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P1638 Manufacturer-specific Basic entry

KAM Voltage Too Low

Updated 2026-10-02 · Manufacturer-specific powertrain

Severity
Medium
Typical repair cost
80 – 900 EUR
System
Powertrain

This is a manufacturer-specific code. Its exact meaning can differ between makes; the definition below follows the most common usage.

What it means

P1638 is stored when the measured value of the KAM Voltage Too stays below the target that the engine control module is trying to reach, despite the control output being at its limit.

This entry is compiled automatically from the standard definition and typical diagnostic practice for this type of fault. A detailed, reviewed article is in preparation.

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Symptoms

  • Check engine light on
  • Rough or unstable idle
  • Hesitation or poor acceleration
  • Increased fuel consumption
  • Possible limp mode with reduced power

Common causes

  1. Weak pump or restricted filter
  2. Leaking or stuck pressure regulator or control valve
  3. Leak in the pressure circuit or hoses
  4. Faulty pressure sensor giving a false reading
  5. Mechanical restriction or blockage

How to diagnose

  1. Compare the commanded and actual KAM Voltage Too in live data at idle and under load
  2. Measure the pressure with a mechanical gauge to validate the sensor
  3. Inspect filters, hoses and the regulator or control valve
  4. Check pump supply voltage and current draw
  5. Replace the faulty component and recheck under load

Frequently asked questions

Can I keep driving with P1638?

Usually yes for a short time, but expect reduced performance or higher fuel use. Have it diagnosed within a few days.

Will clearing P1638 fix it?

No. Clearing only erases the stored code; it returns as soon as the monitor runs again unless the cause is repaired.

Related codes

P1636P1637P1639P1640

Nearby codes in this group