What to Do If Your Prius Won’t Start (But the Lights Turn On)

Mar 24, 2025

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A Complete Diagnostic Checklist for Prius Owners

You get in your Toyota Prius, press the power button… and nothing. The lights come on, but the car doesn’t go into READY mode. You’re not alone — this is a common hybrid-specific issue, and it doesn’t always mean your hybrid battery is dead.

In this guide, we’ll walk you through a step-by-step diagnostic checklist to help you figure out why your Prius won’t start even though the lights and dashboard turn on.

🔍 Step 1: Understand What “READY” Mode Means

In a Prius, the car is not considered “on” in the traditional sense until the dash shows the word “READY.” That’s when the high-voltage system is activated and the car is drivable.

If you don’t see the READY indicator, your Prius hasn’t started — even if the dash lights are on.

⚠️ Step 2: Check for Warning Lights

Triangle of Death (red triangle)
Check engine light
Brake light
Hybrid system warning
Key symbol (immobilizer light)

These can help narrow down the problem.

✅ Step 3: Follow This Diagnostic Checklist

🔋 1. Check the 12V Auxiliary Battery

A weak or dying 12V battery is the #1 cause of a Prius not starting.
Use a multimeter to check battery voltage:
- 12.4V – 12.7V = Good
- 12.2V – 12.3V = Borderline
- Below 12.1V = Weak battery — may not boot up ECUs

Fix: Try jump-starting the car. If it starts, replace the 12V battery.

🦶 2. Are You Pressing the Brake Pedal?

Prius won’t start unless the brake pedal is fully depressed.
Check for a firm pedal feel — a soft or sinking pedal may mean a brake actuator or vacuum issue.

Fix: Press firmly and try again. If no change, check the brake switch sensor.

🗝️ 3. Check the Key Fob (Smart Key System)

Dead fob battery? The car may not detect it.
Try placing the key fob directly against the Start button, then press it with your foot on the brake.

Fix: Replace the fob battery or use the backup method if the battery is dead.

🧪 4. Look for Stored Trouble Codes

Use an OBD2 scanner (preferably hybrid-compatible) to pull any fault codes.
Common codes include:
- P0A0F – Engine failed to start
- P0AA6 – HV system ground fault
- C1241 – Low 12V battery voltage
- U0100/U0111 – Communication loss with control modules

Fix: Address the root cause of the codes.

🔌 5. Listen for the Hybrid System Relays

When you press Start (with brake pedal down), you should hear:
- Clicking noises from the HV battery relay (in the rear)
- Whirring sounds from the brake actuator

Fix: No sound? The high-voltage system may not be initializing due to:

HV system fault
Service plug disconnected
Faulty battery ECU

⚙️ 6. Check the Hybrid Battery Service Plug

Located on the HV battery pack (under the trunk flooring).
If recently serviced, the plug may not be fully seated or locked.

Fix: Turn OFF the car, disconnect 12V battery, then re-seat the service plug correctly.

🧯 7. Look for Signs of Water Intrusion

Moisture in the battery ECU, inverter, or interior fuse box can cause no-start issues.

Fix: Inspect these areas for corrosion, leaks, or moisture. May require ECU replacement or drying out.

🔧 8. Check Fuses and Relays

Check fuse boxes under the hood and inside the cabin (driver’s side panel).
Look for blown DC/DC, AM2, or Hybrid System Control fuses.

Fix: Replace any blown fuses. If they blow again, a deeper electrical issue is likely.

🔁 If All Else Fails

If you’ve checked everything above and the Prius still won’t start:

Try a different key fob
Attempt to jump-start from another hybrid (not a regular ICE vehicle)
Call a hybrid-specialized repair shop
Use Techstream or Dr. Prius app for advanced diagnostics

🛑 Can You Drive the Car in This State?

If the READY light never comes on, you cannot drive the vehicle. Even if lights and accessories work, the electric motors and hybrid system will not engage.

Driving without READY mode is not possible in a Prius.

✅ Final Thoughts

If your Prius won’t start but the lights come on, the issue likely lies with the 12V system, brake sensor, key fob, or communication with the hybrid battery ECU. This situation is rarely caused by a completely dead hybrid battery — but improper voltage, poor communication, or failed components can all block startup.

By following this diagnostic checklist, you’ll avoid unnecessary replacements and get closer to the root of the problem — whether it’s something simple or something more serious.