P1820
Internal Mode Switch A Circuit Low Voltage
What it means
P1820. Internal Mode Switch A Circuit Low Voltage. This Chevy-enhanced code applies to Impala, Malibu, Monte Carlo, Silverado, Sierra.
Findings, check these first
- Faulty internal mode switch inside the transmission
- Misadjusted or damaged shift cable holding the switch between detents
- Short to ground in IMS signal A circuit
- Worn manual valve linkage or detent assembly
- Corroded AT inline connector
Standard procedure
- Observe IMS A/B/C/P parameters in each shifter detent with a Tech2 scan tool
- Verify shift cable adjustment before opening the transmission
- Test IMS signal A circuit for a short to ground
- Inspect the manual valve link and IMS for wear or contamination
- Replace the internal mode switch if circuits and adjustment test normal
Look up another code
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Sources and review
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Last updated 2026-09-13.
Report a correction for P1820. Include your vehicle, the issue and a source if you have one.