P0620 Generator (alternator) control circuit fault: the engine computer sees a problem in the circuit that controls how much the alternator charges

Generator control circuit malfunction / alternator control circuit

P0620 in short

P0620 means the engine computer found a fault in the circuit that controls the alternator. Charging can then run too high or too low. Wiring and connector damage, a bad battery, or a failed alternator are the usual suspects. Get it checked soon, because loss of charging can lead to stalling.

Delaware's inspection program lists P0620 as a generator control circuit malfunction, and TroubleCodes.net describes it as the computer detecting a fault in the circuits that control the alternator's operation or output current.

If the control signal is wrong, system voltage can run abnormally high or low. TroubleCodes.net lists the warning light, hard starting or no-start, rough or fluctuating idle and harsh automatic shifts as possible effects, with power loss or shutdown in severe cases.

The same page names damaged or corroded wiring, a defective alternator or its control module, battery defects, voltage spikes from improper jump-starting and CAN bus wiring damage. It says isolating faults in the bus can need professional equipment.

Most likely causes

These are general causes for this code across vehicles. None is specific to one make or model.

  1. Damaged, corroded or disconnected alternator wiring or connectors

    Common general cause
    Why it ranks here
    TroubleCodes.net lists wiring and connector faults among the causes, and they are the cheapest to check first.
    What would change this
    Rises if the harness shows damage or corrosion or the code returns when a connector is moved. Falls if wiring and connectors test good.

  2. Defective alternator or alternator control module

    Common general cause
    Why it ranks here
    TroubleCodes.net says replacing the alternator resolves many cases where testing confirms a defect, so it ranks after wiring and battery checks.
    What would change this
    Rises if wiring and battery test good but charging output is wrong. Falls if the code returns after a new alternator.

  3. Battery defect or internal short, or a voltage spike from a bad jump-start

    Possible — needs testing
    Why it ranks here
    The reference page lists battery defects and improper jump-starting as causes. It gives no figures, so this ranks below the main causes.
    What would change this
    Rises if the code appeared after a jump-start or the battery fails a load test. Falls if the battery tests good.

  4. Damaged CAN bus wiring

    Possible — needs testing
    Why it ranks here
    The page lists CAN bus wiring damage and says it needs professional-grade equipment to isolate, so it comes last.
    What would change this
    Rises if other communication codes are stored. Falls if no other module codes are present.

What to test first

Start at step 1. Each result tells you what it means and where to go next.

  1. Read all stored codes, including other modules if your scan tool can, and measure battery voltage with the engine off and then running.

    Low riskTool: OBD-II scan tool, multimeter

    • IfVoltage stays near battery level (about 12 V) with the engine running

      It meansThe system is not charging. Check the battery and wiring next.

      Go to step 2

    • IfVoltage is in the normal charging range and the code is stored but not current

      It meansThe fault may be intermittent. Check the wiring for movement-related faults.

      Go to step 2

  2. Test the battery condition, look over the alternator wiring and connectors for damage or corrosion, and check fuses for continuity.

    Low riskTool: multimeter, battery tester

  3. Have a shop test wiring resistance and the alternator against the maker's specifications, and check the CAN bus if other codes are stored.

    Best left to a shopTool: scan tool, multimeter

    Warning:Alternator output terminals carry battery current and can arc if shorted; testing near the belt and pulley risks injury. Leave live testing to a shop.

Symptoms

  • Charging or battery warning light on
  • Abnormally high or low system voltage
  • Hard starting or no-start
  • Rough or fluctuating idle, or harsh automatic shifts
  • Power loss or engine shutdown (severe cases) Gets more serious: stop and have it checked.

How to fix

  1. With the engine off, look over the alternator wiring and connectors for damage, corrosion or a loose plug.

  2. Have the battery and charging system tested, and ask a shop about the alternator if the wiring is fine.

On your vehicle

Ford

  • 2003–2004 · Expedition

    Supported by vehicle data

    Ford bulletin 17738 covers an intermittently lit charging system warning indicator with possible P0620 on the 2003-2004 Expedition. Whether it is still current is not verified.

  • 17738Status unverified

    Charging system warning indicator intermittently on, possible DTC P0620

    2003–2004expedition · issued · NHTSA 10008238

    Ford bulletin for a charging system warning indicator that comes on intermittently with possible DTC P0620. The summary does not give the repair.

    We couldn't confirm whether this bulletin is still current, so it doesn't count as confirmation.

Buick

  • 2000–2001 · LeSabre

    Supported by vehicle data

    GM bulletin 000604055 describes intermittent or no start, a low voltmeter reading or check charge system message, and possible P0620 on the 2000-2001 LeSabre. Whether it is still current is not verified.

  • 000604055 (buick)Status unverified

    No start or intermittent start, low voltmeter or check charge message, P0620 may set

    2000–2001lesabre · issued · NHTSA 622729

    GM bulletin: customers may report a no-start or intermittent start, a voltmeter reading in the low red zone or a check charge system message in the driver information center, and DTC P0620 may be set. The summary does not give the repair.

    We couldn't confirm whether this bulletin is still current, so it doesn't count as confirmation.

Pontiac

  • 2000–2001 · Bonneville

    Supported by vehicle data

    GM bulletin 000604055 describes intermittent or no start, a low voltmeter reading or check charge system message, and possible P0620 on the 2000-2001 Bonneville. Whether it is still current is not verified.

  • 000604055 (pontiac)Status unverified

    No start or intermittent start, low voltmeter or check charge message, P0620 may set

    2000–2001bonneville · issued · NHTSA 622729

    GM bulletin: customers may report a no-start or intermittent start, a voltmeter reading in the low red zone or a check charge system message in the driver information center, and DTC P0620 may be set. The summary does not give the repair.

    We couldn't confirm whether this bulletin is still current, so it doesn't count as confirmation.

No curated relationships for P0620 yet.

All Charging and starting codes

P0620 questions

Can a jump-start cause P0620?

TroubleCodes.net lists voltage spikes from improper jump-starting as a possible cause.

Sources and review

  1. On-Board Diagnostic Trouble Codes (generic code title list) — Delaware Division of Motor Vehicles
  2. P0620 trouble code page — TroubleCodes.net
  3. 17738: Charging system warning indicator intermittently on, possible DTC P0620 — Ford
  4. 000604055: No start or intermittent start, low voltmeter or check charge message, P0620 may set — General Motors
  5. 000604055: No start or intermittent start, low voltmeter or check charge message, P0620 may set — General Motors
Last reviewed
Written by
Saurabh K.
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