P1684 Code: Transmission Module Power Recently Disconnected (Manufacturer Code)

What the P1684 code means

This is a manufacturer-specific code, which means its interpretation depends on the brand. There are two well-documented definitions, and you need to identify which one applies before you do anything.

On Chrysler group vehicles, which includes Dodge, Jeep and Ram, the P1684 code is stored when the transmission control module loses its battery power or its ground. The most common cause is not a fault: someone disconnected the battery to replace it, to make a repair or to clear codes.

The module itself stores that event because, when it loses power, it also loses its diagnostic data and its learned values. The code is its way of warning you that this information is no longer available.

On General Motors group vehicles, which includes Buick, Cadillac, Chevrolet and GMC, the same number corresponds to the performance of the transmission module startup temperature sensor, which is a completely different matter.

That is why the first step with this code is always the same: identify the brand. The repair, if any is needed, has nothing in common between one case and the other.

It helps to know why the module bothers to record something that is not a fault. When it loses power, it also loses its adaptation values, which are the fine adjustments it has learned about how that particular transmission behaves: pressures, fill times, clutch wear. Without them it goes back to factory values and takes a few miles to recover them.

The code lets a technician who finds somewhat harsh shifts know the cause is this and not a new problem. It is useful information, not an alarm.

P1684 code symptoms

• Shows up as a history code after the battery is disconnected or replaced.
• The vehicle works completely normally in most cases.
• Check engine light on.
• Stored together with other codes that also set when power was lost.
• Somewhat harsh shifts during the first few miles, while the module relearns.
• If power was lost while driving, a momentary drop into limp mode.
• Somewhat harsh shifts during the first few miles after the battery was disconnected.

Likely causes of the P1684 code

1. Battery recently disconnected or replaced. The most frequent cause on Chrysler group vehicles.
2. Codes cleared with a procedure that cuts power to the module.
3. Loose or sulfated battery terminals, which cause momentary power cuts.
4. Module power fuse blown or making poor contact.
5. Module connector with backed-out pins or corrosion.
6. Loose or rusty module ground.
7. Damaged power wiring.
8. Battery in poor condition, whose voltage drop during cranking leaves the module without power.
9. Module startup temperature sensor, in the interpretation that applies to the General Motors group.

How to diagnose the P1684 code step by step

1. Identify the brand of the vehicle first. The definition changes completely between the Chrysler group and the General Motors group.
2. Ask whether the battery was disconnected recently. On the Chrysler group, that explains the code with no further investigation.
3. Clear the code and see whether it returns. If it does not come back after several ignition cycles, there was no fault.
4. If it returns, check the terminals and the battery condition by measuring voltage at rest and during cranking.
5. Check the module grounds and their voltage drop.
6. Verify the power fuse of the transmission module.
7. Inspect the module connector for backed-out pins and corrosion.
8. Look for other codes, because the module lost its data and some useful information may be gone.
9. Treat later harsh shifts as part of the relearn, not a fault, if the code appeared after the battery was disconnected.

How to fix the P1684 code

• Clear the code with the scan tool and drive for a few days. In most Chrysler group cases that closes the matter.
• Clean and tighten the battery terminals if they are sulfated or loose.
• Replace the battery if its voltage drops below the acceptable level during cranking.
• Replace the module power fuse and find out why it blew.
• Clean or replace the module connector and protect it with dielectric grease.
• Correct the module grounds down to bare metal.
• Repair damaged power wiring.
• Explain to the customer that the first few miles may feel somewhat harsh while the module recovers its adaptation values.

After you clear the code, the module relearns its adaptation values over the first few miles. Somewhat harsh shifts during that period are normal and smooth out on their own.

Technical reference data

• Definition on the Chrysler group: the transmission module lost its battery power or its ground.
• Definition on the General Motors group: performance of the transmission module startup temperature sensor.
• P1xxx codes are reserved for each manufacturer: their meaning is not universal.
• Minimum battery voltage during cranking to avoid power codes: above 9.5 volts.
• When power is lost, the module loses its adaptation values and learns them again while driving.
• When power is lost, the module loses its learned pressures, fill times and wear adjustments.

Brand-specific notes

If you just replaced the battery on a Chrysler group vehicle and this code appeared, do not look for a fault. It is expected behavior, it clears with the scan tool and it does not come back. Many drivers arrive at the shop worried about a code that only describes what they did themselves.

The exception that does deserve attention is the code coming back when nobody has touched the battery. In that case there are real power cuts, and the suspects are the terminals, the battery itself, the fuse and the module grounds.

P1684 on Chrysler Group Vehicles

On Dodge, Jeep and Ram, the code is stored when the transmission control module loses its battery power or its ground, most often after a battery disconnect.

P1684 on General Motors Group Vehicles

On Buick, Cadillac, Chevrolet and GMC, the same number refers to the performance of the transmission module startup temperature sensor.

Frequently asked questions about P1684