Code: P0146

O2 Sensor Circuit – Bank 1 Sensor 3 – No Activity Detected

What it means 

The downstream oxygen sensor (after the catalytic converter) on Bank 1 is not showing voltage changes. The ECU expects this sensor to switch slowly, but it’s seeing flat or no signal.

Bank 1 = side of engine with cylinder #1

Sensor 3 = after the catalytic converter (on vehicles that have 3 O2 sensors on one bank)

Most Common Causes (in real-world order)

1. Bad O2 sensor (very common)

2. Sensor heater circuit failure

3. Wiring damage (melted on exhaust, corrosion, broken ground)

4. Exhaust leak near the sensor

5. Catalytic converter issue (less common)

6. ECM issue (rare)

Quick Professional Diagnostic Steps (Shop-Friendly)

1. Verify Sensor Activity (Scan Tool)

Engine at operating temp

Monitor B1S3 voltage

Normal: ~0.1–0.9V slow movement

Faulty: stuck at 0V, ~0.45V, or no movement

👉 If flatlined → proceed

2. Check Heater Circuit (VERY IMPORTANT)

Many “no activity” codes are actually heater failures

Key ON, engine OFF

Check for:

12V supply to heater

Ground continuity

Typical heater resistance:

3–15 ohms (check manufacturer spec)

❌ Open circuit = replace sensor or repair wiring

3. Visual Wiring Inspection

Check:

Melted wires near exhaust

Oil or water intrusion at connector

Broken or pulled wires

🔧 Repair before replacing sensor

4. Exhaust Leak Check

Leak before or near sensor will skew readings

Listen for ticking or use smoke test

5. Replace Sensor (If Tests Fail)

If:

Heater power/ground OK

Wiring OK

No exhaust leaks

Sensor voltage flat

✅ Replace Bank 1 Sensor 3

⚠️ Use OEM or high-quality aftermarket — cheap sensors cause repeat codes.

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