Check engine light problems in 2003 Honda Civic

Hi all, I bought a 2003 Honda Civic EX Coupe with almost 132,000 miles a little over a month ago and continue to experience headaches with it, even though I had a mechanic check it out before buying. I think I’ve partially resolved a squealing noise that only comes on when the fan is running. However, there was also an intermittent hesitation I would experience when taking off - no one could diagnose it because it was so intermittent. Last week, however, it became really bad - the car began hesitating and surging really badly and the check engine light came on. Since it was Saturday, my usual mechanic wasn’t in and the only place open in my small town was a Meineke car care center so I took it there. They diagnosed it as a PO135 oxygen sensor bank 1 problem. It was Monday before they could fix it. I picked it up late Monday morning and all was well - the light was off and the car ran perfectly for the next two days. On Wednesday morning, however, the check engine light didn’t go off at all after starting but the car still ran fine. On Thursday, I took it back to Meineke because they were the ones who worked on it. The called me back and had gotten some code about it needing a new catalytic converter and wanted to install one for about $600 and maybe another sensor making it a little over $700. I haven’t let them do that just yet - wanted another opinion. Went back to my usual mechanic who doesn’t do catalytic convertors but put it on his scanner. He got a code saying “catalyst was at low threshold” but explained that may or may not mean that it needs a new convertor. The muffler shop next door to him also started up the car and revved it and said it runs too well to need a convertor. The mechanic turned off the light and said to drive it for now. Last night, after about 15 to 20 miles of driving in town, the dang light came on again! The car still runs well though. Does anyone have any ideas about this? The nearest Honda dealer is 45 miles away and I really don’t want to pay their prices if I don’t have to.

I got exactly same problem. Mileage is about 22-24. Used to get 30-32.

Longprime - I don’t have any gas mileage issues at all - at least not yet. When I was this morning running errands, I still have the light on but the car runs well still. I dropped by both Auto Zone and Advance Auto and each store tried 3 times with their scanners and couldn’t get anything to show up!


I have a few suggestions

Replace the rear oxygen sensor (you already replaced the front)

Cat codes can also occur if the front sensor is switching too slowly and/or the rear is switching too fast.

To really simplify it, the cat code can set if the rear sensor is switching almost as fast as the front.
That’s not very technical, but it’s worded so that all can understand it

Another thing to consider . . . have somebody perform a backpressure test. It will be quite obvious if the cat is plugged

Another thing to consider . . . have the Honda dealer reflash the PCM with the latest software. Sometimes the original programming is a tad too sensitive. This should cost you about 1 hour of labor, and, if nothing else, you’ll know the software is no longer an issue.

Here’s some questions for you

Was there ever a misfire?
Misfires can quickly damage a cat

Any head gasket leaks or overheating?

In the short time I’ve owned the car, there has been no head gasket leaks or overheating, no misfires, just the hesitating and surging I mentioned and the worst of that was last weekend. I’ll ask my regular mechanic about the backpressure test or the other oxygen sensor. Honda dealer is a last resort.

Update everyone - I haven’t had time since Saturday to get my car checked again - the light remained on until this morning as I was going to work and it went off! I’m happy about that, but never know when it’s going to come on again. The car is running fine. Could this be related somehow to the reset process or the fact that I got gas yesterday for the first time since the engine light came back on?

You need to get the exact code and post it here, but code P0420 is Catalyst System Efficiency below threshold, so I am guessing that that is what you are getting. It means that the rear O2 sensor and the front O2 sensor read the same thing all the time.

The voltage from the front sensor should be switching from low to high about once a second. The rear sensor should be switching about once every 8 seconds. If they both switch at the same rate, then the computer thinks the cat is not doing its job. But the problem can be due to the rear sensor almost going bad or the front sensor switching slow. It can also be caused by a leak in the exhaust system before the rear sensor.

If you can find a shop with the proper equipment to monitor the outputs of both sensors, they can figure out the exact problem and that can be a lot cheaper than just replacing parts until the problem goes away.

If there are no codes that indicate there’s a problem with the O2 sensors, then there’s not a problem with the O2 sensors. OBII is pretty smart that way.

A code that indicates that the catalyst efficiency is below threshold means just that. The O2 sensor after the catalytic converter is detecting too much oxygen in the exhaust gasses that should have been consumed during the catalyst process. This means the catalyst isn’t reacting with the oxygen.

This can be caused from a contaminated catalytic converter. And this can happen on a vehicle with high mileage. Before they suggest replacing the catalytic converter, give this a try.


Tester, there have been numerous reports of the rear O2 sensor clearing up a P0420 code on Toyota’s, it did so on my daughter’s 03 Corolla. Replacing the cat is down at the bottom of the list for suggested repairs.

Thanks everyone - the light came back on last night after a couple of days being off but the car still runs fine. I’ve been extremely busy and really won’t have time to get it checked out until this coming Tuesday. Hopefully someone can get to the root of the problem then. I’ve never had a car like this where the light goes on and off and also so soon after another related repair. Any other time I’ve had an issue withe CEL, the light stays on until the problem is resolved - which has always taken one visit to the mechanic - and then that is the end of it.

I have the same exact problem with my 1998 Civic. In order to qualify as a CARB certified low emissions vehicle (or an ultra LEV), the computer expects a lot from your catalytic converter. Yours simply isn’t working well enough to make the computer happy.

If you live in a state that doesn’t do inspections or emissions testing, you might (like I have) decide to live with the “check engine” light (CEL) being on. If anything seriously goes wrong, the CEL will flash, at which point you should pull over and call for a tow truck.

If you live in a state where they test your emissions and inspect your vehicle, you’ll need to bite the bullet and get this fixed, which will probably require either installing a new catalytic converter or figuring out why it isn’t doing the job as well as the computer thinks it should. There might be a problem with the air/fuel mixture that is interfering with the catalytic converter’s ability to catalyze the exhaust gases. It could be something as simple as the mass airflow sensor (MAF sensor). You need a real automotive diagnostician (preferably one who wears a white coat) to figure this one out.

I think you now know why the previous owner decided to get rid of this car.

@wncgirl … what you have is a so-called “drivability problem”. In newer cars like yours, there are a wide assortment of sensors monitored by computer which all together are supposed to run in harmony and keep the engine purring. But when one or more fail, drivability problems are what you end up with. The problem is that these sensors and computers run in what is called a “closed loop”. Think of it like a big circle drawn on a piece of paper, with arrows showing the direction the circle is rotating. When something breaks, all you know is the circle is broken and not rotating properly, but you don’t know which segment of the circle the problem is occurring.

It is possible to find which segment is broken, but with this kind of problem you need someone with the correct diagnostic tools and experience. This isn’t the kind of problem like the car doesn’t crank. That kind of problem doesn’t need speciali zed diagnostic tools or a lot of make/model/year specific experience.

So my advice – if the above good recommendations don’t solve the problem – is to bite the bullet and to find a mechanic who specializes in Hondas. Doesn’t have to be a dealer. Better if it isn’t a dealer usually. But they does have to have the Honda specific scan tool. Ask if they are able to measure real time engine operating parameters and fuel trim on your engine. They may be a bit more expensive, but in the long run you’ll be money ahead by relying on the right shop for the job.

To add a little to what George said, this is called diagnostics and because this can take an hour or more of the mechanics time so you can expect to pay $100 to 120 for this service. But when you consider the cost of a replacement cat, it would be nice to know that it is needed, and they might find out that it is something cheaper so it will save you money in the long run.

Hi all, it’s been a few days and I am just now getting back on here and just now have had time to get the car looked at a little more. For the record, the code this time is the P0420 Catalyst Below Threshold. I may end up having to pay $97 to go to my Honda dealer about an hour away for proper diagnostics. I wish there was someone in my region other than a dealer who works exclusively on Hondas but there just isn’t. I did go to one other independent mechanic in my town when I had a minute at lunch one day but it was just the code reader thing and he referred me to a muffler shop in a neighboring town that said they would replace the CAT for $125! This can’t be real! I really think these people are pushing something from the junk yard on people!


Check this out. Perhaps you just need the software update

BTW . . . A factory scan tool (or an enhanced aftermarket tool) would retrieve P1420, because that is a factory code

A cheapo code reader would retrieve the P0420 code, which is the generic code

P0420 and P0420 are the same thing . . . cat efficiency below threshold


I’m sorry

That bulletin does not apply to your car, because you don’t seem to have a hybrid

Thanks db4690 - I looked at the link - it looks like the 2003 Civics mentioned on there are hybrids - mine is not a hybrid. Can there be a relation between these problems in the hybrids and the gasoline model? Would be nice if a software update is all that is needed. I called the cheapo muffler shop back to inquire about warranty and whether these convertors are new. The owner is out and the person who answered doesn’t know about warranty, but he said they order new convertors from Eastern Catalytics, and the owner is someone who isn’t out to make a lot of money but just wants to help people. He said if they put a new one on, they would give me my old one to prove that it is indeed bad. Just don’t want to get pressured into buying their name brand products or stuff that I don’t really need. I looked them up and they seem to be a viable company with different models - the one in the $250 is designed to keep the CEL off, but they have a $79 one that doesn’t mention keeping the light off. I wonder if that is what this shop is putting on cars. I still haven’t decided but am thinking that breaking down and driving to the dealer and paying $97 for diagnostics may be my best option.

Still not sure what option to pursue.

I have a similar problem with a 2003 Civic with 142,000 miles, except the check engine light does not go one. It happens 2-3 times a week and during the first 10 minutes of driving. There is a total loss of power and feel like I am in a go-cart. After that it runs fine. 2 garages have told me it may be one or both of the oxygen sensors but were certain the check engine light would have gone on. It never has so they do not want to replace them and have the problem still be there. I haven’t taken it to a dealer because I expect if the light is not on they will want to see the problem repeat itself and it’s so intermittent it could be there a while. Does it seem correct to focus on the oxygen sensors as the problem?

Stulest, I am still learning my car’s ways because I just got it about 6 weeks ago. The first time I started noticing symptoms - the engine light didn’t come on - it was just intermittent hesitation and no one could diagnose it because it was so intermittent. It had to start hesitating and surging really badly 3 weeks ago before the light came on and I got the sensor replaced. Now the light is on (came back on 2 days after repair), but unlike you, I am not noticing a loss of power. I am just getting this “catalyst below threshold” code and everyone seems to automatically think the catalytic convertor needs replacing. But from what I read on here and in other sources, that may or may not be the case. You may be right about it just be the other sensor and in my case, the light may just be alerting me that it’s gradually going bad - I don’t know.

Understand your OBD II system. The malfunction indicator lamp (MIL) is labelled as “check engine” on your instrument panel. The codes stored in the computer are called DTC for diagnostic trouble code.

Most DTC’s are two trip codes. That is they have to be detected twice within two drive cycles. A drive cycle is a cold start, driven till fully warmed up and then off till cold again. If you are having an engine problem, you could have a pending DTC show up with a diagnostic scan even thought your MIL is not on. Most autoparts stores will scan your computer for free.

If a DTC is not detected a second time by the end of the second drive cycle, it is deleted from the computer. After the second detection, the MIL is turned on. If the DTC is not detected in the next three drive cycles, the MIL will go off but the DTC will remain stored in the computer for the next ten drive cycles, then it will be deleted if still not detected.

There are some one trip codes that will turn on the MIL as soon as they are detected. These are a little more serious, but you can’t tell a one trip code form a two trip code just by the MIL light. a really serious code will cause the MIL to flash though.

wncgirl, you do not have to go to a Honda dealer for the diagnostics for a P0420 code. Most shops have the capability of tracking both oxygen sensors in real time to see if one of the sensors has gotten lazy. An independent shop may charge less for this diagnostics than the dealer so it is a good idea to do some comparison shopping.

Car repair is a business transaction. If you treat it like one, getting second opinions, getting multiple estimates and verifying the business (BBB, recommendations from friends and coworkers etc) you are more likely to have a good outcome at a fair price. Be leery of any estimates that are out of line, either too high or too low.