No POST, no chime
MacBook Pro diagnostic

Power rail baseline
measurement protocol

No POST and no startup chime indicates the SMC (System Management Controller) is not initializing the boot sequence. This always means DC-in power is arriving but core voltage rails are either missing, collapsed, or unstable. Start by isolating which power domain has failed.

Connect a USB-C power adapter (or 85W MagSafe for older models). Do NOT force power by shorting. Measure these rails first with the logic board unplugged from batteries:

If PP5V_S5 is missing entirely, suspect ISL6259 under-voltage lockout or a shorted output FET. If PP3V3_S5 is weak, the buck converter TPS51125 is likely damaged or its input is starved.

SMC reset and boot
sequence validation

If all rails measure correctly, perform an SMC reset to force a clean power sequencing event. This resets internal SMC firmware state without erasing NVRAM or data.

SMC Reset Steps (All Models)

1. Power off completely. Disconnect charger for 30 seconds.
2. Reconnect charger. Do NOT open the lid yet.
3. Hold Shift + Control + Option (left side) + Power button simultaneously for 10 seconds.
4. Release all keys. Wait 5 seconds.
5. Press Power button normally. Listen for startup chime and observe fans ramping.

The SMC reset forces a re-initialization of power rails and clears temporary fault flags. If the startup chime appears after this, the fault was transient—likely due to a stuck SMC state from an improper shutdown.

If no chime after SMC reset and all voltage rails are present, the fault is in the PMU (Power Management Unit) communication bus or SMC itself cannot initialize the CPU/GPU power rails. This requires component-level repair.

Buck converter load testing
under no-chime condition

When power rails are present but the system refuses to boot, the secondary power rails for the CPU and GPU may have insufficient margin or current capacity. Measure buck converter output impedance and load regulation.

Testing TPS51125 and ISL6259

Use a digital multimeter in DC voltage mode. Probe the output sense pin (typically labeled SENSE or FB) and the raw output node simultaneously while applying a 1-ohm external load resistor across the output for 2 seconds. Voltage should not sag more than 50mV. Excessive sag indicates series resistance in the output stage—likely a shorted output MOSFET.

For the ISL6259 primary rail buck on A1502, A1707, and A1989 models, measure the feedback divider resistors R7124 (typically 100k, top) and R7125 (typically 30k, bottom). Resistance out of spec indicates a burnt resistor, which will cause rail collapse under load.

Inspect the MOSFET gates on the buck stage for bias voltage. Both high-side and low-side gates should show activity (0.5–2.5V ripple) when probed with an oscilloscope during the load test. Dead gates = dead PWM driver IC or failed gate driver capacitors.

If a buck rail collapses under 1-ohm load, the output stage is shorted. Identify the failing MOSFET using thermal imaging (typically one FET will show 15–20°C hotter than surrounding components) and replace. Common part numbers for output MOSFETs: AO3404 (30V), BSS138 (20V).

Shorted rail detection
isolation and component-level repair

A rail reading 0V or severely collapsed when power is applied may indicate a shorted load. Measure resistance from each rail to ground with the charger disconnected and all power removed. Expected values for high-impedance rails:

• PP5V_S5 to ground: >10 kΩ (opens during sleep)
• PP3V3_S5 to ground: >8 kΩ (opens during sleep)
• PP1V8_S5 to ground: >6 kΩ (opens during sleep)

If resistance is <1 kΩ, a load circuit is permanently pulling the rail to ground. This blocks the buck converter from raising voltage and triggers under-voltage protection in the ISL6259 or TPS51125.

Isolating the short

Use a current-limited bench power supply set to 5V / 0.5A. Connect positive to the suspected shorted rail (test point) and negative to ground. Measure current consumption. A short will draw maximum current immediately. Current under 100mA is typically a leaky capacitor or bias network; over 300mA suggests a shorted IC or FET.

Once you identify which rail is shorted, visually inspect the schematic sections feeding that rail. Check for burned capacitors (bulging, darkened pads), water damage around IC pins, or obvious solder bridges. If the short is under the IC (e.g., U6001 power sequencer pin damage), the IC must be replaced.

For detailed A1502, A1707, and A1989 repair procedures, refer to our MacBook Pro A1502 / 820-3476 / 820-4924 repair guide and , which include component placement diagrams and fault-specific replacement procedures.

SMC firmware lockout
recovery and NVRAM reset

Occasionally, SMC firmware becomes corrupted after battery drain or power loss during an SMC update. In this case, all power rails present correctly, but the SMC will not respond to reset commands and no chime is heard. This is a firmware-level issue.

Attempt an NVRAM reset (requires power and chime first), then SMC reset. If both fail, the SMC firmware is likely corrupted and requires a full SMC reflash using Apple Diagnostics in Recovery Mode (Command + D at startup). If you cannot reach Recovery Mode, the issue is a hardware short or dead RAM.

If all diagnostics pass but the board still shows no boot, perform a final check: measure PPBUS_G3H (main system rail at ~12V). If this rail is stable and the SMC is reporting errors, suspect a dead CPU or GPU die. In that case, replacement of the entire SoM (System-on-Module) or logic board is required.