Hybrid No READY Light: 5 Causes and What to Check
Five things have to happen between pressing POWER and the READY light. Here's what fails at each stage, and why a weak 12V battery causes most of it in Arizona.
There are five separate stages between pressing POWER and the READY light coming on, and a failure at any one of them leaves you with a lit dash and a car that won't move. On Toyota and Lexus hybrids in Arizona, a tired 12V battery is far and away the most common stopping point — but a pack that's fallen below its minimum closing voltage, a stuck main relay or a bad brake light switch all produce the same empty dash.
What actually happens when you press POWER?
Understanding the sequence tells you where your car is stuck.
- The 12V battery wakes the computers. Nothing in the high-voltage system moves until the low-voltage side is alive. Dash lights up, ECUs boot, the network comes online.
- The control modules run a self-check and talk to each other. The hybrid vehicle control ECU, battery ECU, inverter and engine ECU all have to report in over the vehicle network.
- The battery ECU checks the pack. Total voltage, individual block voltages, temperature, and isolation from chassis ground. If any of those fail the check, it refuses the next step.
- The system main relays close. A precharge resistor eases current in, then the main contactors snap shut with an audible click from under the back seat or cargo floor. This is the moment the car goes live.
- READY illuminates and the shifter unlocks. Only now will the shift-by-wire system accept D or R, and only with your foot on the brake.
If you press POWER twice without the brake, you get IG-ON — dash lights, radio, no READY. That's normal. The problem is when you hold the brake and READY still never appears.
Which failure stops it at which stage?
| What you see | Where it stopped | Most likely cause |
|---|---|---|
| Nothing — dead dash, no chime | Stage 1 | 12V battery flat or disconnected, main fuse |
| Dash lights dim, everything flashes, clicking | Stage 1 | 12V battery too weak to hold the ECUs up |
| Full dash, no relay click, warning lights on | Stage 3 | Pack voltage low, block imbalance, isolation fault |
| One click then everything shuts back down | Stage 4 | Main relay or precharge circuit fault |
| READY comes on but shifter stays in P | Stage 5 | Brake light switch, shift interlock, park switch |
The audible click is the single most useful clue you can gather in your driveway. Quiet car, foot on brake, press POWER and listen toward the back of the vehicle. A clean double-click followed by READY means the high-voltage side closed. Silence means the car never got permission to go live, and the answer is on the battery or 12V side.
Why is it almost always the 12V battery in Phoenix?
Heat. The 12V battery in a third-generation Prius lives in the right rear of the cargo area, and in a car parked outside in Phoenix that compartment sees interior temperatures well past anything the battery was designed around. A 12V AGM that might last five or six years in a mild climate often gives up in two to three here.
The cruel part is that a dying 12V doesn't crank an engine, so you get no warning. It holds 12.3 volts sitting still, the dash lights up normally, and then the moment the system tries to pull current to close the relays the voltage collapses and the whole boot sequence aborts. Owners describe it as "everything works but it won't go." Flickering lights, a 12V battery warning light, the radio resetting, or the car going READY on the second or third try are all the same story.
If a jump pack on the under-hood jump terminal brings the car to READY immediately, you've found your answer. 12V battery replacement runs $100 to $400 depending on the vehicle and the battery type.
When is it the hybrid pack instead?
The pack has a minimum voltage below which the battery ECU will not allow the main relays to close. Get there and the car simply won't go READY, no matter how strong your 12V is.
Three patterns point at the pack:
- It happened gradually. Fuel economy slid, the state-of-charge bar swung from full to empty in a mile or two, the engine ran more and more, and then one morning no READY. That's nickel-metal-hydride modules going out of balance, and Arizona heat drives it hard — the pack is cooled by cabin air, and cabin air in July isn't cooling much.
- The car sat. Three weeks at the airport in summer can drop a weak pack below its closing threshold. P0A7D — state of charge low is common after a long park.
- There's an isolation code. P0AAC6-family isolation faults mean the system detected high voltage leaking to chassis ground, often from coolant intrusion or a degraded module, and it will lock the car out on purpose.
How do you tell them apart without guessing?
By reading the hybrid system data rather than parts-swapping. A proper hybrid system diagnostic pulls the stored codes, loads the 12V battery, watches system voltage during the boot attempt, and reads individual block voltages from the pack so you can see whether one block is dragging the whole string below threshold. That distinguishes a $100-$400 12V problem from a pack replacement in a single visit, and it's the difference between fixing the car and buying the wrong part.
Our published battery prices are set for the Prius and CT200h and include installation at our shop: $1,000 for a Standard rebuilt pack with 1 year warranty, $1,400 for an Extended rebuilt with 2 years, and $2,100 for a new aftermarket pack with 4 years. Other models vary on install labour. Most installs take about 30 minutes. If the car can't be driven in, mobile service across the Phoenix metro is a $250 add-on — which is how most no-READY cars get handled, since they can't be driven anywhere.
A car that won't go READY isn't going anywhere on its own, so start with a diagnostic rather than a tow to the wrong place. Book a hybrid battery diagnostic or call 623-323-0737 and we'll come to the car.