DTC P0139 – O2 Sensor Circuit Slow Response (Bank 1 Sensor 2).

✅ P0139 – O2 Sensor Slow Response (Bank 1 Sensor 2)

Meaning:
The downstream O₂ sensor (after catalytic converter) is responding slowly when the ECU commands rich → lean or lean → rich.
This sensor mainly monitors catalytic converter efficiency, not fuel control.

🔧 Common Causes

1. Faulty O₂ Sensor (Bank 1 Sensor 2)

Sensor aging

Contaminated by oil/coolant/carbon

Slow switching

2. Exhaust Leaks (before sensor)

Small leak at flex pipe, flange, manifold, catalytic converter inlet

3. Rich or Lean running condition

Injector leak

High fuel pressure

Vacuum leak

Misfire

4. Wiring / Connector Problems

High resistance in signal ground

Damaged wires near exhaust

Poor connector pin contact

5. Catalytic Converter issues

Degraded cat leading to abnormal sensor readings

🧰 Step-by-Step Diagnostic Procedure

1. Check for additional DTCs

Misfire codes (P030X)

Fuel trim codes (P0171/P0172)

Other O2 sensor codes

If present → fix those first.

2. Visual Inspection

Check sensor harness for heat damage

Check connector pins (corrosion, loose fit)

Check exhaust leak before the downstream O2 sensor

If leak present → repair → clear code → recheck.

3. Live Data Check (Scanner)

Look at O2 Sensor B1S2 voltage pattern:

🔸 Normal (good cat):

Slowly fluctuates

Range: 0.65–0.8 V (rich) and 0.1–0.2 V (lean)

Should NOT switch fast like upstream sensor

🔸 Bad (slow response):

Stays stuck for too long

Delayed switching

Voltage responds very late to throttle snap test

4. Perform Throttle Snap Test

1. Idle engine

2. Snap throttle 2–3 times

3. Observe B1S2 sensor reaction

If sensor takes > 1000 ms to respond → sensor slow → replace.

5. Check Wiring

Measure resistance at signal wire

Check ground reference

Check continuity from sensor → ECM

Inspect for insulation melted near exhaust

6. Catalytic Converter Test

If sensor is good but still slow:

Converter might be worn

Downstream sensor will switch too fast (almost like upstream)

🛠 Common Fixes

✅ Replace O₂ Sensor Bank 1 Sensor 2 (most common)

✅ Repair exhaust leak

✅ Fix fuel trim issues (rich/lean)

✅ Repair wiring or connector

✅ Replace catalytic converter (if confirmed faulty)

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