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Severity: Moderate Code type: Generic (all makes)

P0625 Code: Generator Field/F Terminal Circuit Low

The PCM sees the alternator's field-monitor signal stuck low – usually the regulator, its plug or the wire.

At a glance

Severity: 3/5 – Moderate3/5

Severity: Moderate

Meaning
Generator Field/F Terminal Circuit Low
System
ECU & Outputs · Powertrain
Safe to drive?
With caution
Typical repair cost
$50–$1,500
DIY difficulty
Moderate

On many GM, Ford and some Asian designs the voltage regulator inside the alternator reports how hard it is working over a field-monitor wire (GM calls it the F terminal, Ford uses a GEN MON circuit). The signal is a duty cycle, roughly the percentage of time the rotor field is switched on. P0625 means the PCM reads that signal as missing or stuck near zero, which is what it sees when the wire is broken or grounded, or when the regulator has stopped exciting the field altogether. If the battery voltage drops at the same time, the alternator has most likely quit charging; if running voltage is still normal, suspect the monitor wire, the plug on the alternator or the regulator's reporting output. The exact monitoring logic differs slightly between manufacturers.

Can I keep driving? If the battery reads above about 13.5 V with the engine running, you can drive it to a shop. If the charging light is on and voltage is sliding toward 12 V, the car is living off the battery and can stall within an hour – avoid night driving and highway trips.

How to read P0625

How to read P0625P10263254
  1. P Powertrain
  2. 0 Generic (SAE)
  3. 6 Computer & outputs
  4. 25 Specific fault

Symptoms

  • Battery light or a 'Service battery charging system' message
  • Check engine light, on some models only the charging warning
  • Dim headlights and a slow blower motor at idle
  • Voltmeter or voltage display reads low and keeps dropping on long drives
  • Engine stalls or won't restart after a drive because the battery ran flat
  • Idle speed no longer rises when heavy loads such as the rear defroster switch on

Possible causes

Ranked by how often each cause is behind this code (editorial estimate).

  1. Worn brushes or a failed voltage regulator inside the alternator 35%
  2. Corroded, loose or oil-soaked regulator plug on the back of the alternator 22%
  3. Field-monitor wire broken or chafed through to ground along the engine 18%
  4. Blown fuse on the regulator's ignition or sense feed, where the design uses one 10%
  5. Replacement alternator with the wrong regulator version for this PCM 9%
  6. Failed field-monitor input inside the PCM 6%

Diagnosis step by step

Diagnostic flow: P06251Split the problem with a voltmeter2Watch the field duty cycle on a scantool3Inspect the regulator connector4Check the monitor wire end to end5Test or replace the alternator6Clear and verify under load

  1. Split the problem with a voltmeter

    Battery at rest should show 12.4–12.7 V; at 2,000 rpm with headlights and blower on, 13.5–14.8 V. Low running voltage means the alternator isn't charging; normal running voltage with P0625 stored points to the monitor circuit.

    Tools: Multimeter

  2. Watch the field duty cycle on a scan tool

    Look for a PID such as 'generator F terminal' or 'field duty cycle'. At idle with no loads it typically sits around 15–40 % and should climb clearly when you switch on high beams, blower and defroster. A reading frozen at 0 % while the battery is charging confirms a lost signal rather than a dead alternator.

    Tools: OBD-II scanner

  3. Inspect the regulator connector

    Disconnect the battery negative first – the alternator output stud is always live. Unplug the small regulator connector and look for green corrosion, spread terminals or engine oil wicking in from a leaking valve cover or oil filter housing.

    Tools: Flashlight, Screwdriver

  4. Check the monitor wire end to end

    With the alternator plug and the PCM connector both unplugged, the field-monitor wire should measure under about 1 ohm end to end and show no continuity to ground. Pay attention to spots where the harness rubs on brackets or crosses the exhaust manifold.

    Tools: Multimeter

  5. Test or replace the alternator

    If charging voltage is low or the regulator doesn't report while the wiring is good, have the alternator tested or replace it. Match the part to the original regulator version and connector; a generic unit with a different regulator can leave the code in place.

    Tools: Torque wrench

  6. Clear and verify under load

    Clear the code, start the engine and switch loads on and off while watching the duty cycle and voltage. The percentage should follow the loads and the code should stay away through a full drive cycle.

    Tools: OBD-II scanner

Where the parts are

Charging system and control modules – P06251234567
Charging system and control modules
  1. 1Battery
  2. 2Alternator
  3. 3Engine computer (ECM)
  4. 4CAN bus
  5. 5Transmission
  6. 6Wheel speed sensor
  7. 7High-voltage battery

Repair options & costs

RepairDIY?Estimated cost
Clean or replace the alternator connector pigtail Yes $50–$160
Repair chafed field-monitor wire No $100–$260
Replace alternator (remanufactured, installed) No $350–$800
Replace a battery that was run flat Yes $150–$300
PCM repair or replacement with programming No $600–$1,500

Rough US estimates for parts and labor; shop rates vary by region. Not a quote.

Notes by brand

Chevrolet

GM trucks and SUVs use the F-terminal duty cycle in their Regulated Voltage Control strategy, so a field-monitor fault usually comes with a 'Service Battery Charging System' message. The alternator itself and its small connector are the usual finds; check the wire before suspecting the ECM.

Emissions / smog test impact

Depends

Some vehicles show only the battery light for P0625, others also turn on the check engine light. With the check engine light on the car fails an OBD emissions test; with only the charging warning it normally passes, but fix it anyway before the battery lets you down.

Frequently asked questions

The parts store says my alternator tests fine – why do I still have P0625?

A bench or on-car output test checks whether the alternator charges, not whether it reports its field duty to the PCM. With good charging voltage, the monitor wire or the connector on the alternator is the more likely fault.

Can I ignore P0625 if the voltage looks normal?

It won't strand you immediately, but the PCM loses its view of alternator load and falls back to a fixed charging strategy. Idle compensation, battery charging and fuel-saving voltage control suffer, and if the regulator is starting to fail you'll get no early warning.

What's the difference between P0625 and P0626?

P0625 means the field signal is missing or stuck near zero – often a broken wire or a regulator that stopped working. P0626 means the signal is stuck high, as if the alternator were at full output all the time, which points to a heavy electrical load, a bad battery or a short to voltage.

Will a new battery fix it?

No. A new battery only helps if the old one was drained by the charging fault. Replace or repair the alternator circuit first, or the new battery will be run down the same way.

Sources