A minor accident worth noting is I ran over something on the interstate at 70 MPH and I ended up denting the left side of my bumper.

Yes, my radiator fans constantly run when the engine is turned on.

 

Do they come on almost immediately? Or after you have some temp in the engine? Just asking, as it'll help narrow down the problem (might be a bad fan switch).
On my own Accord, I replaced the cat converter instead of the O2 sensor. I haven't had a PO420 since.

 

When I turn my engine on, they don't come on immediately. It takes around 1 second for the fans to start spinning after the engine gets going.

That is the fear with what I will have to do with my car. From my research, I can see that a catalytic convertor replacement can cost up to a grand or two for parts and labor. And with my car already getting up in miles, I'm uncertain that the replacement would be worth the investment.

 

One second from a cold-start isn't enough time for the engine to have any real temperature increase. But the fans might be running from the AC being on? Your AC might be switching on if you select the defroster as the air-flow direction.

Cars are getting more sneaky about the AC being on when you don't specifically switch it on. From auto-climate-control to the defroster setting, there's a few different ways for the AC to run. That in turn switches on the fans. So the fans running may or may not be related to the temperature or the P0128 code. One way to be sure is watch if the compressor clutch starts spinning when the fans come on.

 

I went with a stainless steel Magnaflow cat on my own car. It currently has 236K on it. I also went with a Magnaflow cat back system in stainless, as I didn't really want to have to get back under it again. Although this summer I did, because I had to replace 2 rusty brake lines.