Symptom
Lights flickering or dimming
Headlights or dashboard lighting that pulse, flicker or dim — often in time with the engine, or when a heavy load like the heated rear window switches on. This is a charging or supply symptom rather than a lighting one: the lamps are simply the most visible thing running on an unsteady voltage. Worth noticing whether it changes with engine speed, which points at the alternator's regulation, or with load, which points at a connection that cannot carry current. A poor earth strap produces both and is the cheapest thing on the list to check.
Also described as: flickering lights, dimming headlights, lights pulsing, dash lights flickering, headlights dim at idle, چراغها کم و زیاد میشوند, نور چراغ نوسان دارد, luces parpadean, luces bajan intensidad, luces titilan, الأنوار ترمش, الإضاءة تخف وتقوى, اللمبات تضعف
What this usually means
This is a charging or supply symptom rather than a lighting one: the lamps are simply the most visible thing running on an unsteady voltage. Nothing is wrong with the bulbs, and replacing them changes nothing.
Worth noticing whether it changes with engine speed, which points at the alternator's regulation, or with load, which points at a connection that cannot carry current. That one observation splits the diagnosis before any instrument comes out.
A poor earth strap produces both and is the cheapest thing on the list to check. An earth that has corroded raises resistance in the return path, so voltage sags whenever current rises — with engine speed as the alternator's output varies, and with load as more current is drawn.
Measure the drop rather than the voltage. A resting reading looks fine on a bad joint, because a meter's own test current is milliamps; voltage-drop testing under load across the battery leads, the main earths and the engine-to-body strap is what finds a joint that only fails when current flows.
And test AC ripple, not just DC output. A rectifier with a failed diode still produces roughly the right voltage while injecting ripple that makes lamps pulse in time with the engine — more than about 0.5 V AC at the alternator's output under load points there, and it is invisible to a voltmeter reading DC.
On a car with LED lighting, flicker can also be a driver module rather than supply — but that affects one lamp rather than all of them, which is the discrimination.
Working through it
| What you find | Likely cause | How to confirm it | What fixes it |
|---|---|---|---|
| Pulses in time with engine speed | The alternator's regulation, or AC ripple from failed rectifier diodes | Measure charging voltage and AC ripple at the alternator output under load: more than about 0.5 V AC points at the diodes. | Renew the alternator or its rectifier. Ripple is invisible on a DC reading, which is why it is missed. |
| Dims when a heavy load switches on | A connection that cannot carry current — a terminal, an earth strap or a main cable | Voltage-drop test under load rather than checking continuity at rest. Test the earth path as well as the positive. | Clean and remake the joint or renew the cable, then re-measure to prove the drop has gone. |
| Both — pulses with revs and dims with load | A poor earth strap, which produces both and is the cheapest thing to check | Voltage-drop the engine-to-body strap under load. This is the single most productive measurement here. | Clean the contact faces to bare metal and remake, or renew the strap. |
| Flicker on one lamp only | That lamp's driver module or connection rather than the vehicle's supply | Scope matters: all lamps is supply, one lamp is that circuit. | Repair the driver or the connection for that lamp. |
| Flicker with other unrelated warnings appearing | The supply fault affecting modules as well as lamps — one root cause | Test battery and charging before diagnosing any of the warnings individually. | Repair the supply, clear the codes and re-scan. Only what returns is real. |
| Started after battery or alternator work | A terminal or earth not remade properly | Retrace the work and re-torque what was disturbed. | Remake the connections. Cheapest cause and it dates itself precisely. |
-
Pulses in time with engine speed
- Likely cause
- The alternator's regulation, or AC ripple from failed rectifier diodes
- How to confirm it
- Measure charging voltage and AC ripple at the alternator output under load: more than about 0.5 V AC points at the diodes.
- What fixes it
- Renew the alternator or its rectifier. Ripple is invisible on a DC reading, which is why it is missed.
-
Dims when a heavy load switches on
- Likely cause
- A connection that cannot carry current — a terminal, an earth strap or a main cable
- How to confirm it
- Voltage-drop test under load rather than checking continuity at rest. Test the earth path as well as the positive.
- What fixes it
- Clean and remake the joint or renew the cable, then re-measure to prove the drop has gone.
-
Both — pulses with revs and dims with load
- Likely cause
- A poor earth strap, which produces both and is the cheapest thing to check
- How to confirm it
- Voltage-drop the engine-to-body strap under load. This is the single most productive measurement here.
- What fixes it
- Clean the contact faces to bare metal and remake, or renew the strap.
-
Flicker on one lamp only
- Likely cause
- That lamp's driver module or connection rather than the vehicle's supply
- How to confirm it
- Scope matters: all lamps is supply, one lamp is that circuit.
- What fixes it
- Repair the driver or the connection for that lamp.
-
Flicker with other unrelated warnings appearing
- Likely cause
- The supply fault affecting modules as well as lamps — one root cause
- How to confirm it
- Test battery and charging before diagnosing any of the warnings individually.
- What fixes it
- Repair the supply, clear the codes and re-scan. Only what returns is real.
-
Started after battery or alternator work
- Likely cause
- A terminal or earth not remade properly
- How to confirm it
- Retrace the work and re-torque what was disturbed.
- What fixes it
- Remake the connections. Cheapest cause and it dates itself precisely.
Codes that produce this symptom (8)
Listed with the codes that most typically present this way first — the likeliest place to start.
- P0560 System Voltage Typical of this code Moderate
- P0561 System Voltage Unstable Typical of this code Moderate
- P0562 System Voltage Low Typical of this code Moderate
- P0620 Generator Control Circuit Typical of this code Moderate
- P0622 Generator Field/F Terminal Circuit Typical of this code Moderate
- P0563 System Voltage High High
- P0625 Generator Field/F Terminal Circuit Low Moderate
- P0626 Generator Field/F Terminal Circuit High Moderate
Information, not instruction
CarGeek is a reference, not a service manual. Its pages are written from public sources and from what other people have found, never from a manufacturer's own service information, so they can be wrong or incomplete for your particular vehicle. Some of what they describe carries real risk: high voltage, fuel under pressure, stored energy, hot and moving parts.
Have anything you intend to act on checked by a trained person against the manufacturer's own procedure for your vehicle. Nothing here replaces that procedure.
Others with this symptom
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What makes one worth reading:
- The car — make, model, year and engine. The same fault behaves differently on each.
- What you measured, not only what you replaced.
- What actually fixed it — or that nothing did, which is worth knowing too.
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