Engine light may or may not indicate a disaster
QI remember you said there can be dozens, if not hundreds, of reasons my “check engine” light might come on. And since there's no way for me to know which one it is without it being checked by a repair shop, how do I know it's a serious problem or not? I think I heard somewhere the gas cap being loose is a common cause; what others are there? — Vinnie Salcida
AAside from a flat tire or dead battery, an illuminated “check engine” light — also known as a malfunction indicator lamp, or MIL — is probably the most dreaded and common automotive quirk to wreck someone's day. This amber lamp indicates the emission control system, engine or transmission has incurred a fault that will cause exhaust emissions to rise above allowable values. In many cases, the vehicle will drive flawlessly. In other cases, misfiring, stalling or other drivability symptoms can lead to bigger problems.
In most cases, a continuously glowing MIL with normal engine and transmission performance can be prioritized as a sometime-this-week visit to a technician. A flashing MIL is a different story: The vehicle should be driven as tenderly and briefly as possible, as a severe engine misfire is occurring, which can damage the catalytic converter, among other issues. If the lamp stops flashing and misfire symptoms, such as shuddering or a thumping engine, abate, one may tread further.
According to the California Bureau of Automotive Repair, the most common “check engine” causes and diagnostic trouble codes are:
“Catalyst system efficiency below threshold” (P0420): This means the catalytic converter is no longer functioning efficiently and probably requires replacement. This will not adversely affect vehicle operation, but exhaust emissions aren't being properly treated.
“System too lean” (P0171 or P0174): Lean means the engine is not receiving enough fuel or is admitting too much air, and may be indicated by stumbling, pinging or a loss of power. Engine damage is possible with this situation if the vehicle is driven under demanding conditions.
“Exhaust gas recirculation flow insufficient detected” (P0401): This can cause combustion temperatures to rise excessively, leading to rattling, knocking sounds or pinging. Reducing speed and avoiding hills and hot-weather driving can help mitigate possible engine damage prior to repair.
“Random/multiple cylinder misfire detected” (P0300): If noticeable engine shuddering or reduced power is evident, get this fixed as soon as possible. Engine misfire causes a host of problems, as noted above.
“Evaporative emission control system leak detected” (P0442, P0455, or P0440): This means fuel tank or vapor storage canister vapors are escaping to the atmosphere. Check for a loose gas cap, but remember that it may take several days for the lamp to go out. Vehicle performance is unaffected.
“O2 sensor heater circuit” (P0135 or others): A faulty heater in one of the oxygen sensors is prolonging sensor warm-up or allowing cool-down. Vehicle performance is unaffected.
A neighbor with an inexpensive OBD-II scan tool or code reader can retrieve the stored diagnostic trouble code, and a quick Internet search of the code will yield sufficient information to allow repair prioritization.
Brad Bergholdt is an automotive technology instructor at Evergreen Valley College in San Jose, Calif. Readers may send him email at email@example.com.
Show commenting policy
TribLive commenting policy
You are solely responsible for your comments and by using TribLive.com you agree to our Terms of Service.
We moderate comments. Our goal is to provide substantive commentary for a general readership. By screening submissions, we provide a space where readers can share intelligent and informed commentary that enhances the quality of our news and information.
While most comments will be posted if they are on-topic and not abusive, moderating decisions are subjective. We will make them as carefully and consistently as we can. Because of the volume of reader comments, we cannot review individual moderation decisions with readers.
We value thoughtful comments representing a range of views that make their point quickly and politely. We make an effort to protect discussions from repeated comments either by the same reader or different readers.
We follow the same standards for taste as the daily newspaper. A few things we won't tolerate: personal attacks, obscenity, vulgarity, profanity (including expletives and letters followed by dashes), commercial promotion, impersonations, incoherence, proselytizing and SHOUTING. Don't include URLs to Web sites.
We do not edit comments. They are either approved or deleted. We reserve the right to edit a comment that is quoted or excerpted in an article. In this case, we may fix spelling and punctuation.
We welcome strong opinions and criticism of our work, but we don't want comments to become bogged down with discussions of our policies and we will moderate accordingly.
We appreciate it when readers and people quoted in articles or blog posts point out errors of fact or emphasis and will investigate all assertions. But these suggestions should be sent via e-mail. To avoid distracting other readers, we won't publish comments that suggest a correction. Instead, corrections will be made in a blog post or in an article.
- Sluggish wage growth may sap retail spending during winter holidays
- Last-minute China worries derailed Fed’s rate hike plans, minutes reveal
- Volkswagen executive Horn sidesteps blame in emissions scandal
- CMU showcases its lengthy list of fledgling companies at venture event
- Other segments nudge Alcoa to slim profit
- Fed insight gives stocks room to run; S&P 500 regains 2,000 mark
- Rice, Gulfport team on Utica shale pipeline system
- PNC fined for paperwork errors on municipal bond offerings
- Credit bureau Experian keeps info on cellular firm’s customers
- Alcoa supplying parts for military jets under $1.1B pact with Lockheed Martin
- Energy efficiency goes mainstream with help of regulations, demand