What causes a SPN 1661 code?

SPN 1661 can be triggered by several issues — see the verified probable causes and step-by-step diagnostics shared by mechanics and owners on OBDIY.

Can I drive with a SPN 1661 code?

Drivability depends on the severity. Check the severity rating and verified driver reports on this page before continuing to drive with SPN 1661 active.

How do I fix SPN 1661?

Follow the step-by-step diagnostic on this page, then read the verified fixes contributed by other Heavy Truck owners — the answer marked "Verified Cause" worked for the original poster.

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J1939 SPN

SPN 1661

Transmission Mode 2 Indicator

What it means

Circuit fault for the visual indicator representing the second transmission mode state.

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