Warning lights and codes: colors, the check engine light, and when to stop

Which light colors federal rules fix and which each maker chooses, the lights makers say mean stop now, the difference between a steady and a flashing check engine light, and what a code does and does not tell you.

Light colors: what federal rules fix and what makers choose Understand

A dashboard warning light is what the federal safety standard calls a telltale: a light that shows a system is working, is switched on, or has a fault. Many people read the colors as a traffic signal: red means stop, yellow means caution, green means fine. Federal rules support that reading for only a handful of lights.

Federal Motor Vehicle Safety Standard 101 sets the symbols and colors for a short list of telltales in its Table 1 (49 CFR 571.101, Controls and displays). Everything not in the table is the maker's choice, provided it does not use a color that hides the required ones.

An anti-lock brake system (ABS) keeps the wheels from locking in a hard stop. Electronic stability control (ESC) brakes individual wheels to help keep the car on its path. A tire pressure monitoring system (TPMS) warns when a tire is low; Tires explains how low and how late.

Two of the lights people worry about most, oil pressure and engine temperature, have no federal color. Neither does the check engine light. So the same symbol can be red on one car and yellow on another, and Mazda says the same light can change color or flash depending on how serious the problem is (Mazda, Car Dashboard Lights Explained).

What makers say their colors mean

Makers publish their own color conventions, and they do not match each other exactly. Product-specific

• Toyota says red icons indicate "a condition that requires immediate attention," such as low oil pressure, and yellow icons mark an item of importance that is "not necessarily a warning" (Toyota, Dashboard Warning Lights Explained).
• Ford tells owners to take the car promptly to a dealer if a warning light is orange or red, and says yellow means service may be required (Ford, What do the warning and indicator lights in my Ford mean?).
• Volkswagen sorts its lights into a red group, many of them marked stop, and an amber group, and says the meaning of lights may vary across models (Volkswagen, warning and indicator lights).

So "yellow can wait" is one maker's wording, not a rule. Ford's yellow means service may be required; Toyota gives low fuel as its example of a yellow item, and federal rules make the low tire pressure warning yellow on every car. The default for every light is the same: read what your own manual says for that light, on your car. Ask the authority

Lights at start-up

Most lights come on for a moment when you turn the car on and then go out. Mazda calls this a bulb check: it shows the light works. A light that should come on at start and does not is itself a fault. Toyota says that if the ABS or airbag light does not come on at start, those systems may not be available and the dealer should inspect the car right away; Ram's 2022 2500/3500 owner's manual says to have the check engine light checked promptly if it does not light at key-on (Ram 2022 2500/3500 owner's manual, check engine light). Product-specific

Your manual's lights page is the one page in it you need before anything goes wrong. It usually sits in a section called warning lights, indicators or instrument cluster. Write its page number, or the web address of the online manual, on the car facts sheet under "Where the owner's manual is". If you do not have a manual, Your maintenance schedule says where makers publish them. The warning lights table lists the common lights with what several makers say; it is a guide to reading your manual, not a substitute for it.

The lights makers say mean stop now

A handful of lights carry the strongest instructions makers give: pull over, turn the engine off, do not drive, plan for a tow. The table puts several makers' instructions side by side, paraphrased from their own pages. They agree on the action for the first three rows; they differ in detail, and your manual gives the version for your car. Product-specific

Sources: Chevrolet, warning lights; Hyundai, warning lights; Ford, Toyota and Mazda as linked above; Ram's manual page as linked above; AAA Club Alliance, Identify Car Leaks Like a Pro.

Lights makers say need service soon

A second group carries "soon," "as soon as possible" or "have it inspected" rather than "stop." These words are not permission to put the visit off; they are the maker's wording for how fast to get the car seen. Product-specific

• Check engine light, steady. Hyundai: have the car inspected soon. Chevrolet: get it serviced as soon as possible. The next section covers it.
• Charging system or battery light. Ford: switch off electrical loads you do not need and see a dealer. Hyundai: possible alternator or charging fault; have it inspected as soon as possible. Volkswagen puts its alternator light in the red group.
• Airbag light staying on, flashing, or not coming on at start. Chevrolet: needs service immediately. Ford: the system has a malfunction. Hyundai: have it inspected. A car with an open airbag recall may carry a do-not-drive warning; Recalls, warranties and service contracts covers the Takata recalls and how to check by VIN.
• ABS light. Hyundai says the regular brakes still work with the ABS light alone. With the ABS and brake lights on together, it says to avoid high speed and sudden braking.
• Tire pressure light. Tires covers what it means and how to set pressure from the door placard.

Chevrolet adds one first step for a light on a gasoline car: turn the car off, leave it off for 2 minutes and restart, because some lights clear on their own. For an electric car, Chevrolet says not to do that restart, because some failures can stop the car from turning back on. That is one maker's troubleshooting step for its own cars. A light that clears still had a cause, so it still goes on the symptom log. Product-specific

Stopping safely, and what not to open

"Pull over" in a maker's instruction means where it is safe to do so; Chevrolet's brake light instruction says to pull over carefully. Once stopped, the instruction is the manual's, not a reason to start looking under the hood. Two warnings from the makers apply here. Kia's 2024 manual says never to remove the radiator cap while the engine is running or hot, because hot coolant or steam can cause serious injury (Kia owner's manual, checking the coolant level), and Mazda says not to open the coolant reservoir cap while the engine is hot. Never open a radiator or coolant cap on a hot or running engine. Physical principle If the manual says to check the oil level, Oil, fluids and filters covers how.

The check engine light: steady or flashing

The check engine light is the malfunction indicator lamp (MIL): the light the car's onboard diagnostics (OBD) system turns on when it detects a fault it monitors. Makers name it differently. Ram calls it the Engine Check/Malfunction Indicator Light, Ford's page describes a Service Engine Soon light, and Volkswagen calls it engine control. Its symbol is usually an engine outline. Federal rules set no color for it, and Mazda says it can be yellow, orange or red depending on the car.

The current federal rule on onboard diagnostics for new vehicles requires the system to detect malfunctions in the emission control system, store trouble codes and alert the driver (40 CFR 86.1806-17, Onboard diagnostics). That is why so many makers describe the light in terms of emissions. Ram's manual says the system monitors the engine and automatic transmission control systems. Convention

Steady

A steady check engine light means the system has found a fault. In the makers' words: Hyundai says there may be a malfunction in the emission control system, engine or powertrain and to have the car inspected soon; Chevrolet says it means a malfunction in the engine or propulsion system, to get it serviced as soon as possible, and that the light may come on before you notice anything wrong; Ford says the system has detected an error in the emission control system. Product-specific

Some causes are small. Ram and Mazda both name a loose or missing gas cap, and Ford's page lists a separate loose gas cap light that it says you can check yourself. Ram adds poor quality fuel. If your car has a cap, checking that it is on and clicked is a reasonable first look. It does not replace the visit: the light has a code stored behind it, and the code says what the system saw. Test it

The default this manual gives for a steady light is the makers' own: get the car seen soon, write the light on the symptom log the day it comes on, and note anything else you notice (a rough idle, a smell, less power, worse fuel economy). If a stop-now light or a flashing check engine light joins it, the stop-now instruction applies.

Flashing

A flashing check engine light is a different message. Volkswagen's guide puts it in two words: lights up, engine control malfunction; flashes, misfire. A misfire is a cylinder that is not firing as it should. Ford says a continuously flashing light means a misfire may be occurring and that power may drop or the engine may stop. Ram's manual is the most direct: a flashing light warns of conditions that could lead to immediate loss of power or severe catalytic converter damage, and immediate service is required. The catalytic converter is the part of the exhaust system that cleans the exhaust gases. Product-specific

Ram's manual adds a fire warning: a malfunctioning converter can run hotter than normal and can start a fire if you drive slowly over, or park on, dry plants, wood or cardboard. So when you stop for a flashing light, stop on pavement, not on dry grass. Physical principle

We found no primary source that gives a speed to keep under while a check engine light is flashing, so this manual gives none. Follow your manual's instruction, which on many cars is to reduce speed and get service right away, and on Mazda's color-coded cars is to pull over and call for assistance. Ask the authority

Emissions inspections and warranties

In places with an emissions inspection program (inspection and maintenance, or I/M), the station plugs into the car's diagnostic port. Ram's manual says the check confirms the light works, is not on with the engine running, and that the diagnostic system is ready; it says plainly that the station can fail your vehicle because the light is on, and a failure can keep the car from being registered (Ram 2022 2500/3500 owner's manual, emissions inspection). Whether your state or county inspects, and how, is set locally. Ask the authority

On a newer car, a check engine light may lead to a part covered by the federal emissions warranty. EPA's question-and-answer sheet says catalytic converters are among the specified major components covered for 8 years or 80,000 miles, and that a valid claim costs the owner nothing for diagnosis, labor or parts, at a facility the maker authorizes (EPA, Emissions Warranties for 1995 and Newer Cars and Trucks). Recalls, warranties and service contracts covers which parts, which periods and how to claim. Ask the shop, before diagnosis, whether the fault could be a warranty item. Ask the authority

Codes: where a diagnosis starts, not where it ends

When the onboard diagnostics system turns the light on, it stores a diagnostic trouble code: a short code, such as P0300, that names the fault the system detected. A code reader or scan tool is the device that plugs into the car's diagnostic port and reads it. Mazda says a reader at a dealer, a service facility or most chain auto parts stores shows "what the car thinks is wrong," and Ford tells owners to contact a dealer to learn which code turned the light on.

A code is not a diagnosis. Pat Goss, who presents the Goss' Garage segment on MotorWeek, a Maryland Public Television program, puts it this way: "codes don't actually tell you what to replace" (MotorWeek, Code Readers). For each code the service manual has a matching test procedure. His example is P0300, a random misfire: the code leads to a test that works through each thing that could cause a misfire until it isolates the one that did. A misfire can have more than one cause; the test works through them. Reading a code and replacing the part it seems to name can mean paying for a part the car did not need. Convention

A diagnosis is that testing: the technician's work from the code, through the test procedure, to the cause. It takes time, and the Federal Trade Commission notes that many repair shops charge for diagnostic time (FTC, Auto Repair Basics). The FTC suggests asking whether there is a diagnostic charge if you take the repair elsewhere. Reading a repair estimate covers how a diagnosis line appears on an estimate and what you authorize.

What to ask, and what to write down

A free code read at a parts store is a useful first look: it tells you, and the shop, where the system saw a fault. Write the code on the symptom log exactly as printed. Then, at the shop, ask four things and write the answers on the symptom log or the questions for the shop sheet. This manual's convention

1. Which code or codes are stored, and is any of them the same as the parts store read?
2. Which test did the code lead to, and what did the test find (a measurement, a part that failed a test, a photo)?
3. What part or repair does that result point to, and why that one rather than the others the code could mean?
4. What does diagnosis cost, and does it count toward the repair?

The answers go on the service log later: the code and the test result are the cause, and the repair is the correction. That pairing is what tells the next shop, or you, what was already tested.

See it: Gloria's flashing light See it

• Chevrolet, warning lights: what to do for each light, including the tow instructions and the electric-car exception.
• Ram owner's manual, check engine light: steady and flashing in one maker's words, with the converter fire warning.
• MotorWeek, Code Readers: why a code leads to a test procedure, not a part.
• FMVSS 101 on eCFR: the federal rule behind the few light colors that are fixed.

Practice: sort your manual's lights Practice

Minimum: the lights page found and its location on the car facts sheet, the red and yellow lights marked, and any light on now written on the symptom log with a next step. Time: about 30 minutes. Materials: your owner's manual, printed or online; a pen or sticky notes; the car facts sheet and the symptom log (or paper).

1. Find the lights page. Look in the index for warning lights, indicators or instrument cluster. Write the page, or the web address, on the car facts sheet.
2. Mark the colors. For each light, note whether the manual shows it red, yellow or orange, green, or blue. Where the manual describes a light changing color or flashing, note both.
3. Sort each light into three groups. Stop now: the manual says stop, pull over, turn off the engine, do not drive or tow. Soon: the manual says have it inspected or serviced. Information: the light shows a feature is on. The grouping is this manual's; the words in each group are your maker's. This manual's convention
4. Find the check engine entry. Underline what it says for a steady light and for a flashing one. If it says nothing about flashing, write "not stated" so you know to call the shop or the maker if it happens.
5. Log any light that is on now. Date, odometer, which light, steady or flashing, what else you noticed, and the next step: the shop you will call and when.

Your car Your car

Check your work Check your work

Criteria

• Does the car facts sheet say where the manual is and which page or web address has the warning lights?
• Can you name, from your own manual, which lights mean stop now on your car, and what it says to do for each?
• Do you know what your manual says for a steady and for a flashing check engine light, or have you written "not stated"?
• Is any light that is on now on the symptom log with the date, the odometer, steady or flashing, and a next step?
• If a code was read, is it written exactly, with the test it led to marked as open until the shop answers?

Worked example

Gloria's symptom log entry names the light, when it was steady and when it flashed, the shaking, the tow, and the next step. Her car facts sheet names the lights page. It passes. The parts store's misfire code is written as a code, not as "bad spark plug," and the diagnosis result (failed ignition coil, from a one-hour test) is written as the cause on her service log once the shop gives it. The converter is marked "watch" with a recheck, which is the right way to record an open fact: written as open, with who settles it. Sam's sheet passes too: the lights page address is written, and the high-voltage warnings are marked "maker's service network" rather than guessed.

Common mistakes

• Reading yellow as "it can wait." That is one maker's wording for its own lights. Your manual's sentence for that light is the instruction.
• Treating a steady and a flashing check engine light the same. Makers describe a flashing light as a misfire that needs service right away.
• Buying the part a code seems to name. A code leads to a test; the test names the part.
• Clearing the light and calling it fixed. The fault is still there, and the car may not be ready for an inspection.
• Opening the radiator cap to check an overheating engine. Kia and Mazda both warn against opening it hot.

What is still unresolved

What a light means on your car is settled by your manual. What is wrong is settled by the shop's diagnosis, and the cost by its written estimate, read with Reading a repair estimate. Whether a fault is covered by an emissions or factory warranty, and whether there is an open recall, is in Recalls, warranties and service contracts. A light that comes with a sound, a smell, a leak or a change in how the car drives is described the way Sounds, smells, leaks and feel shows, and what to say yes to on the shop's list is in Discussing recommendations.

Knowledge check and scenario

Checklist

• The car facts sheet names the manual page (or web address) for warning lights, and any light that is on now is on the symptom log with a next step.
• I marked which lights on my car are red and which are yellow, and which my manual says mean stop now.
• I know what my manual says for a steady and for a flashing check engine light, or I wrote "not stated."
• I know to ask the shop for the code, the test it led to and what the test found, and what diagnosis costs.
• I watched the start-up check and looked up whether my car needs an emissions inspection. (Go deeper; optional)