P1B77: High-voltage battery precharging fault
Hyundai hybrid high voltage battery precharge fault
Severity: Inspect soon
This is a high-voltage hybrid system fault, and the owner we read described losing drive in traffic and being towed, so treat it as a shop visit rather than a drive-and-see code.
It gets more serious if:
- Stop: Stop driving and have the car towed if the hybrid warning light is on together with loss of drive, a no-start, or the car coasting to a stop.
P1B77 in short
On Hyundai hybrids, P1B77 means the high-voltage battery precharge step failed when the system tried to start. Hyundai bulletins tie it to a loose power relay assembly connection and to a battery software update on the Ioniq Hybrid. It can mean a no-start, so a hybrid-trained shop should diagnose it.
Before the hybrid system connects full battery voltage to the rest of the car, it precharges the circuit through a resistor. If that precharge does not reach the expected voltage in time, the computer stores a precharge fault and will not close the main relay.
Hyundai bulletin 20-01-003H says that on 2016 and 2017 Sonata Hybrids a loose connection in the power relay assembly or high-voltage cable can cause a no-start, or a check engine and hybrid warning light, with this code stored. Hyundai bulletin 19-01-023H covers a battery management software update to the diagnostic logic for this code on the 2019 Ioniq Hybrid. We read only the summaries of both.
Owner complaints for the 2014 Sonata Hybrid pair this code with P1B25 and a blown 150 amp fuse. This page covers Hyundai only; Kia owners report the same code with a high-voltage fuse, but we did not find a second independent source for a Kia meaning.
Pick your make
P1B77 is a manufacturer-specific code: its meaning depends on who built the car.
Other makes: meaning not verified. Check with a scan tool that reads manufacturer definitions.
Most likely causes
Cause 1: Loose connection in the power relay assembly or high-voltage cable
Supported by vehicle data- Why it ranks here
- Hyundai bulletin 20-01-003H names this for 2016 to 2017 Sonata Hybrids, and the dealer invoice in a 2014 Sonata complaint called the power relay assembly faulty.
- What would change this
- A no-start or intermittent hybrid warning light on a 2016-2017 Sonata Hybrid raises this; the inspection needs high-voltage procedures.
Cause 2: Battery management software logic (Ioniq Hybrid)
Supported by vehicle data- Why it ranks here
- Hyundai bulletin 19-01-023H updates the battery management diagnostic logic for this code on the 2019 Ioniq Hybrid.
- What would change this
- Applies mainly to that model; a dealer can check whether the update is installed.
Cause 3: Blown high-voltage fuse
Possible — needs testing- Why it ranks here
- A 2014 Sonata owner reported a blown 150 amp fuse alongside this code, but the complaint does not show the fuse was the cause.
- What would change this
- A fuse that blows again points to a deeper high-voltage fault.
What to test first
Start at step 1. Each result tells you what it means and where to go next.
Step 1: Have a Hyundai hybrid-capable shop read the hybrid battery data, check for open bulletins or software updates, and inspect the power relay assembly and cable.
Warning:Hybrid high-voltage systems can deliver a lethal shock. Only trained technicians with insulated tools and the safety plug removed should open them; never touch orange cables.
IfThe relay assembly or cable connection is loose or faulty
It meansThat is the likely cause.
Points to: Loose connection in the power relay assembly or high-voltage cable
IfThe model is a 2019 Ioniq Hybrid and the software is out of date
It meansThe software update applies.
IfA high-voltage fuse is blown
It meansFind out why before replacing it.
Symptoms
- Check engine and hybrid warning lights on
- No-start Gets more serious: stop and have it checked.
- Loss of drive while driving Gets more serious: stop and have it checked.
How to fix
This is a safety-critical system, so we explain the code but don't give repair steps. Have it inspected by a qualified technician.
On your vehicle
HyundaiYour car
2014–2017 · Sonata
Supported by vehicle data2016-2017 Sonata Hybrid: Hyundai bulletin links this code to a loose power relay assembly or cable. 2014 Sonata Hybrid complaints show it with P1B25 and a blown fuse.
2019 · Ioniq
Supported by vehicle dataThe 2019 Ioniq Hybrid has a battery management software update for this code.
20-01-003HStatus unverified
Sonata Hybrid power relay assembly and high voltage cable inspection
2016–2017sonata · NHTSA 10170894
Summary-only record: a loose connection in the power relay assembly or high-voltage cable can cause a no-start or warning lights with P1B77 and P1990; the bulletin describes inspecting and replacing the parts. We did not read the procedure.
We couldn't confirm whether this bulletin is still current, so it doesn't count as confirmation.
19-01-023HStatus unverified
Battery management software update for DTC P1B77
2019ioniq · NHTSA 10165457
Summary-only record: a battery management software update changes the diagnostic logic for this code on the Ioniq Hybrid. We did not read the procedure.
We couldn't confirm whether this bulletin is still current, so it doesn't count as confirmation.
Related codes
Sources and review
- NHTSA complaint ODI 11546978: 2014 Hyundai Sonata Hybrid, codes P1B25 and P1B77
- P1B77 High Voltage Battery Precharging Fault (Hyundai)
- Hyundai bulletin 20-01-003H: Sonata Hybrid power relay assembly and high voltage cable
- Hyundai bulletin 19-01-023H: BMS software update to diagnostic logic for DTC P1B77
- NHTSA complaint ODI 11532527: 2014 Hyundai Sonata Hybrid, codes P1B77, P1B96, P1B70, P0A0D
- Last reviewed
- Written by
- Saurabh K.
- Editorial review
- Pending
- Technician review
- Technician review pending
This page explains what a code usually means; it is not a substitute for a professional inspection. How we source and review · Report an error