Mobile Diagnostic Mechanic in Chicago

What does the check engine light mean?

The light is a messenger, and it is not very specific.

Every car and truck sold in the United States since 1996 has an onboard diagnostics system, known as OBD-II. The EPA's brochure on onboard diagnostics describes an advanced engine computer that watches the engine and emissions systems. When a reading stays outside what it expects, the computer stores a diagnostic trouble code and turns on the malfunction indicator lamp, which most people call the check engine light.

The light says a fault exists. It does not say how serious it is. The same amber symbol can mean a gas cap that is not tight or an engine that is misfiring badly enough to damage expensive parts. That is why the first useful question is whether the light is steady or flashing.

What does a flashing check engine light mean?

A flashing check engine light usually means a severe misfire. The Illinois Air Team says a prolonged misfire can quickly damage emissions components, especially the catalytic converter, and that prompt service is required. The EPA's brochure says you can still drive safely but should minimize your time on the road, and avoid high speed and heavy loads such as towing. Kelley Blue Book goes further and says to shut the engine off and call a tow truck. We lean cautious: reduce load, get the car checked promptly, and stop if it shakes badly, loses power or overheats.

How long can I drive with a steady light?

No official source gives a number of miles or days. The EPA calls a steady light a request to seek attention soon. An ASE spokesperson quoted by Consumer Reports, as reported by SlashGear, says a steady light does not mean you must pull over and call a tow truck, but it does mean you should get the car checked as soon as possible. SlashGear's own list of reasons to stop instead: an overheating engine, a strong fuel smell or a fuel leak, or a severe loss of power.

Why does the light come on for a second when I start the car?

That is a bulb check. The EPA's brochure says the light should flash briefly when you turn the key to run, which shows the system is ready, and then stay off. A light that never comes on at all, even for that moment, is worth a look, especially on a car you are thinking of buying.

One look-alike is worth separating out. A check engine light means the car stored a fault. A maintenance required or wrench light is a service reminder that counts miles since the last reset, usually for an oil change, and it is not tied to a fault at all. Only the first kind matters for an emissions test.

Gas cap, fill-ups and when the light goes out

Two of the most searched questions about the light, answered together.

Will tightening the gas cap turn off the check engine light?

It can, but not instantly. The EPA's brochure tells you to make sure the gas cap is not loose or missing, and says that if the light does not go out after a few short trips, you should have the car serviced soon. The car turns the light off itself once it re-checks the system and finds the fault gone. The Illinois Air Team describes that as three consecutive evaluations, with some exceptions. Kelley Blue Book lists a cap that is missing, damaged or not sealed properly as the most common cause of an evaporative leak code, and says a strong gas smell near the tank is a reason to see a technician right away.

How do I reset the check engine light?

The honest answer is to fix what set it. A scan tool can clear the code and turn the light off, and so can disconnecting the battery on many cars, but both also reset the car's readiness monitors to not ready, and the light comes back if the cause remains. If an Illinois emissions test is coming, that is the worst moment to do either. The next sections explain why.

Other warning lights and symptoms

The check engine light is not the only one that sends people looking for a diagnosis.

Not every dashboard light is a check engine light. A red oil pressure light or a red temperature warning means pull over and turn the engine off. A red brake light is a brake question, so see mobile brake repair in Chicago. The rows below are the amber lights and the symptoms people ask about next. A/C not cooling is a symptom, not a warning light, and it is here because it comes up so often.

Pick the same light in the picker above for the full advice. What a scan tool can read depends on the car and the tool, and the mechanic says what they can check.

Before the visit: what helps a diagnosis

A few minutes of notes can save a mechanic an hour of guessing.

• Say whether the light is steady or flashing, and whether any other light is on.
• Note when it happens: a cold or warm engine, at what speed, under load, in wet weather, and how often. Spark plug maker Champion's diagnostic advice for a misfire is to keep a log of exactly these things.
• Note what changed recently: a fill-up, a repair, a jump start, a new battery.
• Leave the codes alone. Do not clear them or disconnect the battery, because the codes and the freeze-frame data are clues, and clearing resets the readiness monitors.
• Tell the mechanic about anything plugged into the diagnostic port, such as an insurance dongle or a tracker. The Illinois Air Team requires those to come out before an emissions test.
• Say where the car is parked and how to get to it, including permit zones and garage rules.

A fault that comes and goes is the hardest kind. It may not be present when the mechanic arrives, which is why a precise description of when it happens matters, and why some faults need another visit or a drive to catch.

Questions to ask before paying for a car diagnostic

These work with any mechanic or shop.

1. What does the diagnosis include: reading codes, testing parts, or both?
2. If the repair is done with you, is the diagnosis fee credited toward it? And is there a diagnostic charge if I have the work done elsewhere?
3. What happens if you cannot find the cause?
4. Will I get the codes and findings in writing, or on video?
5. Will you tell me before any price changes, and ask my approval before doing extra work?
6. Which of the findings are urgent and which can wait?
7. Did you check for open recalls and service bulletins on my car?

With us, the visit covers both reading codes and testing, you get a video walkthrough and a report, and a price change is shown to you before you pay more. The fee and refund rules are in the cost section above, and how a visit works walks through the rest.

Check engine light and the Illinois emissions test

In the Chicago area, the light, the test and your license plate renewal are tied together.

Illinois runs a vehicle emissions testing program in the Chicago and Metro-East St. Louis areas. Most gasoline-powered cars, SUVs and light trucks from model year 1996 on are tested once they are four years old, and hybrids are tested too. After that, even model years are typically tested in even calendar years and odd model years in odd years, in the month your license plates expire. Some vehicles are exempt, including diesels, electric-only vehicles and motorcycles. The Vehicle Eligibility Check tool tells you from your VIN or plate whether your car is due. The test has to be passed, or the car otherwise in compliance, before the plates can be renewed.

When and how to test

You can be tested up to four months before your plates expire. Earlier than that and the car is rejected. The Illinois EPA asks drivers to test several weeks before expiration so there is time for a retest, and the plates can only be renewed within three months of the expiration date. You do not need a test notice, because the stations can test your car without one. Everyone must exit the driver's compartment during the test, and a test at a self-service kiosk averages three to five minutes. The Air Team's site lists the stations, kiosks and mobile testing units.

What the test actually reads

For most cars the test is a plug-in check of the onboard computer, not a tailpipe sniff. The tester connects a scan tool and looks at whether the car's self-checks are complete, whether the check engine light is commanded on, and which codes are stored. A car passes when the computer is working, the required self-checks are done and the light is not commanded on. A transmission code counts too: the Air Team says a light commanded on for a transmission code is emissions related. It fails when the light is on with codes stored, or when the computer is not working or the connector is missing, damaged or blocked. Unplug anything sitting in the diagnostic port first, such as an insurance dongle, a tracker or a scan tool, because testers ask for those to come out.

Rejected is not the same as failed

The self-checks have a name: readiness monitors. Each one tests a system, such as the oxygen sensors, the catalytic converter or the evaporative system, and reports ready only after the car has seen the right kind of driving. If too many are not ready, the car is rejected, not failed. The Air Team's rule is more than two of the non-continuous monitors not ready on 1996 to 2000 models, or more than one on 2001 and newer. A rejection does not count as a test. Often the car just needs more driving, but not always, because a fault elsewhere can stop a monitor from ever running. A car can also be rejected for a missing repair data form or for arriving more than four months early.

Clearing codes or pulling the battery cable

This is the trap. Clearing codes with a scan tool resets the readiness monitors to not ready, and disconnecting the battery does the same on most cars. A scan tool cannot set them again. The car has to run its own tests during a drive cycle, a specific way of driving that the manufacturer designs for each monitor, and the catalyst and evaporative monitors need more driving than the rest. If the cause is still there, the light comes back too. Switching the light off just before the test does not earn a pass.

How long to drive afterward depends on the car. The Air Team's test-day tip is to drive for several days before the test if the car was recently serviced or the battery was disconnected. Outside Illinois, the California Air Resources Board suggests about a week, and California's Bureau of Automotive Repair says a week or two. A car that is rarely driven may take longer.

On the light itself: a lit check engine light with stored codes fails the test. The Air Team says a steady light means get service as soon as possible, not stop at once, and a flashing light means a severe misfire that needs prompt service because it can quickly damage the catalytic converter. A light caused by something simple can go out by itself, because the car turns it off after it re-checks the system a few times and finds the problem gone. A loose gas cap is the classic case, and the light should clear after a few trips once it is tightened.

After a failed test: catalyst codes, repair data and the waiver

A failed test comes with a Vehicle Inspection Report that lists the stored codes. Like any code read, it does not name the failed part, which is why repair shops charge for diagnosis. You also get a Repair Data Form. The repair technician can submit it online, or you bring it to the retest, and a retest without complete repair data is rejected. If any of the codes was for catalyst efficiency, the catalyst monitor must be ready before the retest or the car is rejected again, so a catalyst repair needs a drive cycle before you go back.

The state also has a repair waiver for cars that keep failing, but it is narrow. The conditions on the Air Team's page include at least two failed tests, a minimum spend on emissions-related repairs done by what Illinois law calls a recognized repair technician, a working check engine light and diagnostic connector, and any emissions warranty repairs you were eligible for already done. Diagnostic labor time does not count toward the minimum spend, and the Air Team's own pages show two different minimums, so read the waiver rules for the current amount before you spend money on repairs you hope will count. The pages do not say whether a mobile mechanic's work counts as a recognized technician's, so ask whoever does the repairs, before they start. A one-year extension also exists for owners whose repairs are blocked by economic hardship.

Check the emissions warranty before you pay

Federal law requires manufacturers to warranty emissions parts, and that coverage applies to used cars too, counted from the first sale. For the catalytic converter, the engine's emissions control computer and the onboard diagnostic device, it runs 8 years or 80,000 miles, whichever comes first. Other emissions parts have shorter coverage, 2 years or 24,000 miles. A separate performance warranty applies if the car fails an approved emissions test in an area with such a program, as Illinois has. To use it, the repair goes through a facility the manufacturer authorizes, and a shop that is not authorized does not have to tell you what is covered, so check your warranty booklet before you approve a catalytic converter job. The maker can deny a claim if misuse or skipped maintenance caused the failure, and state laws can add protection. This is the dealer-first case from the section above.

The order that works

1. Diagnose first. Do not clear the codes to see what happens.
2. Repair what the testing found. If the part might be under emissions warranty, check that before you pay.
3. Drive normally for several days, because the catalyst and evaporative checks need more driving than the rest.
4. Then take the test, several weeks before your plates expire and no earlier than four months before, so there is time for a retest if you need one.

This is general information, not legal advice, and not a promise that any car will pass. A TravelMechanic diagnosis is not a state emissions test, and the program's rules can change, so confirm your car's details with the Illinois Air Team. If your car has failed or you expect it to, a diagnosis first tests to find the cause, so you know what the repair involves before your registration deadline arrives.

Chicago winters and warning lights

• Cold mornings expose weak batteries and slow starters. The US Department of Energy notes that battery performance decreases in cold weather, which makes it harder for the alternator to keep the battery charged. A battery maker, Interstate Batteries, adds that a car might show a check engine light or other warning light because a sensor did not get enough power. A click, a slow crank or no lights each point to something different, so describe which one you hear. See our guide to a car clicking when starting, or a car that won't start in Chicago.
• Short trips in winter keep the engine from fully warming up. Kelley Blue Book says cold-climate winter driving, where the engine cannot reach normal operating temperature, can trigger the thermostat code P0128, and that several small trips could reset it.
• A fuel fill-up is a common moment for a loose or missing cap to set an evaporative code. If you smell gas near the tank, do not wait.

For battery, starter and charging problems you can also read about replacing a car battery at home, starter replacement and alternator replacement. For jobs beyond diagnostics, see mobile mechanic services in Chicago.