My car won’t start, or the engine light came on. What do I do?

Five pages in this library bear on this question. One is about a check engine light, one is about jump-starting a flat battery, one is about a temperature gauge that is climbing, one is about a spare car key, and one is about the garage you take the car to. Two of them start with a warning about being on the road right now, so those lines come first.

If the check engine light is flashing, if the car is driving abnormally, or if there is smoke or a strange smell, do not drive on to a parts store. Read the code before you pay for a diagnosis gives these lines. AAA in the US says to pull over immediately and shut off the engine in those cases. AAA says a flashing light points to an ignition malfunction, most likely an engine misfire, which means a cylinder is not burning its fuel properly. Some people warn that driving on with a misfire can wreck the catalytic converter, the exhaust part that cleans the fumes, or the engine itself, and that the car should be towed. Get off the road where it is safe, switch on the hazard lights, and call your breakdown or roadside service. If there is smoke or fire, get everyone out by the side away from traffic and well away from the car, and call 911 in the US or 999 in the UK (your local emergency number elsewhere). If you cannot get off the road safely and there is no smoke or fire, do not stand in traffic: in the UK the Highway Code says to stay in the vehicle with your seat belts and hazard lights on and call 999 if you cannot leave it safely. A steady light is different. AAA says it is OK to drive short distances with it as long as nothing seems wrong with the car, and that you should still get the cause looked at soon.

If the temperature gauge is climbing or a red warning light is on, pull over as soon as it is safe, turn the engine off, and do not keep driving. Pull over before the gauge reaches red is for petrol and diesel cars; if your problem is a hot cabin, or you drive an electric car, it is not the page you need. Its safety line: never open the radiator cap, or the coolant reservoir cap, on a hot engine. The cooling system is under pressure. AAA in the US says that if you remove the radiator cap while the coolant is hot, it can spray out and severely burn you; the RAC in the UK says opening it while the engine is hot can cause serious injury from steam and scalding liquid. AAA says to wait about 20 minutes before checking anything under the hood, and the RAC says at least 30 minutes before reopening the engine compartment, and longer in hot weather or after a bad overheat. Both give a minimum, and waiting longer costs nothing. Even when the engine is cold, the RAC says to open the cap slowly with your hand covered. The RAC says to avoid pure cold water on a hot radiator because it can damage the engine; waiting for it to cool by itself costs nothing. Get everyone out of the car and away from the road. The RAC says to switch on the hazard lights, and in the UK to leave by the left-hand door, in hi-vis jackets if you have them. If you cannot get off the road safely, or you are in danger, call 911 in the US or 999 in the UK (your local emergency number elsewhere). Otherwise call your breakdown or roadside service. If you cannot stop right now, because of traffic or a motorway, AAA says to turn the air conditioning off and the heat on while you look for a place to pull over; the heater is a bridge to the place you stop, not an alternative to stopping. Once the engine is cold, the RAC says to check the coolant level in the reservoir and top it up with coolant, or a mix of water and coolant, and to look under the car for leaks if the reservoir is empty. Then do not drive on: the RAC says never to continue driving an overheated car, and AAA says adding coolant may give temporary relief but a professional has to diagnose the cause. The page cannot tell you why your car overheated.

If the check engine light is steady and the car drives normally, get the code read for free first, and take that code to a garage as a clue to the fault, not as a part to order. Read the code before you pay for a diagnosis is about a steady light on a car in the United States. First write down the date it came on, the mileage, and anything the car did that day. Then check the fuel cap. AAA says a loose gas cap can affect a vehicle’s emission control system and set the light off, and says to make sure the cap is screwed on tightly. That is a free first step and not a promise: one mechanic says people often arrive sure it is the cap when it turns out to be a vent valve or a hose leak, and if the light returns after a tight cap, the cap was not the whole story. Many people say the major US auto parts chains will read the code for free, as a service to customers or in the hope of selling you the part. Phone first, because which stores do it varies, and the page cannot list them. One person says that in England it is not free; if you are not in the US, ask a garage what it charges to read the code before you agree to anything. Many people say the other way is a reader of your own, the cheap kind that plugs into the car or sends the code to a phone app. One person says a cheap reader may not show ABS (anti-lock brake), airbag or transmission codes at all.

Some people say a code tells you which system or sensor reported something out of range, not which part has failed, and that several different faults can set the same code. One person gives an example: a code that points at an oxygen sensor can actually come from an intake leak. Some people warn that a simple-sounding code can hide serious damage, and the example they give is a misfire, which can sound minor and still do damage to the engine while you drive. Some people suggest giving the garage the code and the symptoms, and letting it find the cause, rather than naming the part to fit. Some people say not to clear the light before a mechanic has seen the car, because clearing wipes the stored record of the fault, which a mechanic needs for a problem that comes and goes. Write the code down first.

If the car will not start and you think the battery is flat, check your owner’s manual first. Then, if neither battery nor either lead is damaged and both cars are ordinary petrol or diesel ones, switch both engines off, connect the red lead to the flat battery’s plus, then the good battery’s plus, then the black lead to the good battery’s minus, and last of all the black lead to bare metal on the flat car’s engine, not to its battery. Put the last jump lead on bare metal away from the battery gives these steps from the RAC in the UK. Before you connect anything, look at both batteries and both leads. The RAC says that if there is obvious damage to either battery or to the jump leads, do not risk a jump start, and not to jump a battery that is leaking, cracked or badly corroded. Some people say not to jump a battery that is boiling or smells strongly of acid, and to replace it instead. The RAC says to take off rings and metal jewelry and to make sure nothing metal touches the battery terminals. It says the last black clip goes on metal parts of the engine, not on the battery itself, to avoid sparks when the leads are connected. The RAC also says not to use a hybrid or electric vehicle as the helping car unless its manufacturer specifically permits it. If anything smokes, burns or catches fire, get everyone away from both cars and call 911 in the US or 999 in the UK (your local emergency number elsewhere). Your owner’s manual has the steps for your own car, and it comes first.

The order, in the RAC’s steps:

1. Switch off the lights and anything electrical in the flat car.
2. Park the helping car close to the flat one without the cars touching. Switch both engines off, take the keys out and open both hoods (bonnets).
3. Clip one end of the red lead to the positive (+) terminal of the flat battery.
4. Clip the other end of the red lead to the positive (+) terminal of the good battery.
5. Clip one end of the black lead to the negative (−) terminal of the good battery.
6. Clip the other end of the black lead to a good earth point on the flat car (a ground, in the US), which the RAC says is usually a solid metal part of the engine, and not the battery.
7. Start the helping car and let it run for up to five minutes, the RAC says.
8. Try the flat car. If it does not start, the RAC says to wait another couple of minutes and try again.
9. If it starts, leave it running for about five minutes, the RAC says.
10. Switch both engines off, then take the leads off in the reverse order: black from the flat car’s engine metal, black from the good battery, red from the good battery, red from the flat battery. The RAC says to check that the engine starts again.

People disagree about why the last clip goes on metal. Some people say a spark near the battery could set off hydrogen that a battery can give off. Others say the risk is very small, that the hydrogen clears quickly once the hood is up, and that the real hazard is a clip touching metal where it should not and shorting. Some people say they clip the black lead straight to the battery and have never had trouble. Those are what they say, and following the RAC costs a minute of looking for bare metal. If you cannot find bare metal, your manual or your breakdown service is the next step, not the battery terminal. If it still will not start, the RAC says there may be another fault, and you should call a breakdown service or a mobile mechanic. One person says repeated cranking drains the helping car’s battery too, so wait between tries. One person points out that on a push-button car, a dead key fob can be a different problem from a dead battery. That page is not for a cracked, leaking or boiling battery, an electric or hybrid car, or a battery hidden in the boot, under a seat or behind a panel, where the connection points may be somewhere else on your car and the manual shows where. Its steps are from the RAC, a UK body; elsewhere the roadside numbers are your own.

If a child or an animal is shut inside a locked car and in distress, do not wait for a key or a locksmith: call 911 in the US or 999 in the UK. That is the safety line of Have a spare car key made before the last one is lost. The RSPCA, the UK animal welfare charity, says to dial 999 immediately for a dog in a hot car showing signs of heatstroke and to stay with the dog until help arrives.

Get a spare car key made while you still have a working one. The same page says some people think an emergency replacement costs more and is more stressful. Some people note you need at least one working key in hand to copy or to pair a new one to, and some people warn that with only one left you probably cannot program a new key yourself and will need a dealer or a scan tool, which is expensive. Some people say a local locksmith is frequently much cheaper than a dealer, and others say a dealer can come out cheaper, because it may waive a fee or has free access to key codes that a locksmith has to pay for. It depends on your make, your model and your area, so ring a locksmith and the dealer and ask both, before you are stuck. One person says many locksmiths found through a quick online search are scams that charge a lot by using subcontractors, and says to call local shops in advance to find a fair one. Some people say a reputable locksmith asks for your registration and photo ID before it cuts or programs a key, so bring both. Many people say to keep the spare somewhere secure and separate from the main key, such as a drawer at home, so one loss does not take both; one person warns that a chip key left hidden in the car is a security risk. If your key is a battery-powered fob, one person says to check whether the fob simply has a dead battery before you replace it. If you are locked out right now, many people say roadside help through insurance or a membership is often cheaper and quicker than a locksmith. The page gives no prices, and it does not cover getting police to unlock a car. If you have already lost every key, it says one person reports a locksmith with the right equipment can program a key when all the originals are missing, and another says a push-button car whose remote is lost may need to be towed to the dealer; it cannot say what that costs.

Take your small jobs to one independent garage and keep going back, because a mechanic who expects to see you again has a reason to be straight with you. Give one garage the small jobs first says to take your small jobs there, the oil and the yearly check, until you are a face and not a stranger. People disagree about a garage’s fee for reading a code. Some say it is unfair to charge a hundred dollars for a one-minute read. Others, including a technician, say the fee pays for the work after the read: checking that the code is accurate, looking for related damage, pricing parts and writing the estimate. One person says a shop often waives the fee if you let it do the repair, so ask what the fee covers and whether it counts towards the work. The page gives four checks that need no car knowledge: ask to see the old part; for any big quote, take the car elsewhere before you agree; check the work afterwards, with fluid levels or, one person says, a dot of nail varnish on the bolt that should have moved; and look up what the labour should take before you agree. People argue whether the strategy always works, and one person puts it plainly: loyalty to a bad garage is just paying a bad price for longer, so pick by research and word of mouth first and stay only once it has earned it.

Who these pages are not for. The check engine page is about a steady light on a car in the United States. It checked only AAA, a US body; it cannot say which stores read codes for free, what each state allows or whether a lit light fails an inspection where you live, and it does not teach repairs or tell a simple cause from a serious one without a mechanic. The overheating page does not cover electric cars, which cool differently. The jump-start page does not cover electric or hybrid cars or vehicles larger than a car, and it cannot say how to test whether a battery is worn out or what drains one. The key page gives no prices. The garage page covers no law: what you can do about unauthorised work or a refusal to return a part is not there, and one person’s route is to dispute the card payment and find whoever regulates repair shops where you live. The check engine, overheating and jump-start pages each say that for your own car the owner’s manual and a mechanic outrank them.