Car Won’t Start Yellow Engine Light (Top 5 Reasons)


There’s never a good time to break down. For me, it always seems to happen at the worst possible time. The last time it happened was on the way to a birthday party with a car full of now, very disappointed kids.

There are many possible reasons your car won’t start with the engine light on. Here are the top five reasons:

  1. Faulty battery
  2. Fuel pump failure
  3. Coil failure
  4. MAF sensor failure
  5. Crank sensor failure

I’ve been a mechanic for over twenty years, and when your car doesn’t start, experience has shown me it’s probably one of the top five.

Engine light on

In this post, we’ll look at why each of these top five can prevent your car from starting and what you can do to fix it.

What Does The Light Mean?

An engine light “On” means a specific fault has been flagged by your car’s onboard computer. Also known as MIL (Malfunction Indication Lamp). Each fault has a unique code, and that code is logged in your car’s computer.

Reading the fault codes, therefore, is always going to be the best place to start faultfinding.

Now I know you may not have a fault code reader, and that’s OK for now. We can still do some detective work and eliminate some of the top five faults.

However, we may arrive at a point where we need to read the codes. That’s not all bad as a code reader is a very useful tool and not as expensive as they once were. You can check out a basic code reader and the shop level model I use here on the Mechanics tools page.

Faulty Battery Causes A No Start

Battery

A faulty or flat battery is the single biggest reason for all no starts, and it’s most common in the winter months. Batteries don’t like the Burrr conditions. If your battery is going to fail, it’ll be on a frosty morning.

But how do we know for sure it’s a battery problem? Without a code reader, it’s not definitive but tries this simple test of the battery:

  • Turn on the headlights and the wipers
  • If the wipers are moving slow or slower than normal and the lights are dim, you can bet you found the problem, a soft/flat/faulty battery

Your battery might have enough power to crank over the engine, but it just won’t run. That’s because the battery must have enough power to turn over the engine fast enough to cause a sensor on the flywheel known as a crankshaft sensor (more on it later) to generate a signal and send it to the ECU (Engine Control Unit – computer).

The ECU sees this signal as a sign to send fuel and spark to the engine and fire it up. Without sufficient engine cranking speed, the sensor never sends that signal, and so nothing happens.

How To Jump Start Your Car From Another Car

Battery terminal check
Tight & Clean Terminals are Mission Critical

If you suspect a weak battery, you can quickly eliminate it from our list of possible problems. Borrow a neighbors car or consider buying a jump pack. The little NOCO boost/charger pack is about the best I’ve seen, and I’ve been a mechanic for over twenty-five years.

It’s small enough to fit in a glove box and powerful enough to start a diesel engine, I know because I tried it. Anyway, you can check it out here on the Auto electrical tools page.

Go ahead and attempt to jump-start your car. If she starts up, you know your battery is probably on its last legs.

Many car batteries only last about three to four years. Budget to change it partway through the third year. More modern vehicles use a battery control module to optimize battery performance. Vehicles fitted with this technology are easier on batteries and can expect greater life expediency.

Jump starting is easy and using a good quality set of jump leads makes the job go smoothly. Cheap skimpy leads will cause problems, so if you are buying a set of jumpers, spend a few extra dollars and buy a good set.

You can find a great set here on the Trunk essentials page, or consider using a smart charger which you can find here on the Auto electrical repair tools page.

Use the info-graphic above to fit and remove the jumpers. If you have any doubt about finding the correct terminals, check out “Unmarked car battery terminals”.

Connecting The Jumper Cables

Positive jumper cable first – Connect Red positive (+) to the red positive battery post of the flat battery (1), then to the red donor positive (2).

Now for the black negative jumper cable – Connect the negative (-) black clamp to the donor negative battery post (3) and the final clamp to a ground (GRD) on either the chassis or engine. A good ground (GRD) is any bare metal.

The whole process looks like this:

  1. Move a donor car close enough to connect booster cables
  2. Put the jumper cables on in sequence 1, 2, 3, 4
  3. Start the donor car and leave it idling
  4. Now start your car and leave the cables attached and your car idling for a few minutes before removing
  5. With the engine running, remove the jumpers in reverse order 4, 3, 2 and 1

Testing The Battery With A Volt Meter

Battery testing

A more scientific approach to testing your car battery will require a simple voltmeter known as a DVOM (Digital Volt Ohm Meter). You’ll find a Voltmeter listed here on the Auto electrical repair tools page.

Set your voltmeter to 20v DC (Direct Current). Place the red voltmeter probe on the red positive terminal of the battery and the back probe on the black negative terminal.

The reading of the battery should read more than 12.4 volts, less than this, and your car may have trouble starting.

The following values represent percentage state of battery charge:

  • 12.65 – 13.2 volts is 100% charged
  • 12.4 volts is 75% charged
  • 12.2 volts is 50% charged
  • 12.0 volts is 25% charged
  • 0 – 11.9 volts is Discharged (Flat)

However, it’s worth noting that even a battery showing a full 12.65 volts may not be healthy, a little confusing, I know. The best test of a car battery is to stress it, and that means performing a crank test.

Car battery crank test

This test is a real-world test of a battery. It’s a straightforward test, but you’ll need a helper and a voltmeter. The battery should be fully charged for this test, and so obviously, if your battery is below 12.4 volts, it won’t be a useful test result. You’ll need to charge your battery first.

A household battery charger is a useful tool. Modern smart chargers are clever enough that they can be left plugged in without the risk of overcharging.

They can be used on any 6 or 12-volt battery – Cars, Trucks, Motorbikes, Lawnmowers, Quads, Camper-vans, Boats, Jet-skies, Snowmobiles, etc.

Battery crank test

Step One:

The car battery must be above 12.4 volts. If not, charge it.

Step Two:

Attach the voltmeter probes red to positive and black to negative, just as per the info-graphic above.

Step Three:

Have a helper crank over the engine.

Step Four:

Take note of the lowest voltage reading during the crank or use the low reading capture function of your voltmeter. Any reading below 9 volts indicates a faulty battery, go ahead and replace it.

Fuel Pump Failure Causing A No Start

Every car will have a fuel pump. It moves the gas from the gas tank to the engine. Modern cars will have an electric fuel pump inside the gas tank. Fuel pumps work pretty hard and fail regularly.

Fuel pump
Running gas tank low damages pumps

Without a working fuel pump, your car won’t run. When pumps give trouble they either stop pumping altogether or continue to pump, but without producing the correct volume.

Worth checking your gas tank fuel level. Many cars are programmed to prevent the pump from operating if the gas tank is critically low on fuel, this protects the pump from dry-running damage.

A pump that sort of works will require a pressure test to confirm a problem, but a pump that doesn’t work at all is the more common fault and usually pretty easy to confirm.

Listen for pump noise

With the ignition key in the “On” position, the pump will make a “hum” or “buzz” noise. It will be most noticeable at the rear of the car where the gas tank lives.

If you do hear a humming noise your fuel pump is turning on and so is likely working OK.

If you don’t hear any noise, cycle the key again as the pump will turn off after it reaches the desired pressure.

You can try tapping on the bottom of the gas tank, this often gets the pump going again, but the pump will need to be replaced.

Checking car hand book

Check also the fuel pump fuse. You’ll find the location listed in your driver’s manual listed in the index under fuses. You may find this post useful “How to check fuel pump fuse?”

A blown fuse suggests a possible fuel pump problem. You may find this post helpful ” Where is my fuel pump located?”.

Fuse chart

Coil Failure Causing A No Start

Plug Coil

The coil is an important component. It produces the high voltage needed to fire the spark plugs.

All modern motors run either a single Coil pack, paired or COP. If your vehicle is fitted with individual coils per cylinder (we call them COP – Coil Over Plug), we can eliminate them as the source of the no-start. The reason is, it’s unlikely all coils dropped out at once.

If however, you have a single or twin coil pack, (depends on the type of engine you have & the number of cylinders, etc.) then a failed coil will cause a no-start.

Checking for a failed coil will require a spark testing tool or the use of a code reader. Check out this inexpensive, simple-to-use code reader on the Mechanics tools page. Check also your driver’s manual for the ignition system fuse location and check the fuse.

You may find this post useful “Where is my coil located?”.

MAF Sensor Failure Causing A No Start

MAF sensor

The MAF (Mass Air Flow) sensor’s job is to let the ECU know how much air has entered the engine, and also the temperature of that air. The ECU can then calculate how much gas to inject.

The ratio is important. Too rich or too lean can cause a no-start.

Maf Sensor cleaning

The MAF sensor is usually located in the intake pipe from the air filter housing. Cleaning the MAF sensor with an electrical cleaner or MAF cleaner. Check out MAF cleaner here on the “Consumables page”.

As a test, try disconnecting the MAF sensor and attempt to start the engine. This causes the ECU to use a default value. Note, however, disconnecting the sensor will set a code in the onboard computer, so reading this as a fault later may send you on the wrong path.

Crank Sensor Failure Causing A No Start

The crankshaft sensor is a sensor we spoke of at the beginning of this post. Its job is to let the ECU know that the engine is turning and also the location of the top dead center.

CKP sensor

This information is crucial for starting and firing the plugs at the correct time, it’s also used to trigger the fuel pump activation after crank. If the crank sensor isn’t working, the car won’t start.

Crankshaft sensors are pretty durable but do fail. They often suffer from heat soak, meaning they fail as the engine warms up.

The crankshaft sensor is either located at the transmission bell-housing or at the crank pulley (Harmonic balancer).

It is a small sensor pointed at the pick-up and will have a small plug-in wire connector. It’s worth checking that the connector is in place and the wiring is undamaged.

Obviously, all of these faults will be a lot easier to diagnose with a code reader. OBD code readers were once an expensive item, but not today. For the price of a tank of gas, you can get hooked up with a good unit that will have you well on your way to fix your problem.

A workshop manual for your vehicle is always a good plan. They only cost a few dollars but will save you a packet. Good manuals cover replacement and repair diagrams, wiring diagrams, system operation overview, troubleshooting sections, fastener torque specs, and sequences, etc., all mission-critical info.

You can check out all the manuals and tools I use here on the Auto electrical repair tools page, and you can check out inexpensive quality scanners on the Amazon link below.

Amazon ODB Scanners

Related Questions

Can it be too cold for my car to start? Cars are rigorously tested in extreme heat and cold. If your car is having trouble starting in the cold, have your battery and fuel system checked.

John Cunningham

John Cunningham is an Automotive Technician and writer on Rustyautos.com. I've been a mechanic for over twenty-five years, and I've worked for GM, Volvo, Volkswagen, Landrover, and Jaguar dealerships. My passion is cars. I use my knowledge and experience to write articles that help fellow gear-heads with all aspects of car ownership, including buying advice, maintenance, and troubleshooting.

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