OBD Guide
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P1840

Internal Mode Switch 'S' Circuit Low Voltage

P1840 is an OBD-II diagnostic trouble code meaning: Internal Mode Switch 'S' Circuit Low Voltage. Common causes: malfunction of the internal mode switch, open or short circuit in switch circuit. Estimated repair cost: $17–56.

Severity
⚠️ Medium
Can you drive?
Limited driving only, diagnose soon
Approx. repair cost
$17–56 (est.)

Symptoms

  • Incorrect display of the current transmission mode
  • Gear shifting problems
  • Check engine light comes on
  • The gearbox may go into emergency mode
  • Difficulty starting the engine in some modes

Causes

  • Malfunction of the internal mode switch
  • Open or short circuit in switch circuit
  • Poor contact in connectors
  • Problems with the transmission control unit
  • Switch contact corrosion

How to Fix

  1. Check the integrity of the mode switch circuit
  2. Inspect and clean switch contacts
  3. Check voltage at switch
  4. If necessary, replace the mode switch
  5. Check and reflash the transmission control unit

Related codes

P1839P1841P0705

FAQ

What does the P1840 code mean?

P1840 is an OBD-II diagnostic trouble code that indicates: Internal Mode Switch 'S' Circuit Low Voltage

What causes a P1840 error code?

The most common causes of P1840 include: Malfunction of the internal mode switch; Open or short circuit in switch circuit; Poor contact in connectors; Problems with the transmission control unit.

How do I fix a P1840 diagnostic trouble code?

To fix P1840: Check the integrity of the mode switch circuit. Inspect and clean switch contacts. Check voltage at switch. For a complete diagnosis, use an OBD-II scanner.

Is it safe to drive with P1840?

Limited driving only, diagnose soon

How much does it cost to fix P1840?

The estimated repair cost for P1840 is $17–56. Actual cost depends on your vehicle, location, and labor rates.

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See also: Russian version · NationStat