P1604
Severity: MediumCode Word Unregestered
Last updated: July 2026
System
Powertrain
Subsystem
Computer Output Circuit
Code Type
Manufacturer-specific
Code Word Unregestered
Last updated: July 2026
P1604 is a manufacturer-specific powertrain diagnostic trouble code. It is defined as "Code Word Unregestered". It falls under the computer output circuit area of the vehicle’s powertrain system, and its exact meaning can vary between manufacturers.
The onboard diagnostic system sets P1604 when it detects a condition matching "code word unregestered" in the computer output circuit circuit or system. When the fault is confirmed, the module stores the code and, in most cases, turns on a warning light.
This code is generally moderate. The vehicle is often still drivable, but the underlying fault should be diagnosed before it leads to further problems.
P1604 means "Code Word Unregestered". It is a manufacturer-specific powertrain trouble code in the computer output circuit area.
The vehicle is often still drivable with this code, but you should have it diagnosed soon so a moderate fault does not turn into a larger one.
There is no single fix. Start by confirming the code with a scan tool, then work through the likely causes and checks above — inspecting wiring, connectors and the relevant components before replacing any parts. On your specific make, confirm the diagnosis before repairs.
You can clear the code with an OBD-II scan tool, but if the underlying fault is still present it will usually return. Clearing without repair is only useful to confirm whether the fault is active.
This information is provided for general reference only. It is not repair advice and may not apply to every make or model. Manufacturer-specific codes can differ. Always confirm a diagnosis with a scan tool and, where needed, a qualified technician before replacing parts.