Code
B1588
MITSUBISHI
B — Body
B/UP capacitor up converter
Views:
UK: 18
EN: 23
RU: 23
AI status
Completed
Completed
100%
Causes
- Faulty up‑converter (DC‑DC) module that charges/conditions the backup capacitor
- Failed or degraded backup capacitor (low capacitance, high ESR)
- Open, short or high resistance in wiring or connector to converter/capacitor
- Blown fuse, tripped relay or poor power/ground connection
- Low vehicle battery voltage or charging system problem
- Intermittent contact/corrosion at connectors
Symptoms
- DTC B1588 stored and MIL or body warning lamp may be illuminated
- Loss of backup/retained power functions (memory loss of some modules after key off)
- Intermittent electrical anomalies or reoccurring body control warnings
- Possible inability to maintain certain systems when key is off (depending on vehicle)
- Vehicle may reboot or modules may report additional communication errors
What to check
- Read/record all stored DTCs and freeze frame data with a capable scan tool
- Visually inspect converter module, backup capacitor, harness and connectors for damage, corrosion, heat or water intrusion
- Inspect related fuses and relays; check for blown fuses and correct relay operation
- Measure battery voltage (key ON and cranking) and charging system output
- Check continuity and resistance of power and ground leads to the converter and capacitor
- If accessible, measure converter input and output voltages and waveform with a multimeter/oscilloscope
Signal parameters
- Battery voltage: nominal ~12.0–14.5 V (vehicle off/on as applicable) — verify per vehicle spec
- Converter input: should follow battery voltage when powered (no large drop under load)
- Converter output: stable regulated voltage within manufacturer specification (measure at connector) — no large ripple or dropouts
- Charging current pulse when capacitor is being charged (measurable with clamp meter) — not continuous excessive current
- Capacitor ESR: low for a healthy capacitor; high ESR indicates degraded capacitor (measure with ESR meter or equivalent)
- CAN/serial comm lines (if used): idle voltages ~2.5 V and proper recessive/dominant levels (no bus errors)
Diagnostic algorithm
- Connect scan tool, record B1588 plus any accompanying codes and freeze frame data. Clear codes and attempt to reproduce condition.
- Visually inspect the up‑converter and capacitor area for physical damage, oil/water ingress, overheating or loose connectors.
- Verify battery state and charging system. A weak/low battery can cause converter faults — repair charging/battery issues first.
- Check fuses/relays that feed the converter; replace any blown fuses and verify relay operation and control circuit.
- With vehicle safe and capacitors discharged per safety procedures, check wiring harness continuity and resistance from battery/ignition feed and ground to the converter and capacitor.
- Measure converter input and output voltages during key ON and during an attempt to charge the capacitor. Compare to manufacturer values; look for excessive ripple or no output.
- If equipped, measure capacitor capacitance and ESR. Replace capacitor if capacitance is below spec or ESR is high.
- If wiring, fuses and capacitor are good but converter output is missing or abnormal, suspect the up‑converter module — consider replacement or bench test per service manual.
- After repairs, clear codes, verify operation over several key cycles and road/usage conditions. Recheck for reoccurrence and for other related DTCs.
- Safety note: High‑energy capacitors can retain a dangerous charge. Follow manufacturer service manual procedures to discharge and isolate capacitors before handling.
Likely causes
- Up‑converter module failure
- Backup capacitor failed (loss of capacitance or excessive ESR)
- Open/short in power or ground wiring to the converter
- Blown fuse or faulty relay feeding the converter
- Corroded/loose connector or ground
Fault status
Status
B1588 — Backup capacitor up‑converter malfunction: abnormal voltage/charging behavior or electrical fault detected in up‑converter/capacitor circuit.
Repair difficulty: Medium
Diagnostic time: 1.0-2.5 hours
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