Battery and alternator via OBD-II: P0562, P0620, diagnose in 5 minutes

A weak winter battery → car won’t start, everyone knows what to do. But alternator problems get diagnosed too late — after the battery is worn out 6 months early, or after “overcharge” fried an ECU. OBD-II sees both sides through one parameter — System Voltage. Below: how to interpret it.

What OBD-II sees

Standard PID 0x42 (Control module voltage) — ECU supply voltage. That’s effectively battery voltage since the ECU is wired directly.

Norm with engine running (alternator working):

• 13.8 - 14.4V — ideal
• 14.5 - 14.7V — upper limit
• 13.5 V and lower — alternator not holding
• 15.0 V and higher — voltage regulator dead, overcharge

Norm with engine off (battery alone):

• 12.6 V and higher — charged
• 12.0 V — half state of charge
• 11.8 V and lower — discharged, “borderline start”

Typical DTCs

Code	Meaning	Cause
P0562	System voltage below spec	Weak alternator, old battery, broken wire
P0563	System voltage above spec	Voltage regulator dead, short
P0620	Alternator control fault	Wire/relay to alternator field
P0621 / P0622	Field winding fault	Internal alternator failure
P0625 / P0626	Field circuit errors	Voltage regulator / aging alternator

How to diagnose in 5 minutes

1. Engine off → read Control module voltage (0x42)
   • Norm: 12.4 V after 5-10 min
   • < 12.0 V → battery needs charge or replacement
2. Start the engine → read again
   • Norm: 14.0 V ±0.4
   • < 13.5 V → alternator weak (check belt + brushes)

   • 14.7 V → regulator

3. Turn on loads (headlights, rear defrost, A/C) → read again
   • Norm: shouldn’t drop below 13.5 V
   • Drop to 12.5 V or lower → alternator can’t hold under load

Classic patterns

”Battery doesn’t hold” (actually — weak alternator)

Symptom: morning start is fine, but after 3-4 days sitting — no start.

OBD reads:

• Engine off (after 30 min sitting): 12.2 V
• Engine on: 13.4 V
• Under load: 12.6 V

Diagnosis: alternator doesn’t fully charge. Battery drains in 2-3 days. Replace diode bridge ($10-25 (US benchmark $40-100)) or whole alternator ($150-400).

”Overcharge” — the silent electronics killer

Symptom: everything looks fine, but headlights burn very bright, bulbs blow often, radio died.

OBD reads:

• Engine on: 15.4 V (!!!)

Diagnosis: voltage regulator. Urgent — otherwise ECU, BCM or TCU will burn ($500-1500 each). Replace regulator or whole alternator.

”Battery dies in 2 years”

Symptom: battery replaced every 18-24 months instead of 5-7 years.

OBD reads:

• Engine on: 14.8-15.0 V (slightly above spec but not critical)

Diagnosis: systematic “overboiling” of the battery — regulator slightly above spec, electrolyte evaporates faster. Replace regulator.

”Won’t start in cold”

OBD reads:

• Engine off at -15°C (5°F): 11.8 V

Diagnosis: battery on the edge. Capacity drops 30-50% in cold. Charge or replace.

What AI agent adds

Our Prediction Agent tracks the voltage trend via daily automatic log:

• Notices when V slowly drops (battery aging)
• Notices when V oscillates non-specifically (regulator flaky)
• Warns 1-2 months before the “won’t start” surprise

Prevention

• Annually — battery stress test at a shop ($10)
• Monthly — check OBD voltage at start (5 minutes)
• Long sits — turn off head unit, don’t leave USB chargers in
• EFB/AGM battery for start-stop systems — don’t replace with plain lead-acid (will fry the alternator)

Try it on the free tier — pair an ELM327 adapter and read PID 0x42. Trend monitoring — on paid tier.