Stop Pretending Not to See That Check Engine Light. Ignoring It Is Costly

Check engine light on dashboard of car
baloon111/Getty Images

When I bought my 2007 Toyota Yaris used, it came with a 30-day warranty on the engine. On day 31, the check engine light turned on. Typical luck for me, but I tempted fate and ignored it for days. It turned off on its own, but for months after, it would periodically come on and then go off after a few drives.

I eventually took it to my Toyota dealership, and though the issue ended up being a faulty gas cap, the mechanic said it could have been disastrous. Like a stern father, he told me to never ignore a check engine light: Not only was I putting myself in danger, but I could be looking at costlier repairs down the road.

I feel much more secure in my 2017 Subaru Crosstrek these days, but I know the day will come when my check engine light illuminates again. So, I sought some professional advice from Ryan Kooiman, Director of Training at Standard Motor Products. In that role, Kooiman oversees the training of 65,000 North American technicians every year.

What to Do When the Check Engine Light Comes On

First things first, I wanted to know if the mechanic at my Toyota dealership was just a drama queen, or if it really was crucial that I take my car in right away. I don’t think anyone would be surprised to learn that Kooiman agreed with my mechanic: If your car has a check engine light on, you should take it to a technician pronto.

The check engine light is an indication that there is an emission-related fault, so it is important that the driver get it diagnosed by a certified technician as soon as possible,” explained Kooiman. “The same could be said for any warning light on the dash. The manufacturer put them there for a reason — primarily the safety of the customer, as well as the protection of the vehicle, along with the government mandates related to air quality.”

Kooiman also emphasized his use of the term “certified technician.” According to him, many customers believe that anyone who plugs in a scan tool and gets the diagnostic trouble code that triggered the check engine light can diagnose the issue. Using a certified technician gives you peace of mind knowing the person working on your vehicle has the training required to correctly read the scan data and perform the battery of tests required to correctly diagnose the issue and determine what caused the failure.

“Many times, I’ve seen people treat the symptom and not address the actual cause of the problem. This results in a repeat failure, frustration and possible unnecessary repairs being performed. Finding a qualified technician to accurately diagnose the problem the first time is the best tip I can give anyone looking to save money on a car repair.

Signs That Check Engine Light Is a Serious Issue

Okay, so Kooiman made it pretty clear that you should have a certified technician diagnose that check engine light as soon as possible, but are some issues more pressing than others and, if so, are there clear indicators that the issue is urgent?

According to Kooiman, a flashing check engine light indicates there’s a catalyst-damaging misfire, which can damage your catalytic converter in just a few miles. In short, abandon your errands and head straight for your mechanic or risk even costlier repairs.

Unusual sounds and smells or symptoms, like lack of power or jerking, can also heighten the urgency. When your check engine light comes on, look for other signs that something’s amiss.

Most Common Check Engine Light Causes

Kooiman told me that every make and model is different when it comes to the check engine light, so a certified technician needs to inspect the car. However, there are some common issues a check engine light tends to point to. Here are a few codes to look out for:

P0300 Random Misfire Detected

“This trouble code indicates that there are multiple cylinders misfiring or that the engine control module is unable to pinpoint exactly which cylinder is misfiring,” explained Kooiman.

There’s a variety of faults that can cause this issue — mechanical engine malfunction, fuel system fault, ignition system fault, etc. — and a certified technician can perform tests to pinpoint the cause.

If left untreated, this fault can cause catalytic converter damage, and this can be considerably more expensive than repairing the misfire.

P0456 EVAP Large Leak Detected

“This trouble code indicates that there is a problem with the evaporative emissions system — the system that collects fuel vapors and recycles them, rather than allowing them to evaporate into the atmosphere and cause air pollution or smog,” said Kooiman.

In newer cars, the emissions system can test for leaks as small as 0.010 inches in diameter. “That is smaller than the diameter of a human air,” Kooiman noted.

This issue typically won’t result in costly repairs if left untreated, but its environmental impact is large enough to merit a repair when you can afford it.

P0171/P0174 Fuel Trim Lean Bank 1 (P0171) or Bank 2 (P0174)

Kooiman explained that this check engine light issue arises when the oxygen sensors or the air-to-fuel ratio sensors sense an incorrect air-to-fuel ratio. Common causes of this issue include fuel injector contamination, a weak fuel pump, faulty sensors or unmetered air leaking into the engine.

“Any one of these scenarios can cause a lean condition in the engine,” Kooiman told me, resulting in “internal engine damage, damage to the catalytic converter(s), poor engine performance and excessive tailpipe emissions.”

Thus, addressing this issue immediately could save you thousands in the future.

P0420/P0430 Catalyst Efficiency Below Threshold Bank 1 (P0420) or Bank 2 (P0430)

If a scan turns up one of these codes, one or more of your catalytic converters failed its self-test.

Because a fair amount of these codes concern the health of the catalytic converter, Kooiman explained how crucial this component is to a car: “The catalytic converter is a device in the exhaust system that holds and releases gases that come out of the engine, mixing them in the least harmful combination to the atmosphere. If there is a fault such as a misfire or any contamination such as coolant or oil, the life of the catalytic converter is quickly shortened.”

He added, “With the amount of chemicals and precious metals involved in the catalyst, these end up being relatively expensive repairs. In some cases, the catalytic converter will fail so badly that it eventually gets plugged up. Since an engine is essentially an air pump, if the exhaust is plugged up, it can’t run properly. This will quickly lead to a lack of power or start and stall condition.”

Check Engine Light Repair Costs

When my Yaris’ check engine light lit up on my 31st day of ownership, I didn’t choose to skip the mechanic out of laziness. As an employee at a frozen yogurt shop paying his way through college, I simply didn’t have the money — without dipping into my beer fund.

Reflecting on that time in my life, I asked Kooiman about the typical costs for the repairs he mentioned. Because the severity of the issues can vary, and because every model is different, Kooiman couldn’t assign a number.

But he did say, “Delaying a repair doesn’t result in a less expensive repair bill. Ignoring a problem almost always results in additional damage to the vehicle and a higher repair bill.

Kooiman added, “Follow the regularly scheduled maintenance plan set by the manufacturer. In my days as a technician, the cars that came in on a regular basis for an inspection, fluid change tuneup, etc. were the ones that had the smaller repair bills. Plus, the work was done at the convenience of the owner, rather than following an emergency breakdown or unexpected failure. The people who ignored their vehicle’s maintenance and drove it until it was broken were the ones who ended up paying a tow bill and a larger repair bill.”

So the lesson I learned — luckily not the hard way, though it very well could have been — is if your check engine light comes on, take it to a certified technician. Do whatever you can to make it work financially when it happens, or you very well could find yourself in a deeper financial hole down the road.

Timothy Moore, a Nashville-based editor and writer, has written for the automotive industry for five years. He currently drives a Hyper Blue 2017 Subaru Crosstrek and wouldn’t trade it for the world, but maybe for a Tesla.