I have an 03 Civic EX with 153K miles on it. I have taken very good care of this car since I got it brand new. Always have used full synthetic oil and never have had a problem with it. If you lift the hood the engine is very clean. Just making the point that I do take care of the car. Long story short, my battery light came on, had a mechanic change the alternator. Light went off. Let my sone use the car and as he was driving the battery light came back on. He didn't let me know. He drove the car until the battery was completely dead and the car shut down and I had to get it towed back to my house. Was able to charge the battery and take it back to the mechanic. Turns out the "new" altenator was bad and had a new one installed again. I was not charged. The problem is now, and less than 5 miles down the road from picking it up, the Malfunction Indicator Lamp comes on. Went back to have the codes read and the lamp reset. I figured since my son had driven it until the car just died completely maybe a bogus code had just popped. But a few miles down the road it came on again. I checked the gas cap, changed my forward O2 sensor a few months ago and just went and purchased, and installed, the downstream sensor. Had the lamp reset and 7 to 10 miles later it came back on. (by the way, I don't remember the exact codes the car set but it led me to believe, by the codes meaning, that the problem was the downstream sensor, the mechanic concured, and this was before I changed the downstream sensor) By the way, the car is running great like it always has. Any idea what the problem might be? Is there anything that can be bumped or broken during an altenator change that could make this Malfunction Lamp come on? I also think its may be more than just a coincidence that this lamp just happened to start coming on AFTER I took it to the mechanic. That being said, is there anything that can physically be done to make this light come on? Guess I'm not too trusting but its just too much of a coinsidence in my mind. Over the last few days of trying to figure this out, once the Lamp came back on it stayed on until I went and got the codes cleared. On a reach I pulled the ECU fuse to check it and it was fine. Then I turned the car on and the Lamp did not come on (have the car now running to see how long it takes for the lamp to come back on). Does this fuse have anything to do with the MIL? I appreciate any and all input. Love my Civic and want it back to itself!! Thank you.

Gene Wiedenbeck
U.S. Navy Chief (ret)

 

I thought it might be me but my wife noticed the same thing..... my exhaust smells quit a bit more than it ever has. Matter of fact I do not remember ever smelling the exhaust from my car. Thanks for your help!

 

Pulling the fuse resets the ECU and any codes will be cleared.

You need to know the exact code. The downstream sensor exists only to check the performance of the cat converter, so sometimes a bad converter will cause codes related to the downstream sensor.

 

P0420 is the Code. Had the GM at Advanced Auto read it. He said the same code was "pending" and "confirmed". He said at 153k miles, he would put money on it that it is the cat converter and the fact that it popped right after I got the altinator changed was just a coincidence. Any thoughts on that before I drop 300 on a new converter?

 

P0420 is the Code. Had the GM at Advanced Auto read it. He said the same code was "pending" and "confirmed". He said at 153k miles, he would put money on it that it is the cat converter and the fact that it popped right after I got the altinator changed was just a coincidence. Any thoughts on that before I drop 300 on a new converter?

P0420 "Catilist efficency below threshold"

 

Okay, so it would seem that my son bottomed out my Civic and messed something up with the cat/exhaust system. And the fact that I now smell exhaust like I never had before tells me that an exhaust leak may be my P0420 problem. First off, how can I pinpoint the leak in my stock 03 EX exhaust system? And how do I determine if the problem is my cat? There is a fresh scrape on the protective cage of the cat which tells me he bottomed out... thank you for your help!!!

Gene

 

Idle the engine and block the tailpipe with a rag. Leaks in the exhaust system will make a hissing noise of exhaust leaking out. Air leaking in will also lead to the P0420 code so don't replace the cat just yet.