Failed smog check, hi NOx, no error lights - What to fix?

Hi CarTalk gurus,

I need some plain-English advice. MUCH obliged in advance for any pointers!

1995 Honda Civic CX failed smog, with a NOx reading that was 2x the California limit. There have been no error lights that I have ever seen, although even my “low gas” lamp doesn’t turn on, so who knows. There’s also been a drop in gas mileage from about 325 miles/tank to 250 miles/tank in the last two years.

Is a Diagnostic Trouble Codes (DTC) check going to help, or are the errors likely to be too vague?

When it’s high NOx and no error codes, what’s the likely culprit? Sparkplugs? Wires? Distributor cap and rotor? O2 sensors? Catalytic converter? I’ve never touched any of the above in the car in the years I’ve owned 'er. (I have to plead ignoramus when it comes to maintenance, and need to learn more about maintenance schedules.)

Can I safely go “generic” with any of these, or is it vital to go OEM with any of the above components?

By the way, this is a system with 2 O2 sensors. The Honda dealer told me that the top-of-engine O2 sensor for the 1995 would cost $200 (which suggests he thinks it requires a full-spectrum O2 sensor), but friends are advising me that a generic 2-wire O2 sensor would be a small fraction of that. I’ve read forum posts where replacing the O2 sensor causes new problems: 1) Stripping the threads, 2) New check-engine lights (“ECM’s don’t like generic O2 sensors - best to go OEM”), 3) Severely failed smog readings.

Finally, what’s the best rust-buster compound to try on fittings like the O2 sensor? This car spent too long in Vermont winters before coming out to sunny California.

Thanks a million!

You’ve never touched the sparkplugs, wires, distributor cap, rotor, and (I assume) filters for years? No wonder you’re failing! No wonder your gas mileage is dropping!

Before you try to “fix” the car, get all of this stuff up to date. Then see what your emissions readings are.

Be advised that your high NOx readings may be a result of your lack of routine maintenance. Excessive NOx is formed by high coombustion temps, and the two primary methods used to control it are the Exhaust Gas Recirculation (EGR) system, which displaces a bit of incoming oxygen with inret exhaust gases when the engine is under load to keep temps down, and the catalytic converter, which contains a catalyst that when heated seperates the nitrogen and oxygen. Both of these systems can be “poisoned” by carbon deposits, which are common byproducts of incomplete combustion…which is a common result of poor maintenance. The catalyst can be coated and made ineffective, and the EGR system can get gunked with carbon and made ineffective.

In short, get all the maintenance u to date. Then, if the readings still fail (and they probably will) let the tech check for trouble codes and determine why. It could be that you’ll end up getting a new cat converter, but don’t guess. Let the diagnosis be done.

Oh, and getting everything up to date should help your gas mileage too.

Of course, a 95 Civic would be OBD I, correct? The check engine light isn’t going to tell you much then…

Generally, but he said his had two O2 sensors. Some makers made the switch in the '95 models in anticipation of the '96 mandate.

Great. Thanks very much.

Again, pardon the gaps in my mechanical knowledge – Which filters are you referring to here?

Would like to know if I can repair any parts of the EGR system myself – seen some videos, but not entirely clear to me yet.

Also, would you go with an OEM cat, or aftermarket?

By the way, timing belt has been replaced recently.

This was – is – my first car, so I’m finding out the hard way about maintenance schedules. What are the best book/film/online resources for taking on DIY procedures?

Keep in mind that the reason I don’t get tune-ups is that I don’t have any spare money to throw into the car.

How many MILES on this car?? Have you ever checked the TIMING to see if it is within specs?


Are you just rolling your eyes at someone who never took a Shop class, or do you have a specific point you want to make? C’mon, lots of us don’t know more about our cars than what we got in Driver’s Ed!

There are 110,000 on it. Wouldn’t the mechanic have adjusted the timing when the timing belt was replaced?

Give it a good basic tuneup with new plugs, wires, cap and rotor (if it uses them), air filter. Clean/test/replace the EGR valve and inspect the vacuum tubing that goes to it. A couple of cans of Techron cleaner run through the fuel system couldn’t hurt too. If you can find a shop that has an older scan tool that will ‘read’ OBD1, you should be able to tell if the oxygen sensor(s) are totally dead, lazy, or working normally before replacement. If they’re dead, replace and retest for smog. A generic sensor would probably be fine if you need one, but don’t quote me on that.

You can get all of this done cheaper somewhere besides the dealer. Retest and see what you get.

"Keep in mind that the reason I don’t get tune-ups is that I don’t have any spare money to throw into the car. "

If you don’t have spare money to keep up on maintenance then you don’t have enough money to own a car…NOT keeping up on maintenance will cost you far more in the long run then if you kept up on maintenance.

“Would like to know if I can repair any parts of the EGR system myself – seen some videos, but not entirely clear to me yet.”

Based on the questions you’ve asked…NO…At least NOT unless you plan on spending some time learning. I wouldn’t just dive into replacing the EGR Valve.

“This was – is – my first car, so I’m finding out the hard way about maintenance schedules.”

Your maintenance schedule is spelled out in detail in the owners manual.

Okay, here it is in plain English…If you failed because of NOx, it’s almost guaranteed to be one of these three reasons:

  1. Ignition timing is advanced too far.
  2. EGR system is not working correctly.
  3. 3-way catalytic converter has failed.

Obtain an Emissions Control Systems Service Manual for your car. This is likely to be a separate manual from the regular Service Manual. It will contain instructions on how to “read the codes” and what those codes mean. It will also outline, in detail, a troubleshooting flow-chart for diagnosing high NOx emissions test failures. A step by step process of elimination.

In regards to the ignition timing Caddyman is referring to, it should be checked after a timing belt job on a distributor equipped car even if in theory the ignition timing should not budge.

The key phrase there is “in theory”. In practice it can move, and maybe move a lot.
One tooth off on a camshaft and it may be way, way off.

Actually, if the timing had been routinely adjusted to spec on a distributor based system, it would probably have wandered off as the belt stretched. The new belt would definitely change it. Adjusting the ignition timing after changing the belt is just plain part of the job.

Check that the passages to the EGR valve aren’t clogged with carbon deposits.

Awesome. Thanks, to everyone for your $0.02 – yes, you too, Caddyman: Much appreciated! Will do all this in order of low-priceyness.

If it’s the catalytic converter, are there big consequences of going with an aftermarket one?

Not if there’s a “direct fit” one available. Some claim that the aftermarket converters are inferior to the OEMs, but in most of these cases I think a “universal” replacement was used. I don’t think a 16 year young vehicle justifies the expense of an OEM converter.

An aftermarket converter will work just fine. For what it’s worth, all converters are aftermarket actually; even the ones the factory installed on the assemblyline. Just like about 80% of the car it’s all farmed out to suppliers.

Last year there was a show on TV (of which I watch little) with a segment about manufacturing catalytic converters. The company (name escapes me) manufactures converters for both car makers and the aftermarket parts houses. In a word, they were all coming out of the same plant. This does not mean that ALL converters originate in the same building; only that 1 company may produce them for many users.

It was pretty fascinating. They had a huge assortment of jigs and fixtures which were changed as needed to produce the next run of converters or converters with pipes attached.