'03 Mits

2003 Mits. Lancer, ES. Service engine light been on 6 months. Had it scoped twice, both reported “warm up catalyst efficiency below threshold bank 1.”
One shop wants 800 bucks for repair. Still getting 30-plus mpg. Are there any other options? janeyschaz

Clear your code…by disconnecting the pos terminal of the batt…give it a few minutes and hook it back up and monitor. If it comes back you may have an issue with an o2 or a mixture sensor.

In the cold most people just take off and force every part of the engine to warm up as well… I see most if not ALL of the O2 sensor issues in cold weather…either the heater circuit this or heater circuit that…etc… I ALWAYS clear the codes to see if they return hard…if they continue to return on the same code I act on it…

Your code points to the efficiency of the cat…the cat is expensive and rarely go bad…this issue is most likely a discrepancy between the pre and post cat O2… This can occur for several reasons… Clear the codes and monitor…see what you get before you act.


Have the codes read again. This time write down the exact code - e.g. “P1234” and post that. Have the codes cleared after reading them to see if the light comes back. For the moment I’ll assume the code is P0421 & the shop is quoting you a price for a new catalytic converter. If that’s the case then one thing you want to do is call a few other shops to get estimates - just tell them the diagnosis & that you’re just looking for a ballpark idea.

But the other thing you want to do is make sure that you’re getting it looked at by a reputable & trustworthy shop known for its expertise in diagnosis as there are a couple of things to be careful about. One is that, as HB noted, the code is actually set because of reports from O2 sensors. A lot of shops will see such a code & just assume a bad converter, but the O2 sensors should be checked first. A second thing is that if it ends up being a bad catalytic converter then someone should find out what killed it. There’s probably a reason & if the underlying cause isn’t addressed you’ll just kill the new one too.

Do you do a lot of short trip driving?

Very true… Cigroller is right… get a good diagnostic shop to look at it…and if you don’t do this yourself…go into the shop pre-armed with the knowledge that cats don’t go bad that often…the code you have can and usually is a discrepancy between two O2 sensors…SAY THAT TO THEM…that way the shop will be less likely to throw you a red herring…

RE Mits. Lancer, thanks ya’ll, very much. Each time I had the code read, they turned the light off. It came back each time within days. Will go forward with 02 sensors, thanks again JS