I have problem with my Mazda 626 19998 LX(Mileage: 115,000). The "Check Engine light" came ON (OBDII P0421: Warm Up Catalyst Efficiency Below Threshold...), two months back. I reset it thinking it may be a false alarm. Meanwhile, I was able to pass Emission/Safety inspection without any problems. THE CEL came back again after 700 miles.

I don't want to spend any more money on the car. My question is if I just ignore it, will it cause any other major problems to the car.

 

Why did the CEL come on again after 700 miles? Same P0421 or different code? Since you cleared the code and then were able to pass an I/M inspection later then the PCM must have run a self test of the CAT and passed. Otherwise you would not have cleared I/M inspection. Would have failed with a P1000 or Self Test Not Complete. If the same code is still a 421 for cat efficiency then no it will not cause any problems with your car outside of failing even worse and not being able to pass the inspection later on. Have the code rechecked at parts store that does a free OBDII code check and if code is same then not a worry. If different then may be a problem that needs to be addressed.

 

Thank you babyhuey. I appreciate your fast response.

I checked the second time as well, the OBD code is still the same, P0421. I am kinda relieved to hear that this code will not cause any major problems with the functioning of the car, and can be safely ignored..

Thanks once again.

 

Just make sure that when the light on, it is always pointing to a failing cat converter and not masking some other code or problem. Would hate to see you driving around for six months with a fuel trim maxed lean condition and only getting 12mpg.

 

Thank you Babyhuey. I appreciate your help..

I have couple of questions:
1) I am currently getting 19 mpg for city driving and 23 mpg for highway, is it normal for 1998 Mazda 626 LX V4(milege 115,500)?

2) I checked the code today also, it is P0421. Is there any way to tell that there could be some other problem hiding behind this problem..

3) I am not planning to fix the problem, it as I don't feel any performance degradation. How often do I need to check the code, to make sure it is a CAT issue?

 

Mileage does sound a little low for the 4 cyl but given that kind of miles on the car, am not overly suprised. A system running rich due to injectors, high fuel pressure, vacuum leaks, or slow responding oxygen sensor could all be contributors to this. These same things could also lead to premature failure of the converter. The biggest killer of cat converters is engine missfire however. Pumping a cylinder full of unburned fuel and air into the converter superheats the cat and can destroy them quite quickly. 115k is not too bad a life expectancy for a warm-up cat converter though so you may not have any underlying problems. The only way to really determine if your fuel control is operating normally is to retrieve freeze data that occured when the code was set. A simple scan too is not capable of getting this information but a more sophisticated one at a repair shop should be able to. A good shop should also be able to monitor fuel trim levels while driving to determine if there are any problems. Long and Short fuel trim should be around +/- 5%. If trim levels get too far out of range for the PCM to make any adaptive changes to correct then a code will be set and stored for that as well.
I would try to check the code at least every month or two.

 

Thank you Babyhuey. I really appreciate your detailed response, it would be very useful for me.