Blown bulb or failed lamp/LED in the instrument cluster; Open circuit, chafed wiring or short to earth or voltage in the lamp control wire; Corroded or loose connector at the instrument cluster or powertrain control module; Fault in the instrument cluster circuit board or lamp driver; Internal lamp driver fault in the powertrain control module; Previous wiring repair or aftermarket accessory interfering with the lamp circuit.

P1664

1-4 Upshift Lamp Control Circuit.

Buick
Powertrain
Manufacturer code
Low severity

How this code is built

P: SystemPowertrain
1: Defined byThe manufacturer
6: AreaComputer and output circuits
64: FaultThe specific fault in that area

What it means

P1664 on a Buick means the powertrain control module has detected a fault in the control circuit for the 1-4 upshift lamp. This lamp, sometimes called the 'Shift Now' light, tells the driver on certain manual gearbox models when to move from first to fourth for fuel economy. The module commands the lamp on or off and sets the code when the circuit voltage does not match the command, which points to an open circuit, a short or a driver fault.

Symptoms

Malfunction indicator lamp or service engine light on
1-4 upshift lamp stays on all the time
1-4 upshift lamp never lights when it should
Instrument cluster lamp flickers or behaves erratically
No change in driveability or gearbox operation

Possible causes

Blown bulb or failed lamp/LED in the instrument clusterlikely
Open circuit, chafed wiring or short to earth or voltage in the lamp control wirelikely
Corroded or loose connector at the instrument cluster or powertrain control modulepossible
Fault in the instrument cluster circuit board or lamp driverpossible
Internal lamp driver fault in the powertrain control modulerare
Previous wiring repair or aftermarket accessory interfering with the lamp circuitrare

How to diagnose

Read all stored and pending codes and note the freeze frame data, then check for other faults that need dealing with first
Watch the instrument cluster bulb check at ignition on and see whether the 1-4 lamp lights briefly
Check whether the lamp is permanently on, permanently off or intermittent, as this helps separate a short from an open circuit
Inspect the wiring between the cluster and the powertrain control module for chafing, damage and past repairs, and check the connectors for corrosion and loose pins
Check the lamp supply voltage and the control wire voltage against the wiring diagram for the specific model and year
Test the control wire for continuity and for shorts to earth or voltage with the connectors unplugged
Command the lamp on and off with a bidirectional scan function if the model supports it, to see if the module output responds
Clear the code after repair and road test to confirm it does not return

How to fix

Replace the failed bulb or lamp unit in the instrument cluster
Repair or replace damaged wiring in the lamp control circuit
Clean, repair or replace corroded or loose connector terminals
Repair or replace the instrument cluster if the lamp driver or circuit board is faulty
Replace or repair the powertrain control module only after wiring and cluster have been proven good, and only if the model procedure requires it

Can I keep driving?

Low severity

The car can normally be driven as the fault only affects a driver shift indicator and not gearbox operation. Book it in at your convenience, and keep in mind the warning light may hide any new fault that appears.

Guide written by AI from the code's definition. Confirm with a diagnosis before replacing parts.