The Saga of the Mysterious Check Engine Light

 Your check engine light may look like any one of these, or something else.
Your check engine light may look like any one of these, or something else.

Many of us have seen it and if you’re like me you’ve been puzzled by it–the Check Engine light on the dashboard of your car or RV. Normally, it illuminates with many other lights when first turning on the ignition, and then it disappears along with most of the others when the engine begins to run. What does it mean, though, when you’re driving merrily along your way and the Check Engine light comes on? Well, it could mean one of many things or point to more than one item that requires attention. The engine has a lot of parts and components, mechanical and electronic. Just which of these things are you supposed to check? Is it something urgent? Should you pull over and turn the engine off, or is it OK to continue driving?

I’m by no means an expert on the subject but I’ve learned a few things during a recent adventure with the Check Engine light on my Subaru that may prove useful to others. First, if the Check Engine light is flashing as opposed to a steady glow, that means to have your engine looked at right away as it could be something serious and it could cost more to deal with it later rather than sooner.

ELM 327 OBDII Wifi Code Scanner
I found this ELM 327 OBDII Code Scanner on eBay for less than $15 delivered. They are available in both Bluetooth and WiFi versions. I’d read somewhere that you need the WiFi version for iPhones.

When the Check Engine light came on in my car I did some research by way of figuring out what it meant and what I should do about it. Pretty much the first thing I discovered is that the Check Engine light can illuminate for many reasons. Your car is full of computers and sensors that run checks on a number of systems. Problems in any one of them can cause the light to come on. So, how do you know where the problem is located? Turns out, when an error is detected, a code is stored in the On Board Diagnostics (OBD or OBDII) system computer. This code may indicate the precise problem or just point in the general direction.

As I poked around the web for information about the Check Engine light I found an advertisement by AAMCO Transmissions for a free diagnosis so I took my car to AAMCO. They plugged it into their machinery and told me a P0420 error code had been set in the OBDII and it meant “catalyst system below efficiency threshold”. That meant that the catalytic converter, part of the emissions control system, wasn’t working as well as it should.

So, I thought I needed a new catalytic converter. My thinking was that many auto repair facilities would have encouraged me to hire them to install one, and possibly do some additional work. When I looked into the cost of a new catalytic converter I was not happy to learn that having one installed could cost well over $1,000 for my car! Being that my car was old and that I was planning to be RVing around the country in the near future without it, I opted to just ignore the Check Engine light for a while. Surprisingly, as I continued to drive my car for some number of weeks before departing on my RV trip the Check Engine went off and on a number of times. Hmmm…

Having returned home from my RV trip–yes, I’m home now–I was forced to attend to the Check Engine light situation when the time to re-register the vehicle came along. In California your vehicle must pass a smog check every other year in order to re-register it and its time had come. I learned that a car cannot pass a smog check in California while the Check Engine light is displayed. It’s an automatic Fail. Since I’d seen the light come and go before I thought that maybe I could wait for the light to go off again then take my vehicle in for a smog check, but what if it wouldn’t go off again? What then? That’s a moot point because the registration had expired and such being the case I couldn’t drive the car except to take it in for a smog check or for service in order to repair it so that would pass the smog check.

FourStroke OBD-II iPhone App
FourStroke OBD-II iPhone App. It’s free!

Because the time had come to re-register my car I couldn’t just ignore the Check Engine light anymore. I began doing some additional research about replacing  the catalytic converter: were they available from different manufacturers, did the prices vary much, could I possibly install one myself? As I poked around online I unearthed some additional unpleasantness–I learned that most of the time the “below efficiency threshold” error code indicates a catalytic converter that has been damaged by a problem upstream of the converter itself. In other words, I may need to repair something in addition to the catalytic converter. One thing that can kill a catalytic converter is a leaky head gasket which allows contaminants to get into the system and foul the converter. I already knew that my car had a leaky head gasket and that repairing it could cost around $1,500. So, I could potentially be saddled with repair costs in the $2,500 – $3,000 area in order to pass the smog test. That’s about what my car is worth on the market. Still, repairing it could be the most economical option in the long run, but would there be a long run? I’m not sure. I was already thinking about selling it in favor of a pickup truck to tow a 5th wheel RV.

I thought perhaps I could install a new catalytic converter myself and that it would last long enough to get the car smogged. Then I could sell it or drive it a couple more years before the next smog check would be due. I found out that it would cost about $500 for an aftermarket converter and there would be no guarantee that my car would pass a smog check once installed. It would be a big gamble.

More research unearthed information that it could be the case that my catalytic converter functions fine much or most of the time and that only under certain conditions it may function below the efficiency threshold thus tripping the Check Engine light. Wow. If that’s the case then maybe my car could pass a smog check and I could avoid the expense of repairing it. Even if that turned out to be true there remained that nagging little detail of the illuminated Check Engine light which would prevent passing a smog check. What to do about that?

I also found out that for less than $15 I could buy an OBDII code scanner. Such a device plugs into the OBD or OBDII port on a car (usually under the dashboard). They are used to both read and delete the error codes stored in the OBDII computer memory of a car. Hmmm… This device could be used to clear the P0420 error code and turn off the Check Engine light, but it wouldn’t make my car pass a smog check, not if the catalytic converter was operating below the efficiency threshold all of the time anyway. I thought that maybe I could pass the smog test if the catalytic converter was functioning properly while the vehicle was being tested, but I’d have to get that darned Check Engine light to go out first.

Other potential solutions I read about included a couple methods of cleaning out a catalytic converter which would require removal (not easy) or pouring lacquer thinner in the gas tank. I didn’t want to try those things because the research I did indicated there was no real evidence to support the efficacy of these potential solutions and I was concerned that pouring a potent solvent such as lacquer thinner into my gas might screw up something else up. Nope… not for me, thank you very much.

Since I had observed the Check Engine light come and go a few times I was hopeful that the catalytic converter in my car was working well enough, at least some of the time, to be able to pass a smog test. So, I purchased an ELM 327 WiFi code scanner on eBay for less than $15 delivered and downloaded a free iPhone app called FourStroke to work with it. When the ELM 327 arrived I plugged it into the OBDII port of my car, cleared the P0420 error code from the onboard memory, and drove my car around for awhile until all the self-diagnosis checks had been run by the onboard systems–you can see all this in the FourStroke app. These self-tests must be completed by the car and the corresponding codes stored in the OBD memory in order to pass a smog check in CA. Much to my relief–perhaps I should say utter glee–when the self-tests were completed the Check Engine light remained off and the P0420 error code remained gone. I rushed down to a smog check station and in five minutes I had a certificate–my car had passed the test! Woohoo!!!

One thing I’ve learned over the years is that sometimes, with the passage of a little time, alternative and sometimes better solutions are found.  I could have headed straight for the repair shop and I probably would have done just that had it been a $50 repair bill I was facing. However, a repair that might have been in the $3,000 area provided the incentive to look for a more economical solution. My persistence and inquisitiveness saved me thousands of dollars that I may never need to spend, and I learned a few things along the way. Knowledge is power, as they say, and in this case it meant saving a bundle.


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