P1684 Code – Meaning, Causes, Symptoms (& How to Fix it)

engine control unit e1623003106373

You probably freaked out the moment you noticed a P1684 error code.

But it shouldn’t be too much to worry about. The P1684 error code means that the power supply to the Powertrain Control Module (PCM) is faulty. One of the reasons for this is a weak battery.

This can often be caused by replacing the car battery or removing the battery cables.

You must also check the wiring around the PCM. The error code does not apply to all types of cars but is limited to Honda, Acura, Chrysler, Dodge, Subaru, and Jeep.

What could cause the P1684 error code?

There are several different things that may cause the P1684 code to appear on your scanner. Here are some of the most common causes.

1. The Battery/Cables Removed

flashing check engine light

The P1684 code is often displayed after you have removed and reinstalled the car battery. The code usually means: The battery has been disconnected within the last 50 starts. This means that when the problem is solved, the code should disappear by itself after 50 successful cycles.

The first thing you notice with the error code is that the engine light comes on. The voltage going to the PCM or TCM is low, hence the error code. The PCM interprets this low voltage as lack of power. Since P1684 is not exclusively due to low battery power, this could also mean that the PCM connectors are defective. You may need to check the wiring and repair loose wires.

2. Defective PCM

engine control unit inside

A defective P1684 could also mean that the PCM is defective. This could mean that you will have to purchase a new PCM. The PCM is the central computer system and is responsible for the function of most vehicle components. If the PCM does not work, there will be interference between the engine and the transmission.

The PCM receives important engine data from various parts; it interprets the signals and sends instructions. This is done via several sensors located in the car. The PCM also plays a crucial role in the air-fuel mixture.

If the PCM is defective, your car may not start at all because it affects the ignition timing of the spark plugs. Incorrect fuel mixtures result in more money being spent on fuel, and this leads to more emissions.

If you have a problem with the PCM, you will find that you get ‘check engine’ fault codes. One of the things that cause the PCM to become faulty is contact with water elements. Even a single drop of water can ruin the entire system.

3. Overload Voltage

PCM devices are also susceptible to voltage overloads. This can come from the circuit or from the solenoid. A good mechanic can diagnose the problem for you, but if the damage is due to water problems, you will be forced to buy a new unit as it is irreparable.

Many people have trouble distinguishing between PCM and ECM. In fact, although the two acronyms are used interchangeably, they are two different components. The ECM works with the cooling system, the exhaust intake and uses the data set to control the fuel injection timing, camshaft and throttle valve positioning, and fuel injection timing. The PCM controls the functions of the ECM.

Other causes of P1684

  • Corrosion of the battery terminals can lead to poor connections of the battery terminals.
  • If the battery has been disconnected while the engine is still running or when the ignition has been switched off.
  • Missing PCM information for correct reprogramming when replacing the battery.
  • Damaged wires between the control modules and the CAN bus system.

What are the symptoms of P1684 error code?

Check Engine Light

The first symptom of P1684 is the check engine light coming on. This may mean that you are experiencing further engine problems. As mentioned above, the problem is not that the battery is not well connected; although this could be the problem, it could be the first sign of further error codes.

The PCM controls many engine functions such as fuel ignition and air-fuel mixture. The simple solution of disconnecting the battery to remove the error code is not recommended until you have carefully diagnosed all potential problems.

Loss of KAM (Memory)

KAM memory

All information concerning the PCM is stored on the KAM chip. Information about idle speed, air-fuel mixture, and other factory settings is stored here. When you disconnect the battery, the chip returns to the default settings. If your car has more problems, you will find that the P1684 comes back.

The PCM performs a number of self-assessment tests, including the emission test. If you try to delete the error code before your emission tests, you will be charged with evading the law. The PCM controls the TCM, and if it is not working properly, you will have problems with loss of performance.

The reason for this is that disconnecting the PCM resets the transmission settings and your car has to relearn how to do it again. If the code is restarted after 50 cycles, you will have to wait until the vehicle has driven 50 to 100 miles for the TCM to relearn the transmission settings.

A power failure in the PCM can affect the Body Control Module (BCM), and if it is not corrected, it can cause the BCM to stop working. A failure of the BCM can cause other vehicle modules to shut down. The power failure may also cause your steering wheel angle sensors to stop working.

This can later lead to a problem with the stability control and you could be involved in an accident due to the loss of control. The steering sensor can be connected to the anti-theft system, and if the system fails, unauthorized intrusion into your car may occur.

How to fix the P1684 error

Since the error code can be caused in various ways, it is important to diagnose the cause quickly. If the battery power is low, you may need to recharge it and check for loose terminals. The problem could also be that the TCM or PCM is defective. In this case, you may need to buy a new TCM or PCM.


Error code P1684 does not always mean that you have bad battery connections. It could be the first symptom of further engine problems. The PCM unit may be faulty due to circuit problems.

It is important that you identify any other engine problems that may be connected to the PCM. Disconnecting the battery can erase all stored PCM memory and you will have problems with transmission settings or emission testing. Record all error codes and consult your manufacturer’s manual for possible causes.

Written by:

Magnus is the owner and main author of MechanicBase. He has been working as a mechanic for over 10 years, and the majority of them specialized in advanced diagnostics and troubleshooting. He has also been a motorsport (drifting) driver for over 5 years.