Failed Emissions - Probable O2 Sensor?

It’s a 1999 Ford Escort. A few months back we had to get the coil pack replaced (it died), but our car continued to get poor mileage. It used to get 31 mpg, then once the coil pack went to hell it got about 18 mpg, and once it was replaced, it still is only getting about 22 - 24 mpg.

The SERVICE ENGINE SOON light would come on for a few weeks, then turn off for a few weeks, etc. etc. It was on when I got my car inspected (woops), and they failed it for emissions. The SERVICE ENGINE SOON light is now off.

All of this is leading me to believe it’s the O2 sensor. I can get a used or cheap one on eBay for about $35 (shipping included)… should I just replace it? What else would be a sign that it’s the O2 sensor?

Why blame the O2 sensor? A failed EGR solenoid will also light up the SES light and also cause you to fail emissions. So can a few other items under the hood. You can replace them one by one until you achieve satisfactory results, if that be your preferred method of DIY auto repair.

Or you can eliminate all guesswork by having the computer scanned for trouble codes. That is how to determine the actual malfunction.

First see what codes are stored in the computer. Some of the chain parts stores will download these for free.

Then ask yourself honestly of you’re way overdue for a complete tuneup including filters. If so, pick up the parts while you’re at the parts store.

You’ve been driving around for with a warning light for a long time. It’s time to find out what it’s trying to tell you.

Oh, and do nothing until you find out what’s wrong first. Driving the car this way could be causing even more problems. And while the internet is a good place to shop for a replacement part, Ebay is not. You want to out new parts in, not used parts that may have problems. And a used O2 sensor is unlikely to be any good. I would not put the time and affort into installing it.

That CEL (check engine light) is just a kid in class waving her hand trying to get you attention because she has the answer. You need to have the codes read. Some places will read them for FREE. Try Autozone or Advanced Auto Parts. Get the exact code (like P0123) not just their translation into English and post it back here.

That little girl may be waving her hand because of another reason. The impression I get, from your statements, is that you’re heavily into maintenance avoidance. These problems might prove to you that you, may, have (forgottenly) put off doing the routine maintenance; but, your engine hasn’t forgotten the need!
Some routine maintenance items: new spark plug and wires, air filter, fuel filter, pcv valve, transmission filter and fluid, timing belt, other belts and hoses, etc. Be sure to get the ‘tow package’ from AAA.

You’re wild guessing and that can get expensive very quickly. Listen to Mr. Meehan about getting the codes read and posting back.

We inherited this car from my in-laws, who did not take very good care of it. We got it when it was at about 35K. I haven’t gotten a tune up because… well I don’t have the money to have someone else do it for me, I know it isn’t that hard, and I just didn’t get around to (or have the money for) the parts, so I’ve been putting it off.

Unfortunately, BOTH of our cars right now are having problems. I’ve also been posting about my Volvo’s electrical gremlin… Ack.

I will do this tomorrow! Check back! (and thanks :slight_smile: )

If it is the O2, or any other part, do you have a recommendation? Auto Zone and similar chains want $60+ for a new one… anyone out there cheaper?

And say what it failed for. Which reading was too high?

Ok, do some inexpensive things: change the spark plugs and the air filter. Auto Zone (and other stores) can get the codes, and erase them. If the check engine light comes on again, have the codes read again, and tell us the numbers.

I don’t really have any parts purchasing suggestions except to buy new and shop around thoroughly. Including the internet.


Now mind you… when I went to Auto Zone, there were no codes in the system. They had been erased (apparently by the place I got it inspected). However, I had the inspection print out, which I showed to the guy, and we looked things up etc.

No code is showing in the system, and check engine light is off and has been off for a while.

I stupidly got it inspected when the check engine light was on, so I suspect I was failed automatically for that. Now that it is off, I feel confident that I could pass, however I need this sort of motivation to get a tune up done. I discovered that the parts I need to do a tune-up come to about $55. Luckily my brother-in-law is a car hobbyist so I will make him help me and show me how to do it so I can do it on my own in the future.

QUESTION: Auto Zone has this “Emissions Pass” stuff you can put in your car that is guaranteed to make you pass emissions. It’s tempting, but again I need the motivation to get the tune up done, so I don’t even want to try it. However, I’m SO curious - does that stuff work??

I’m not sure where you got that code from if the system had the codes cleared?

According to what I could find the P0420 is a catalytic converter efficiency problem. That could be caused by the downstream sensor, the converter itself, or the upstream sensor. If the upstream sensor is going bad it will give the engine computer bad information and the fuel/air mixture will not be correct, this can cause the cat to not work efficiently, if let go long enough this can ruin the cat converter. The downstream sensor is the test to see what is coming out of the cat, and if it’s bad that could cause the same code. Someone needs to do some testing on this to see what the actual tailpipe emissions are doing when the light is off vs. when the light is on. There may be some electrical tests for the sensors and a shop with a scanner may be able to monitor them while the car is running to get a good idea of what the real problem is.