Symptom
Only with the headlights or heater on
The fault turns up when you switch something heavy on — headlights, heated rear window, blower on full, all three on a winter evening — and goes when you switch it off. That points at the supply rather than at whatever is misbehaving. Every one of those is a large current draw, and a charging system that is only just coping, or a connection with resistance in it, will hold up until asked for the extra and then let the voltage sag. Modules misbehave at low voltage in ways that look nothing like a charging fault. It is worth trying deliberately: one accessory at a time, and note which one does it.
Also described as: when the lights are on, with the heater on, when i switch things on, on full blower, with the heated rear window on, worse at night, با روشن کردن چراغ یا بخاری, وقتی مصرف برق زیاد است, solo con luces encendidas, pasa con calefaccion puesta, solo con faros encendidos, cuando enciendo luces, فقط مع تشغيل الأنوار, لما أشغل المدفأة, عند إضاءة الكشافات
What this usually means
That points at the supply rather than at whatever is misbehaving, and it is one of the most useful patterns a driver can notice. Headlights, a heated rear window and a blower on full are each a large current draw. A charging system that is only just coping, or a connection with resistance in it, will hold up until asked for the extra and then let the voltage sag — and modules misbehave at low voltage in ways that look nothing like a charging fault.
The mechanism is worth stating because it explains why the symptom seems unrelated to the cause. A module below its designed voltage does not switch off cleanly; it loses communication, fails its own self-tests, and records faults describing conditions that do not exist. So a corroded earth produces warnings in systems that are working perfectly, and only the correlation with a switched load gives it away.
It is worth trying deliberately: one accessory at a time, and note which one does it. That converts a vague complaint into a measurement — and if every heavy load produces it, the fault is upstream of all of them.
Measure the drop rather than the voltage. A resting voltage reading looks fine on a bad connection, 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 the alternator's AC ripple, not just its output. A rectifier with a failed diode still produces something like the right voltage while injecting ripple that upsets modules across the car; more than about 0.5 V AC at the alternator's output under load points there.
Working through it
| What you find | Likely cause | How to confirm it | What fixes it |
|---|---|---|---|
| Fault appears with any heavy load and clears when it is switched off | Voltage sagging under load — a charging system at its limit, or resistance in the supply or earth path | Try one load at a time and note which do it. Measure system voltage at the battery with each load applied, engine running. | Repair the charging system or the connection the drop test names. Do not diagnose the affected modules individually. |
| Voltage looks fine at rest but drops under load | A high-resistance joint — a terminal, an earth strap, or a main cable | Voltage-drop test under load rather than checking continuity at rest; a meter's own test current will not expose it. 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. |
| Lights dim or flicker with engine speed, or several warnings appear together | The alternator — low output, or failed rectifier diodes injecting AC ripple that upsets every module | Measure charging voltage under load, then AC ripple at the alternator's output: more than about 0.5 V AC points at the diodes. | Renew the alternator or its rectifier. Ripple is invisible to a voltmeter reading DC, which is why it is missed. |
| Only with the blower on full, or only with the heated rear window | That circuit specifically — a resistor pack, a relay with resistive contacts, or its own earth | Isolate by load. A fault that follows one accessory and not others is that circuit rather than the whole supply. | Repair the named circuit's relay, connector or earth. |
| Appeared after an accessory was fitted | The accessory's supply taken from a circuit that cannot carry it, or spliced into a signal wire | Ask what was fitted and where its power came from, then disconnect it and retest. A scan tool cannot see a splice. | Rewire the accessory to a suitable supply with its own protection. See the accessory entry in this vocabulary. |
| Engine idle also drops or wavers when the load switches in | The idle control has no margin left, as well as or instead of a supply fault | Watch idle speed and system voltage together as the load is applied. Which one moves first separates them. | Address the idle control path if the voltage holds; address the supply if it sags. |
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Fault appears with any heavy load and clears when it is switched off
- Likely cause
- Voltage sagging under load — a charging system at its limit, or resistance in the supply or earth path
- How to confirm it
- Try one load at a time and note which do it. Measure system voltage at the battery with each load applied, engine running.
- What fixes it
- Repair the charging system or the connection the drop test names. Do not diagnose the affected modules individually.
-
Voltage looks fine at rest but drops under load
- Likely cause
- A high-resistance joint — a terminal, an earth strap, or a main cable
- How to confirm it
- Voltage-drop test under load rather than checking continuity at rest; a meter's own test current will not expose it. 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.
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Lights dim or flicker with engine speed, or several warnings appear together
- Likely cause
- The alternator — low output, or failed rectifier diodes injecting AC ripple that upsets every module
- How to confirm it
- Measure charging voltage under load, then AC ripple at the alternator's output: more than about 0.5 V AC points at the diodes.
- What fixes it
- Renew the alternator or its rectifier. Ripple is invisible to a voltmeter reading DC, which is why it is missed.
-
Only with the blower on full, or only with the heated rear window
- Likely cause
- That circuit specifically — a resistor pack, a relay with resistive contacts, or its own earth
- How to confirm it
- Isolate by load. A fault that follows one accessory and not others is that circuit rather than the whole supply.
- What fixes it
- Repair the named circuit's relay, connector or earth.
-
Appeared after an accessory was fitted
- Likely cause
- The accessory's supply taken from a circuit that cannot carry it, or spliced into a signal wire
- How to confirm it
- Ask what was fitted and where its power came from, then disconnect it and retest. A scan tool cannot see a splice.
- What fixes it
- Rewire the accessory to a suitable supply with its own protection. See the accessory entry in this vocabulary.
-
Engine idle also drops or wavers when the load switches in
- Likely cause
- The idle control has no margin left, as well as or instead of a supply fault
- How to confirm it
- Watch idle speed and system voltage together as the load is applied. Which one moves first separates them.
- What fixes it
- Address the idle control path if the voltage holds; address the supply if it sags.
Codes that produce this symptom (12)
Listed with the codes that most typically present this way first — the likeliest place to start.
- P0E35 DC/DC Converter Voltage Sensor A Intermittent/Erratic Typical of this code High
- P0E3A DC/DC Converter Voltage Sensor B Intermittent/Erratic Typical of this code High
- P2169 Exhaust Pressure Regulator Vent Solenoid Control Circuit/Open Typical of this code Moderate
- P2208 NOx Sensor Heater Sense Circuit Bank 1 Sensor 1 Typical of this code Moderate
- P24D1 Particulate Matter Sensor Regeneration Incomplete Typical of this code Moderate
- P2503 Charging System Voltage Low Typical of this code High
- P2605 Intake Air Heater B Circuit/Open Typical of this code Moderate
- P26AB Engine Coolant Bypass Valve A Stuck/Open Typical of this code High
- P2AF5 Starter Relay C Stuck On Typical of this code High
- U0017 Medium Speed CAN Communication Bus (-) High Typical of this code Moderate
- U0077 Control Module Communication Bus E Off Typical of this code High
- U0142 Lost Communication With Body Control Module B Typical of this code 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.
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