Can A Bad Alternator Cause False Codes & Limp Mode [Explained]

A failing alternator can not only cause issues electrical issues in your car but also have a negative effect on the overall engine performance. So, you may be wondering – can a bad alternator cause false codes and limp mode?

As there are several sensors that rely on the alternator to get the proper information, a failing alternator with voltage spikes can show false error codes. More often than not, it will also lead to the car going into limp mode.

A failing alternator with voltage spikes can show false error codes & also lead car going into limp mode.

Can bad alternator cause false codes

can a bad alternator cause false codes & limp mode

We’ll discuss what sensors are most commonly affected when you have a bad alternator in this guide. You’ll also learn how to easily diagnose whether your car is in limp mode. Without further ado, let’s dive in!

Can A Bad Alternator Cause False Codes & Limp Mode [Solved]

Modern cars are extremely reliant on getting readings from the different sensors in the car. When there’s a drop in the voltage or any kind of spike, an error message gets passed to the ECU.

car bad alternator

The ECU may interpret the error code as an issue with the sensor and give an error code that it’s the sensor that has gone bad. But in reality, it’s the failing alternator that has caused the unwanted change in the voltage of the sensor.

That’s how the alternator causes the ECU to throw false error codes. These error codes detected by the ECU can cause the car to go into limp mode. Here’s how a bad alternator causes false error codes in your car and causes the ECU to switch the vehicle into limp mode:

Note: You can also read our guide on what to do after replacing crankshaft sensor.

1. Mass Airflow Sensor

The MAF sensor in your car is one of the most important sensors as it measures the amount of air entering your engine. It gives this reading to the ECU of the car.

car mass airflow sensor

The ECU takes that information into account and supplies the right amount of fuel through the injectors. The data on the amount of air getting into the engine is critical as it’s used to achieve the optimum air-fuel ratio.

The sensor sits between the intake manifold and the air filter. It has a couple of wires. One of these wires gets heated due to the power sent from the alternator. When there is a failing alternator in the equation, the MAF sensor wire doesn’t get heated properly.

As a result, the MAF sensor gives a faulty reading to the ECU. The low voltage makes the MAF sensor inaccurately read a lower percentage of air. Voltage spikes can also cause the same issue. High voltage will cause the MAF sensor to read too much air entering the engine.

Further down the line, it can lead to more issues like poor engine performance and rough idling. When the ECU detects the issue with the sensor, it can put the car into limp mode until you fix the problem.

2. Manifold Absolute Pressure Sensor

Just like the MAF sensor, there’s a MAP sensor in your vehicle. It usually sits on the intake manifold of the vehicle and serves a similar purpose to the MAF sensor. It reads the level of the intake air pressure in the engine and passes that information to the ECU.

car manifold absolute pressure sensor

In turn, the ECU has proper air and fuel that would be required to achieve proper combustion in the ignition cylinders. The alternator affects this sensor and when something goes wrong with the alternator, the MAP sensor can give faulty readings.

It leads to the ECU setting up the wrong mixture of air and fuel in the ignition cylinders. The engine either starts running too rich or too lean. In the end, the car can go into limp mode and stay in that condition unless the underlying issue is addressed and fixed.

3. Camshaft Position Sensor

The camshaft position sensor is located in the cylinder head and it has a very important job. It makes sure that the right cylinder is being fired to achieve synchronization with the fuel injectors as well as the coil firing sequence.

car camshaft position sensor

Just like the other sensors in the list, the camshaft position sensor can act erratically when there are voltage spikes in the alternator. It leads to the misfiring of the cylinders and in bad cases, causes the car to go into limp mode.

4. Transmission Gears

There are different types of transmission systems for different types of cars. The transmission is what transfers the energy from the engine to the wheels of the car and gets it to move properly.

car's transmission gears

In cars with manual transmissions, there are cables that help with the transmission. But most modern cars are equipped with automatic transmissions where the person driving the car doesn’t need to take any extra steps to complete the transmission.

Many of these modern cars that have automatic transmissions are electric. And you know what that means. They rely on the alternator and the battery working properly to function well.

When there’s a malfunctioning alternator and the battery can’t make up for the reduction in voltage, the transmission gears won’t be shifted effectively. In this state, the gear changes will be mostly sluggish. In some cases, the gears may not shift at all.

Eventually, the car gets stuck in low gear. The car can get into failure mode such as the limp mode to prevent any further damage to the engine. Though the alternator is the issue here, the ECU of the car thinks that the issue is with the transmission gears.

5. Crankshaft Position Sensor

The crankshaft position sensor is one of the most important sensors in your engine as it has a wide variety of use cases. Besides being used to set the ignition timing in your engine, it also detects the engine RPM and determines the relative engine speed.

car crankshaft position sensor

As you may have guessed by now, it requires receiving the correct voltage from the alternator to work properly. When the crankshaft position sensor doesn’t send accurate readings, the ignition timing of your car will be messed up. As a result, the engine will frequently misfire.

The pistons can be damaged in the worst-case scenario. All these issues can be caused when you have a failing alternator and there’s actually nothing wrong with your crankshaft position sensor.

Getting faulty readings from this sensor will cause the ECU to throw false error codes showing that the sensor has gone bad. When the reading from the crankshaft position sensor goes beyond the normal operating range, it can put the car into limp mode. It’ll be that way unless you fix the problem.

What Causes Voltage Issues In An Alternator

When there are voltage spikes in the alternator, it makes the whole electrical system in your car malfunction. It also affects the sensors that rely on the alternator having a steady voltage to get their readings correctly.

There are a few things that can go wrong with an alternator that can cause these voltage spikes. Here they are:

Faulty Voltage Regulator

An alternator has various components to make sure that the mechanical energy is converted to electric energy. One of the most important parts of doing this job is the voltage regulator.

car faulty voltage regulator

It makes sure that the voltage generated by the alternator stays consistent even when there are spikes in the input level. The voltage regulator is mounted to the back of the alternators in most cars and it controls the spikes to ensure that none of the electrical components in the vehicle are damaged.

When the voltage regulator is working properly, it will generally limit the output voltage to around 14.2 volts. But if it goes bad, it will send a higher voltage and that can cause fluctuations in the readings of different sensors.

If it sends an extremely high voltage, it can even blow fuses as well as the bulbs in your electrical system. You can easily test if you have a bad voltage regulator by testing the alternator with a multimeter.

If the multimeter gives a reading that’s extremely high, that means you most likely have a bad voltage regulator. In some cars, you’d be able to replace just the voltage regulator. But more often than not, you’ll have to replace the entire alternator to fix this issue.

Bad Alternator Grounding

The alternator is grounded to the engine block to prevent the users of the car from getting shocked. The engine block acts as the ground distribution block here. The alternator grounding strap allows the electricity to complete an entire circuit by providing a path of return for it.

car bad alternator grounding

The main parts involved in the completion of this electric circuit alongside your alternator are the battery, engine, and chassis. When the alternator grounding strap is loose, it can immediately cause voltage spikes. It causes the sensors to give readings and the car can even go into limp mode in worse cases.

Check the resistance of the grounding strap with a multimeter. If it’s more than 5 ohms, that’s not a good sign. You can just tighten the grounding strap if it’s loose. But they can often get frayed or corroded. In that case, you’ll have to replace it. It’s pretty cheap and easy to replace. So, you can do it on your own.

What Are The Symptoms Of Limp Mode

You may think of limp mode as a bad thing but in reality, it’s a safety mechanism designed to protect your car from major damage. As many drivers avoid or outright ignore the check engine light, the limp mode is an automatic setting that’s put in place to protect the driver.

The limp mode gets activated when the ECU in your car detects an issue with the engine or transmission. It limits the possibility of doing certain things in your car.

For starters, your car’s engine power will be reduced when it is in limp mode. The RPM can also be limited by lowering the car into limited gears. It can also turn off the AC. The limp mode is your car’s way of telling you that there are issues with your car and something bad could happen if you don’t fix it.

Although there’s no limp mode light in vehicles, you will most certainly be able to feel it when your car has gone into limp mode. Here are the most common symptoms of a vehicle that is operating in limp mode:

Check Engine Light Turned On

More often than not, the limp mode will come on after the driver has ignored the check engine light for a while. When somebody does that, the car switches to limp mode to avoid more damage.

car check engine light turned on

Even if the check engine light wasn’t on before and the limp mode is caused by the sudden failure of any of the engine components, the check engine light will always be on when the car is in limp mode.

But as you’d know, the check engine light can be triggered due to a multitude of different reasons. Make sure you can also confirm the presence of other symptoms before you conclude that your car is in limp mode.

Reduced Power

The limp mode is designed so that you can take your car to the nearest service station as soon as possible rather than stopping the car altogether. The most obvious sign of your vehicle going in limp mode is the power limitations you’ll come across when driving.

The speed will generally be limited to 40 – 45 MPH maximum. The RMP of your car will also be most likely capped at around 3,000 RPM or less. Flooring the gas pedal will also have little to no effect at this stage.

Your car can also get stuck in gears and your transmission will also get messed up when your vehicle is in limp mode. You won’t be able to shift higher than the third gear in most cases.

To make matters worse, your engine may be shaking and trembling. There can also be misfires and the engine may overheat. All these performance issues happening simultaneously are a strong indicator of the limp mode.

Auxiliary Functions Shut Off

While in limp mode, your car only wants to focus on the functions that are necessary. So, it automatically shuts off the auxiliary functions. Things like your windows, AC, and radio could stop working altogether when your vehicle has entered limp mode.

automatically shuts off the car's auxiliary functions

It does so to lower the load on the battery and the alternator and to shift the focus to getting to a service station soon. The risk of any of the electrical components going bad is also reduced when these functions are shut off.

What Else Causes Limp Mode In A Car

The issues that we have already mentioned before which the ECU may read due to an error instead of diagnosing a bad alternator can all cause limp mode in the car. Yes, the MAF sensor, MAP sensor, transmission gears, and crankshaft position sensor can make your car get into limp mode.

If you have a functional alternator, then the limp mode can be triggered by any of these issues. You can replace the sensor you suspect has a higher chance of going bad after doing your diagnosis.

These problems are quite common in causing the limp mode. However, there are a few other issues that are unrelated to the alternator but can still cause your car to go into limp mode. Here are some of these issues in detail:

Oxygen Sensors

Reducing carbon emissions is an important area of focus in most modern cars as we are heading into an era of sustainability. The oxygen sensors in your car help do this job by measuring the amount of unburnt oxygen in the exhaust.

car oxygen sensors

The unburnt oxygen is sent back to the combustion chamber where it gets burned again. It improves the fuel efficiency of your car, makes sure the maximum amount of oxygen is utilized, and helps the ECU figure out the right distribution of fuel for optimum engine performance.

When the oxygen sensors go bad and give faulty readings, the carbon emissions can be increased. You’ll have poor engine performance and poor fuel economy when that happens.

The oxygen sensor isn’t related to the alternator. So, if you suspect having a faulty oxygen sensor and see issues like poor engine performance or rough idling, you can replace it and see if that fixes the issue. If it doesn’t work, keep on diagnosing to get your car out of limp mode.

Low Fluid Levels

You need to make sure that all the fluids required to run your car smoothly are topped up at all times. When you run low on any of them, it can cause issues with the performance of your car. When the issue becomes more serious, your car can go into limp mode.

low fluid levels

We’re talking about all the different types of fluid here including engine oil, coolants, transmission fluid, and steering fluid. Each of them has its unique function and when your engine runs low on any of them, problems happen.

Start by checking the level of all these fluids in your car. Each of these fluids usually has its own dedicated dipstick. If you’re running low on any of them, simply refill them and that should solve the issue.

Besides checking the level of fluid, make sure you also check the quality of the fluids. If they are too contaminated or dirty, they can cause the same issue despite having a sufficient level of them. In that case, drain the existing fluid and top up with the new and fresh fluid.

Wiring Issues

There are so many electronic parts in modern cars. All these parts are connected via wires. Things like water, the acid in your car battery, and getting into accidents can damage these wires. They can also get damaged with time as rodents can get into them and bite them off.

car wiring issues

When the wires are disconnected or cut off due to any of these issues, it wreaks havoc on the sensors. The ECU of your car may come with an error code for a faulty sensor even if it’s just the wires that got disconnected.

Wires that are corroded have the same effect. They are a potential fire hazard and can cause more problems down the line. These damaged wires also lead to engine performance issues and can get the car into limp mode.

Diagnosing faulty wiring can be a difficult job as there are a bunch of wires in your vehicle. If you see an error code for any of the sensors, start from there. Otherwise, you may have to hire a mechanic for this job. When you fix the wiring issue, your car should go out of limp mode if there are no more issues.

Turbo Boost Pressure

This issue will only occur in cars that have turbochargers. Cars that have this function can get more power as the increase in positive pressure allows more air into the engine. More fuel is injected into the cylinders to keep up with the air-fuel ratio. The end result is that your car gets more power.

car turbo boost pressure

However, when there’s something wrong that leads to too low or too high turbo boost pressure, your car can go into limp mode. A malfunctioning turbocharger, boost pressure sensor, boost pipe leak, and boost control valve are all potential reasons behind this issue.

Diagnose which of these parts is causing the issue and replace it to fix your car from being in limp mode. Take it to a mechanic if you need extra help.

Faulty ECU

The ECU is the computer in your car and it works as the brain of the vehicle. Unfortunately, they can also go bad at times. When that happens, it can create all sorts of mess.

car faulty ECU

It may not be able to read the data from the different sensors correctly and send the car into limp mode even when everything is fine. It’s also an expensive component and it’ll cost you a lot to replace it.

Sometimes, you can get lucky and find a used ECU that matches the part number you want and gets it for a fraction of the price. But if you don’t get that, get ready to spend a fortune on replacing your ECU.

Can A Weak Battery Cause Limp Mode

Now you know that a bad alternator and a bunch of other issues can cause the car to get into limp mode. But many people wonder if a bad battery can also be a potential culprit behind this issue.

car weak battery

Remember that a battery doesn’t normally get called upon to support the electrical system of the vehicle when it’s moving. It only happens when the alternator fails to do its job properly.

If the battery gets called upon and starts getting low in charge which takes the car into limp mode, it’s actually the alternator that’s not charging the battery. On the surface, it may seem like the battery is causing the issue. But in reality, the alternator is the main culprit here.


Do alternators fail suddenly?

It’s more common for alternators to go bad over time and give you plenty of warning signals before that happens. However, in rare cases, they can also go bad all of a sudden.

Is it okay to keep driving the car in limp mode?

When the car gets into limp mode, it doesn’t stop but allows you to go to a safe place. If you keep driving your car in this condition, things can get worse over time and the check engine light can start flashing.

Is there a code for a bad alternator?

If you scan your car using an OBD2 scanner and you have a faulty alternator, you’d most commonly see the P0562 error code.

What does it cost to get your car out of limp mode?

It depends on the issue that caused your car to go into limp mode in the first place. Most of the issues can be fixed for less than a few hundred dollars. But you’d be spending in the thousands if you have a bad ECU.

How to bypass the limp mode?

You can bypass the limp mode by disconnecting one of the battery terminals and connecting it again. If the limp mode appears again, it means there’s an issue with your car that you need to fix.


Can a bad alternator cause false codes and limp mode? Hopefully, now you know the answer to that question after completing this comprehensive alternator guide. A faulty alternator can often lead to failing sensors and cause this issue.

But keep in mind that there are plenty of other issues that can get your car into limp mode. Make sure that you diagnose the problem well before attempting any solution. Comment below if you have any questions.

Note: You can also read our ultimate guide on can a bad alternator cause a misfire.

About John M

John contributed as a technical head at an automobile company just 2 years after his post-graduation in Automobile Engineering. He loves to lead a free life, so he left his job & started blogging. Now, he does research on every automotive problem, part & product and seeks a better solution & best products & shares his findings with his readers to help them as well as to minimize their struggle.

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