What Controls The Wait To Start Light (Light Not Coming On)

It seems that the glow plugs and the wait to start light may not be on all engine models. Technology seems to have done away with these features for some engine builders. It all depends on who built your motor and what model it is.

Usually, the ECM should control the wait to start light. That is the way it was on 1998 and 99 Dodge Ram trucks and it may be the same for other diesel vehicles as well. Or it may be the control box that handles this operation.

To learn more about this topic, just continue to read our article. It explores the issue so you have the best information possible about wait to start lights or glow plug lights as they are normally referred to.

Why do You Have To Wait To Start a Diesel?


The reason for the delay is found in the composition of diesel fuel. It is not like regular unleaded or premium gas which needs a spark to combust. This spark comes right away because of the spark plugs in the engine.

You can throw a lighted match in a pool of gas and it will combust right away. However, with diesel fuel, you can throw a lighted match in a pool of it and the fuel will simply put the match out.

Diesel needs heat and the air and the fuel must get hot enough to make the fuel combust. This is done through glow plugs and it takes a while for the glow plugs to get hot enough to create the needed heat to warm up the air as well as the diesel fuel.

The time frame for the glow plugs to get hot enough can take anywhere from 15 to 30 seconds depending on quality and how the engine was made. This feature of diesel makes it a far safer fuel to use than regular gas.

It also helps diesel engines last longer than gas motors because of the lack of a spark and explosion within its combustion chamber.

Do You Need To Wait For Glow Plugs To Warm Up?

When you are starting the engine for the first time that day, then yes, you do need to wait for the glow plugs to warm up the air and the fuel before starting your diesel engine.

You must wait for that glow plug or wait to start the light to go out before turning the ignition to start. It is a short delay so it is not an inconvenience. It is the right way to start the diesel motor.

Some people have started their diesel engines before the glow plug or wait to start lights go out. Do not fret about that. Nothing will happen to your engine if you do this. It is not recommended that you keep doing this but it is not a serious mistake to make.

There will be times during the day when you may not see the glow plug light illuminate when turning the ignition. That just tells you, if there is no problem, that the glow plugs are already warm enough and you can start your motor without any delays.

Warmer weather may also have this effect on your glow plugs and you may not see the light turn on when it is hot out. It all depends as in tropical countries those lights do come on but not always for long.

Can You Drive With a Glow Plug Light On?


Yes and no. The glow plug light will come on or flash if the ECU detects a problem in the system. It will be the nature of the problem that determines if you can drive safely or not with that light on or flashing.

To determine the nature of the problem though, you have to go to a qualified mechanic with a code reader to see what the problem is. It could be serious or the code just needs to be cleared by the mechanic and you can go on your way.

However, do not be surprised if your engine gets derated when you see the light illuminated or flashing. This is a limp or safety mode that is designed to protect your engine from further damage.

It is a judgment call if you drive or not and there will be some situations where you will have to drive. Those situations are when you are out in the rural parts of the country or on highway systems. The warning light is there for a purpose and it should not be ignored.

What that last statement means is that the glow plug will not go out on its own and the problem will not self-correct. You need to deal with it as soon as possible.

What Controls The Wait To Start Light

Some people have said it is the ECM that turns the glow plugs on or off. But others have correctly stated it is the ECU unit that handles that duty. Under normal starting operations, the glow plug light should turn on the moment you turn the key in the ignition to the ‘on’ position.

Then after 15 to 30 seconds or so, it will go out by itself as there are no problems detected in the system at that time. Since the ECU controls a lot of electronic components it will handle that normal response as well.

But that is not the end of the ECU’s duties with the glow plugs. That module monitors the glow plugs and other key parts in the ignition system and when there is a problem, it will turn the glow plug light on to alert you. Or it may have the light flash to get your attention.

Once the light is illuminated, it is up to you to head to a mechanic to see what the problem is. After that, the mechanic will test the glow plugs for power, and then for any damage etc.

Wait To Start Light Not Coming On


This is a common problem as this situation can mean that the light bulb behind the dash has burnt out or has been damaged in some way. Or the fuel bowl fuse has blown and there is no power getting to the light.

There may be a problem with the glow plug relay. You will need to get to that part and check to see if any power is going in and out. If not then either the relay is bad or you have a problem between the relay and the power source.

The glow plug controller is another part that can fail on you. If it is going bad, then the glow plug light should not turn on either. Here is a diagram of the dash and the different parts in it.


020 at the top left-hand side is the glow plug relay and 030, highlighted in yellow, is the glow plug controller. You can start with those two parts to see if they are the source of the problem.

After that, check the codes that the ECU may have set to see where the problem lies.

Wait To Start Light Comes on While Driving

As you know many alert lights come on while you are driving. Those lights, including the wait to start one, let you know that between the time you started your vehicle and the time the light came on, a problem occurred.

In this case, there may be a problem with the glow plugs or in the electrical system attached to the glow plugs. It would be best to pull over and let your engine cool down first before checking the problem.

As you know the hot engine can burn. After the engine cools a little bit, you can restart your engine and see if the light comes back on. If not, then it may be a false problem. If it does light up again, then you know you need to get a code reader.

The code reader will let you know which codes the ECU set and which problems it has detected. After you get that information, the mechanic can fix the problem for you and you can go on your way.

Wait To Start Light Stays on Cummins


There seem to be some differing opinions on what this means. Some people experience chimes at the same time. One response to this situation was that 6 chimes means the FCA failed and 10 chimes means that the injectors are going bad.

However, another explanation went like this- “Wait to Start light staying on after the engine is warmed up on the road indicates failure in the grid heater system. Should produce a code.” (source).

Now, if your engine is an ‘06 model, then you would need a dealer-level fault code reader to find and read the codes set by the ECU. The suggestion was that you will see codes 2122 and 2123 on the code reader.

If that is the case then your injectors are going bad and they need to be repaired quickly before they damage the piston. The chimes and the light may point you in a different direction than the actual source of the problem.

Wait for the codes to be read before taking any action. One step you may be able to take before doing that is to “Start by retorquing the injector connector tubes in the head to 41 lbs-ft.” (same source)

What Are The Symptoms Of Glow Plug Failure?

There are a few sources for this problem. The first one would be that the glow plugs are bad and need to be replaced. Age, overheating, or contamination are usually what causes them to go bad.

A second source would be a malfunctioning temperature sensor. It may send the wrong signals to the ECU which turns the glow plug light on. Next, a faulty glow plug relay may be the problem and we have mentioned that earlier. If not the relay then the controller may be bad

Finally, there may be electrical issues that create a problem with the glow plug and its warning light. Check for loose or damaged wires, etc., and see if that solves your problem.

If you cannot find the problem, then going to a mechanic is not a bad thing. With their code readers, you should be able to solve the problem a lot quicker.

Is It OK To Drive With a Bad Glow Plug?


You can but it Is not recommended that you do so. There are signs telling you that you need to replace those glow plugs before things get much worse. The first sign will be a difficulty in getting your engine to start.

If you get your engine started, you may see or feel the following signs- the engine will misfire, you lose fuel efficiency, have low acceleration, see dark smoke coming from your tailpipe, or white smoke will come from your tailpipe instead.

You will also see the check engine light turn on when any of these problems exist. The glow plugs help ignite the fuel and when it can’t do its job, you will be running pure diesel through your engine. That is never a good situation to be in.

It is best to change those glow plugs or have them checked out, as soon as you have a hard time starting your engine. That way you can avoid those other problems.

A common glow plug fault code is P0380. Or you may also see these fault codes P0384, P0673, and P0677. They are all related to glow plug issues.

Some Additional Words

Glow plugs are essential to a diesel engine’s operation. When they go bad, you can start to lose a little money because you are buying more fuel than usual. The wait to start light is essential to the proper operation of your vehicle.

Wait for it to go out so you can safely start your engine and have it turn over right away.

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