B16A4 – Turn Signal (RH) Bulb Outage

The Mitsubishi fault code B16A4 – Turn Signal (RH) Bulb Outage is a manufacturer-specific diagnostic trouble code stored by the ETACS (Electronic Time and Alarm Control System) on Mitsubishi vehicles. It indicates that the ETACS has detected an abnormally low current draw on the right-hand turn signal circuit — consistent with one or more bulbs in that circuit having failed, been removed, or replaced with an LED that draws insufficient current for the ETACS to register as a valid load. Because B16A4 falls in the manufacturer-controlled B1xxx range, its definition and diagnostic approach are specific to Mitsubishi vehicles.

What Does B16A4 Mean on a Mitsubishi?

On Mitsubishi vehicles the ETACS manages the turn signal circuit directly — it supplies current to the indicator bulbs and simultaneously monitors the return current to verify that all bulbs in the circuit are functioning. When a conventional incandescent bulb fails, the total circuit resistance increases and current draw drops. The ETACS detects this reduced current as a bulb outage and responds in two ways: it stores B16A4 and it increases the flash rate of the remaining turn signal indicators — the familiar “fast flash” or “hyperflash” that alerts the driver to a failed bulb.

The right-hand turn signal circuit on Mitsubishi models typically includes the front right indicator, the rear right indicator, and on some variants a side repeater. B16A4 is set when the combined current draw of these bulbs falls below the ETACS threshold — indicating that at least one bulb has failed or that the circuit has an open or high-resistance fault.

Symptoms of B16A4

B16A4 produces clear, noticeable symptoms that make it one of the more easily self-diagnosed faults on any Mitsubishi:

• Right-hand turn signal indicators flashing at double speed — “hyperflash” or “fast flash” — the classic sign of a bulb outage detected by the ETACS
• One or more right-hand indicator bulbs visibly not illuminating when the right turn signal is activated
• The dashboard turn signal indicator (right arrow) may flash rapidly or not illuminate at all if the fault affects the instrument cluster feed
• B16A4 stored in the ETACS module when scanned with MUT-III or a compatible scanner
• In some cases the fault is intermittent — the bulb makes contact only when warm or when the vehicle is on a flat surface, pointing to a bulb with a failing filament contact rather than a completely dead bulb
• No effect on vehicle driveability — this is an electrical body fault only

Common Causes of B16A4

The root causes of B16A4 are almost always in the bulb or its immediate wiring rather than in the ETACS itself:

• Failed incandescent bulb — the most common cause by far; a blown filament in the front right, rear right, or side repeater indicator bulb causes the current draw to drop below the ETACS threshold
• Aftermarket LED bulb without load resistor — an LED indicator bulb fitted without an inline load resistor draws insufficient current, which the ETACS reads as a bulb failure
• Corroded bulb holder — corrosion between the bulb contacts and the bulb holder creates resistance that reduces current flow without the bulb itself having failed
• Damaged bulb holder or broken socket tab — a cracked or physically damaged bulb holder prevents reliable electrical contact, causing intermittent or permanent B16A4
• Open circuit in the indicator feed wiring — a broken wire, corroded connector, or loose terminal in the wiring between the ETACS relay and the indicator unit
• Water ingress into a light cluster — moisture in the rear or front light cluster causes corrosion on the bulb holder and connector terminals, leading to high resistance and reduced current flow
• Blown indicator fuse — a blown fuse in the indicator circuit removes the supply to one or more bulbs entirely, causing B16A4
• ETACS current sensing circuit fault — rare, but a failed current sensing circuit within the ETACS can generate a false B16A4 when all bulbs are functioning correctly

Common Misdiagnoses

B16A4 is one of the simpler codes to resolve, but a few recurring misdiagnoses waste time before the real fault is found:

• Only one bulb checked when multiple are in the circuit: A common shortcut is to check only the most obvious bulb — usually the rear indicator — while the actual failed bulb is the front indicator or side repeater. Always check every bulb in the right-hand turn signal circuit before clearing the code.
• LED bulb assumed to be faulty: When aftermarket LED indicators are fitted and B16A4 appears, the LEDs are often replaced under the assumption they have failed. In most cases the LEDs are functioning correctly and the fault is that no load resistor was fitted. Replacing the LED with another LED without adding a load resistor will produce the same B16A4 immediately.
• Bulb replaced but socket not cleaned: Fitting a new bulb into a corroded socket will cause the new bulb to fail prematurely and B16A4 to return within weeks. Always clean the bulb holder contacts before fitting a replacement bulb.
• ETACS suspected when the fault is a bulb: Because B16A4 is stored in the ETACS module, there is occasionally a tendency to suspect module failure. ETACS failure as the cause of B16A4 is extremely rare — always exhaust all bulb and wiring checks before considering ETACS diagnosis.

Affected Mitsubishi Vehicles

Tools & Equipment Required

Step-by-Step Diagnosis

This is a visual and electrical inspection job — start with the obvious before reaching for diagnostic tools:

• 1
  Activate the Right Turn Signal & Walk AroundWith the ignition on, activate the right turn signal and walk around the vehicle. Identify which specific bulb is not illuminating — front right indicator, rear right indicator, or side repeater. Note whether any bulb is flashing at normal speed and which is absent. This visual check takes 60 seconds and immediately narrows the diagnosis.
• 2
  Check the Indicator FuseBefore removing any light clusters, check the indicator fuse in the fusebox. A blown fuse will cause all right-hand indicators to fail simultaneously. Refer to the owner’s manual or fusebox lid for the correct fuse location and rating. Replace with an identical fuse if blown — and investigate why it blew before simply moving on.
• 3
  Remove & Inspect the Faulty BulbAccess the failed bulb — on most Mitsubishi models the rear indicator is accessible from inside the boot/tailgate area, and the front indicator from the engine bay or via a wheel arch liner. Remove the bulb and inspect it — a broken or blackened filament confirms the bulb has failed. Also inspect the bulb holder for green corrosion, burnt contacts, or cracked plastic.
• 4
  Verify Socket Supply VoltageIf the bulb appears intact but is not illuminating, use a test light or DMM to verify that 12V supply is present at the bulb holder when the turn signal is activated. No voltage at the socket confirms an open circuit in the feed wiring upstream of the bulb. Voltage present but bulb not illuminating with a new bulb installed suggests a poor ground return for that circuit.
• 5
  Confirm LED / Load Resistor SituationIf aftermarket LED bulbs are fitted, confirm whether load resistors were installed. With the ignition on and turn signal active, measure current draw on the right indicator circuit with a clamp ammeter — a typical incandescent indicator draws 1.75–2.1A per bulb. An LED without a load resistor may draw as little as 0.1–0.2A, which is below the ETACS outage threshold regardless of whether the LED is illuminating.

Scanner Readout Explained

Below is a representative MUT-III diagnostic readout for a first-generation Outlander PHEV presenting with B16A4 due to a failed rear right indicator bulb.

====================================================
  MITSUBISHI MUT-III SE — DIAGNOSTIC REPORT
====================================================
  Vehicle:     2019 Outlander PHEV (GG3W)
  VIN:         JMBXNGA3WKZ0XXXXX
  Date:        2025-03-26  14:33:18
  Technician:  Workshop Bay 1
====================================================

MODULE: ETACS (Electronic Time & Alarm Control System)
----------------------------------------------------
  B16A4  Turn Signal (RH) — Bulb Outage
         Status:       Confirmed / Current
         Freeze Frame:
           Turn Signal Switch:      RIGHT ACTIVE
           RH Circuit Current:      1.82A (front only detected)
           Expected Current:        3.50A (front + rear + repeater)
           Flash Rate:              120 flashes/min (hyperflash)
           Ignition Status:         ON

MODULE: PCU
----------------------------------------------------
  No faults stored.

MODULE: ECM
----------------------------------------------------
  No faults stored.

====================================================
  LIVE DATA (Turn Signal Active — RH)
====================================================
  RH Turn Signal Circuit Current:    1.82A
  RH Front Indicator Status:         ACTIVE
  RH Rear Indicator Status:          NOT DETECTED
  RH Side Repeater Status:           ACTIVE
  Flash Rate Current:                120 flashes/min
  Flash Rate Normal:                 60–90 flashes/min
====================================================

The live data tells the story immediately. The ETACS is detecting 1.82A on the right indicator circuit — consistent with the front indicator and side repeater functioning but the rear indicator absent. Normal full-circuit current with all three bulbs active would be approximately 3.5A. The rear right indicator bulb was found to have a broken filament on inspection. Replacing the bulb, clearing the code, and re-testing confirmed normal flash rate and B16A4 did not return.

Step-by-Step Repair Guide

Repair Path A: Incandescent Bulb Replacement

• 1
  Access the Bulb HolderAccess varies by position. The rear indicator on the Outlander PHEV is accessed by opening the tailgate and removing the inner trim panel to reach the light cluster connector and bulb holder. The front indicator is typically accessed from the engine bay or by removing the wheel arch liner. Consult the workshop manual for your specific model year — most access procedures require only basic hand tools.
• 2
  Clean the Socket & Fit the New BulbBefore fitting the replacement bulb, clean the bulb holder contacts with electrical contact cleaner and a small brush. Apply a thin smear of dielectric grease to the bulb base contacts — not the glass — to prevent future corrosion. Fit the correct replacement bulb (typically PY21W amber for indicators on most Mitsubishi models — verify against the owner’s manual or workshop manual for your specific variant). Ensure the bulb seats fully and locks in the holder.
• 3
  Test & Clear the CodeBefore refitting any trim panels, activate the right turn signal and verify the replaced bulb illuminates correctly at normal flash rate. Clear B16A4 using MUT-III or equivalent. Reactivate the turn signal and confirm the flash rate has returned to normal (60–90 flashes per minute) and B16A4 does not return. Refit all trim panels and light cluster fixings.

Repair Path B: LED Bulb with Load Resistor Installation

• 1
  Select the Correct Load ResistorFor each LED indicator bulb fitted, a load resistor must be wired in parallel with the bulb to replicate the current draw of the original incandescent. A 50W 6Ω resistor per bulb is the standard specification for most PY21W applications — this draws approximately 2A at 12V, matching the ETACS’s expected load. Resistors rated below 50W may overheat under sustained use.
• 2
  Wire the Resistors CorrectlyConnect one load resistor in parallel with each LED indicator bulb — positive terminal of the resistor to the bulb supply wire, negative to the bulb ground wire. The resistor must be mounted on a metal surface to dissipate heat — never mount a load resistor on plastic trim, wiring looms, or rubber components. Use high-temperature wire and proper waterproof connectors for the installation.
• 3
  Verify & ClearActivate the right turn signal and confirm all LED indicators are illuminating and the flash rate is normal. Clear B16A4 using MUT-III and confirm it does not return. Check the resistor mounting points are secure and not in contact with any combustible material before refitting trim panels.

Repair Path C: Wiring or Socket Repair

• 1
  Replace a Damaged Bulb HolderIf the bulb holder is cracked, has burnt contacts, or shows severe corrosion that cannot be cleaned away, replace the holder. Bulb holders for most Mitsubishi indicator positions are available as standalone components and fit directly onto the light cluster wiring harness. Clean all mating surfaces before fitting the new holder.
• 2
  Repair Open Circuit WiringIf no voltage is present at the bulb holder, trace the indicator feed wire back from the bulb toward the ETACS using a DMM. Locate the open circuit or high-resistance section and repair it — replace damaged wire sections using correctly rated wire with waterproof solder-splice connectors. Do not use standard electrical tape in exterior locations exposed to moisture.

Repair Cost Estimates

Prevention & Maintenance Tips

• Check all indicator bulbs at every service: A 60-second walk-around with all indicators active takes virtually no time but catches a failed bulb before it becomes a B16A4 fault code and a potential roadworthiness issue.
• Keep a spare bulb set in the vehicle: Indicator bulbs are cheap and small. Keeping a set of the correct replacement bulbs in the vehicle’s tool kit means a roadside failure can be fixed in minutes rather than requiring recovery.
• Apply dielectric grease to bulb holders at each bulb change: A small amount of dielectric grease on the bulb base contacts prevents corrosion and ensures reliable electrical contact throughout the bulb’s service life.
• Check for water ingress after any rear or front light cluster removal: Water in a light cluster accelerates corrosion in bulb holders and connectors. After any light cluster removal for any reason, verify the seals and gaskets are correctly seated before refitting — a leaking cluster will cause recurring B16A4.
• Always fit load resistors when upgrading to LED indicators: If you choose to upgrade to LED indicator bulbs, fit the correct load resistors at the same time. Without them, B16A4 will be stored on every drive cycle regardless of how well the LEDs are functioning.

Frequently Asked Questions

Can I still drive my Mitsubishi with B16A4 stored?

Technically the vehicle drives normally, but a non-functioning turn signal is illegal for road use in all markets. You should not drive the vehicle on public roads until the failed bulb is replaced. Fortunately this is one of the quickest and cheapest repairs possible — a €3 bulb and five minutes of your time.

Why does my right indicator flash so fast after a bulb failed?

This is the ETACS’s deliberate response to detecting a bulb outage. When current draw drops below the expected threshold, the ETACS increases the flash rate — typically to around 120 flashes per minute versus the normal 60–90 — to alert the driver that a bulb has failed. This behaviour is by design and is common across all vehicles with electronic turn signal monitoring, not just Mitsubishi. The fast flash returns to normal as soon as the faulty bulb is replaced and the circuit current is restored.

I replaced the bulb but B16A4 is still stored — what should I check?

B16A4 will remain stored until it is actively cleared with a diagnostic scanner — replacing the bulb alone does not erase the code from the ETACS memory. If you have replaced the bulb, verified all indicators are working at normal flash rate, but B16A4 is still showing, simply connect a MUT-III compatible scanner and clear the fault. If B16A4 returns immediately after clearing with the new bulb installed, check the bulb holder contacts for corrosion, verify the new bulb is the correct type, and check for an open circuit in the wiring.

Do I need a scanner to clear B16A4, or will it clear itself?

On most Mitsubishi models B16A4 is a stored fault in the ETACS that requires active clearing with a diagnostic scanner. It will not typically self-clear after the repair on its own drive cycle the way some other codes do. Any scanner with Mitsubishi ETACS access — MUT-III, Autel MaxiSYS, Launch X431, or similar — can clear it in under a minute. Many independent garages will clear a single body code for a nominal charge.

Can I use any LED indicator bulb or does it need to be a specific type?

For Mitsubishi vehicles with ETACS current monitoring, any LED indicator bulb must either be used with a correctly rated load resistor or be a CANbus-compatible LED specifically designed for vehicles with electronic load monitoring. A standard automotive LED indicator without these provisions will always trigger B16A4. CANbus LED indicators have a built-in resistor circuit and are designed to replicate the current draw of an incandescent bulb without requiring external load resistors.

Is there a left-hand equivalent of B16A4?

Yes — the equivalent code for the left-hand turn signal circuit is a separate B1xxx code stored by the same ETACS module. The diagnostic approach, repair procedure, and causes are identical — only the physical location of the bulbs differs. If both sides show bulb outage codes simultaneously, check the indicator fuse first as a single blown fuse can affect both circuits depending on how the fuse protection is arranged on your specific variant.

Why does B16A4 sometimes appear and disappear intermittently?

Intermittent B16A4 most commonly points to a bulb with a failing filament that still makes contact under some conditions — typically when the bulb is cold or when the vehicle is on level ground — but loses contact when the filament expands with heat or when vibration disrupts a weak connection. A corroded bulb holder that makes intermittent contact as the bulb expands and contracts with temperature cycling produces the same pattern. Replace the affected bulb and clean the socket — this resolves intermittent B16A4 in the vast majority of cases.

Conclusion

Mitsubishi B16A4 – Turn Signal (RH) Bulb Outage is one of the most straightforward fault codes on any Mitsubishi model. The ETACS has detected insufficient current on the right indicator circuit and stored the code — in the overwhelming majority of cases because a single incandescent bulb has failed, or because aftermarket LED bulbs were fitted without the required load resistors.

The repair is almost always a five-minute job costing a few euros. Walk around the vehicle with the right indicator active to identify the failed bulb, replace it with the correct type, clean the socket, and clear the code with a scanner. Anything more complex than this — wiring repairs, load resistors, or socket replacement — is required only in a small minority of cases.