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99 Civic - P0420 - Fix It or Junk It?

Any thoughts and/or advice on this auto dilemma would be much appreciated. A friend has a 1999 Honda Civic Dx with 153,000 miles. Other than oil changes at 5k mile intervals, she hasn’t kept up with much maintenance. She recently had ignition coils and spark plugs replaced after breaking down on a highway. Shortly afterward, the CEL came on. Code P04020 (or P0420?). Dealer advised her that the catalytic converter, two oxygen sensors, and heat shields need replacement – cost is $1845. Dealer stated this is possibly a result of when the coil failed. Note, dealer is not the mechanic who did coils/plugs, he’s several hundred miles away. She also needs a new timing belt at a cost of roughly $900 (interference engine and belt never changed before).

  1. does the dealer’s explanation of a possible cause make sense – cat failing due to coil/plug problem? I always thought cat problems were caused mainly by prolonged oil consumption.

  2. are the costs reasonable? (My friend is in New York City, so labor is at premium prices.)

  3. Would you keep and fix this car, or shop for something else at this point? The immediate repairs total $2745. She hopes to get at least another 2 years out of it. She drives about 8,000 miles/year.


I noticed that the dealer wants to replace both oxygen sensors when replacing the cat. This code means rear oxygen sensor is following the front too closely, indicating that the cat is not working, but often it is just the rear sensor causing the problem.

A leak in the exhaust system before the cat is actually to most common cause of this code. Rear O2 sensor is a close second. The reason the dealer wants to replace all three is that he wants to make sure he gets the right component. In his defense, this is a difficult code to troubleshoot.

Well, she definitely needs a new timing belt ASAP if it hasn’t ever been replaced.

However, I wouldn’t rush into replacing the catalytic converter. It could be just a bad oxygen sensor. If it were my car, I’d drive it for a while to see if the Check Engine light goes off. If it’s still on after a week or two of regular driving, then I’d take it to a trusted independent mechanic, not a dealer, for further diagnosis. It could be that the engine is just running rich due to a bad coolant temp sensor, which would result in a P0420 code. Or it could be one or both oxygen sensors gone bad.

Here’s a little more info about that code:

But even if she goes with the whole deal, $2750, this car still has a lot of life in it. That’s only $1375/year based on two more years, but it will last longer but might need new axles in the next year or two. When new 13 years ago, she probably paid at least $16k or about $1230/year. Since it will go much longer than the two years, I think she should go for the repairs

But, if she has not been doing transmission oil changes and coolant changes on schedule, and she will be needing new tires and brakes soon, then maybe she should look at the alternatives.

I could be wrong (in which case the experts will be along shortly to correct me), but it seems like a coil or plug failure could lead to raw fuel getting into the converter, which would be very bad for it. Did your friend drive for a while with the check-engine light lit up, as I suspect here? If so, having it checked right away would have prevented this problem.

For the timing belt (and presumably the water pump also), $900 sounds pretty good for New York City.

I’m not sure how to answer your last question. Normally I’d say that if the rest of the car is in decent shape that it’s hard to beat that price for a few more years of transportation. However, as we know, this car has been neglected, so other failures could occur soon. At least she got lucky on the timing belt, as that would have cost her the whole engine had it broken.

The price seems a bit steep to me. A 1999 Civic catalytic converter retails for $953.66 from Honda. Throw in another $30 for gaskets and $287.60 per oxygen sensor from Honda, and I guess you’re looking at a reasonable price for a dealer job.

OTOH, you can get a Walker direct fit catalytic converter for $357.79 and oxygen sensors from Denso/NTK for around $80 each at Rockauto. Labor would be the same, but that just goes to show you that you’re likely looking at ~$1000 in savings by going somewhere other than the dealer.

This thread is also instructive. Many catalytic converters are replaced needlessly because the mechanic doesn’t get to the root of the problem:

The only reason any sane person would take their car back to a dealer would be for recalls or warranty repair work. There ARE good independent mechanics out there, you just need to take the time to find one.

As mentioned briefly by lion9car, I think the choice depends mainly on conditions easily judged - the overall physical condition of the car. If the body, paint, interior, and glass are pristine, then definitely do the repairs (but NOT at a dealer). Same conclusion if the physical condition is good with no rust issues affecting the structural integrity (frame), and tires are a matched set with at least half the tread.

But if the body and paint are rough, ie, dented and rusted with peeling paint etc, and interior cracked or torn, doors, locks, windows not working, or obvious electrical problems, etc, it may be time to move on. Otherwise, do the required repairs only if another diagnosis finds that it needs less than the dealer described (maybe just an 02 sensor for example).

Most likely, the reality is somewhere between these two extremes, so the choice will come down to how much the owner likes the car. If she really likes it and it serves her purpose, find a well regarded independent mechanic and get a better diagnosis, and an opinion on the car overall.

If the decision is made to keep the car and do repairs, definitely do the timing belt FIRST, and RIGHT AWAY. Don’t risk the chance of destroying the engine when the belt breaks.

If I were your friend, I’d find a decent mechanic and get it done with the intent of keeping the car. Agree with the mechanic that the parts are bought from a place like eraser suggests, saving a bunch of money.
Let the total bill be maybe 1000-1300 (making up for NYC prices) and let’s assume that the car last for another two years (it probably will last much longer than that, tho):
If your friend bought a new car, that’s four payments - not including paying lots more for insurance, etc. You will not be able to buy a reliable car for that money.

Thank you all for your advice, suggestions, and background info. I’ll point my friend to this thread and see what she decides. With some luck, maybe the P0420 is just due to a sensor. Though, the scenario of gas in the cat sounds like a possibility in this case. The CEL did come on as she was driving home after the coils/plugs were replaced and she did continue to drive with the CEL on. When it comes to the independent mechanic v. dealer issue, I use a local independent for most work, avoiding the dealer unless there’s no choice. My friend, however, just seems more comfortable going to the dealer.

My impression from the OP is that the owner hasn’t been interested in doing more than is absolutely necessary to keep the car on the road. That’s perfectly normal by the way. I’m not dissing the owner. I had a friend who bought a 1986 new and never once changed even the oil. She topped the oil off only when the level was no longer visible on the dipstick. This car lasted fine for her until 1996. She was completely satisfied with the car for the entire 10 years. At which point she junked it out.

The owner of the car in the OP needs a car that isn’t going to cause her time consuming problems. This car isn’t THAT car. My advice is to trade this car in and purchase a new or newer one. Let some teenage high school kid get a good deal on this car, and he/she can bring this 1999 Civic back to life.

Can’t believe nobody’s pointed it out yet, but this model was absolutely notorious for cracked exhaust manifolds. (I know because I had one).

Other than that, these are really reliable cars, despite the fact that a lot of people mistreat them (I know because I had one).

The O2 sensors may be fine. Unfortunately, if I remember correctly, the catalytic converter is welded to the exhaust manifold.

I don’t know if a muffler shop can/will cut off the existing cat (which I’ll bet is fine) and weld it onto a replacement manifold (aftermarket manifold and cat are available separately and bolt together), but I’d pursue that avenue if it were my car.

If the owner of this car has been admittedly negligent about preventive maintenance, then (unless corrected) I will have to assume that the transmission fluid has never been changed.

If my assumption is correct, that transmission is living on borrowed time, and is on schedule to fail…at any time. Personally, I would not invest any money for the CC-related repairs when a $2k transmission overhaul is…just over the horizon.

These pre-date the Honda Automatic Transmissions woes. (Civics were problematic in the seventh generation, 2001-2005, although early in the 2003 run appears to be when the issue was resolved. I had one of THOSE too).

If the OP does a few changes with new fluid in the trans now, the car might keep chugging along for many tens of thousands of miles more. Or, you’re right, it could die a horrible transmission death tomorrow. But that a risk of any car of this age.