What You Need
Bluetooth OBD2 Adapter
A small dongle that plugs into the OBD2 port under the dash. Look for adapters with a genuine ELM327 v2.2 chip. Owner-tested pick: Veepeak OBDCheck BLE ↗ — works with both iOS and Android, confirmed compatible with Dr. Prius. Avoid the very cheapest clones — they often use counterfeit chips that cause connection failures. Cost: $20–$40 for a quality adapter.
Torque Lite (Android) / OBD Fusion (iPhone)
Android: Torque Lite (free) reads and clears codes — everything you need for this guide. Upgrade to Torque Pro (~$5) for live gauges, battery temp, and data logging. Also good: Car Scanner ELM OBD2 (free tier, highly rated). iPhone: OBD Fusion or Car Scanner — use a Bluetooth 4.0 BLE adapter, not classic Bluetooth. For hybrid battery diagnostics specifically, add Dr. Prius (battery health test ~$12 in-app).
Step-by-Step: Read and Clear a Code
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Plug the OBD2 adapter into the port. You'll usually see a small LED light up on the adapter confirming it has power.
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Turn the car to Ready mode (or at minimum turn the ignition to "On" without starting). The ECU needs to be active for the adapter to communicate.
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On your Android phone, enable Bluetooth and pair with the adapter. Go to Bluetooth settings, scan for devices, and select the OBD adapter. When prompted for a PIN, enter 1234.
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Open the Torque Lite app. Wait a few seconds — you should see the message "Connected to ECU OK" at the bottom of the screen. If it says "Not Connected," verify Bluetooth is paired and the adapter has power.
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Tap the gear/menu icon → "Fault Codes" → "Show logged faults." The app will query the ECU and display any active or stored codes.
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Note the code(s) — write them down or screenshot them. Look up what they mean in the error codes guide before doing anything else.
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To clear: tap the gear icon again → "Fault Codes" → "Clear all logged faults." Confirm if prompted.
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Check the dashboard. The check engine light should be off. Turn the car off and back on to confirm the light doesn't immediately return.
The Most Common Code: P0420
On a high-mileage Gen 2 Prius, P0420 (Catalyst System Efficiency Below Threshold — Bank 1) is the most frequent check engine light cause. The catalytic converter has degraded to the point where the downstream O2 sensor detects it's no longer cleaning exhaust efficiently.
| Code | Meaning | What to do |
|---|---|---|
| P0420 | Catalytic converter below efficiency threshold | Check O2 sensor first (cheaper fix). If sensor is fine, cat needs replacement. Clearing the code temporarily turns off the light — it returns within a few hundred miles if the cat is truly failing. |
Tips
- Leave the adapter plugged in — it's convenient to have always available and draws no power when the car is off
- After clearing codes, drive at least one complete trip cycle before checking if the light stays off — some codes need the ECU to run through monitoring conditions before confirming a fix
- If the same code returns within a day, the root cause is still present and needs actual repair
- Torque Lite is free and covers everything in this guide — upgrade to Torque Pro ($5) if you want live gauges, battery temperature monitoring, and data logging for diagnosing intermittent issues
- iPhone users: the app ecosystem is different — look for OBD Fusion or Car Scanner, and get a Bluetooth 4.0 (BLE) adapter rather than the classic Bluetooth version
Notes from the Field
P0420 appeared regularly in the later years of ownership — every few months, like clockwork. The catalytic converter on a high-mileage Prius eventually stops meeting the efficiency threshold the ECU expects, and the downstream O2 sensor catches it.
Having the OBD2 adapter permanently plugged in and Torque on the phone meant clearing it took under two minutes. It's one of the most practical tools any Prius owner can have — and at $15, it's the cheapest diagnostic tool that will ever save you a $100 dealer diagnostic fee.