📟
Bank 1 Sensor 2 is the downstream sensor - it monitors catalytic converter efficiency, not fuel control. P0136 disables the ECM ability to evaluate converter health. The engine will still run in closed-loop using S1, but P0420 catalyst codes may follow if the converter is also failing. Inspect the converter for rattling or heat damage when replacing S2. See downstream O2 sensors on Amazon ↗
🗺️ Where Is the Problem?
Exhaust diagram - P0136 fault at the downstream (post-cat) oxygen sensor on Bank 1
These are statistical causes across ALL vehicles - your exact car may rank differently
For example, on a Honda 4-cyl the downstream O2 sensor causes P0136 64% of the time, but on a GM 5.3L V8 the catalytic converter is the cause 71% of the time. Get a probability ranking built specifically for your year, make, model, and mileage.
🔎 Get the ranking for my exact car - $5.99 →
🎯 Top Causes & Probability
55%
#1 - Most Likely
Failed Downstream O2 Sensor
The B1S2 sensor element, heater, or internal circuitry has failed. Downstream sensors face an extremely harsh thermal environment and are especially vulnerable to contamination from a degrading catalytic converter. Replacement with OEM or equivalent quality is recommended.
🔩 Part
$30–$150
👨🔧 Labor
$50–$120
⚡ DIY Difficulty
Easy
20%
#2 - Check First
Damaged Catalytic Converter Affecting Readings
A heavily degraded or melted converter can coat the downstream sensor with catalyst substrate material, physically destroying the sensing element. Rattling sounds from the converter undercarriage are a warning sign. Replace both converter and sensor together in this case.
🔩 Part
$200–$800
👨🔧 Labor
$200–$400
⚡ DIY Difficulty
Medium
15%
#3 - Less Common
Wiring Fault
The downstream sensor wiring runs the length of the underbody and is susceptible to corrosion, heat damage, and physical damage from road debris. Inspect the full harness length, especially near heat shields, before replacing the sensor.
🔩 Part
$10–$50
👨🔧 Labor
$50–$150
⚡ DIY Difficulty
Medium
10%
#4 -
Exhaust Leak After Catalytic Converter
A cracked flex pipe or loose exhaust joint downstream of the cat introduces fresh air into the exhaust, altering the downstream sensor signal. Listen for exhaust noise from the mid-pipe area when diagnosing.
🔩 Part
$30–$200
👨🔧 Labor
$100–$300
⚡ DIY Difficulty
Medium
🚗 Most Affected Vehicles
🔧 Step-by-Step Diagnosis
- Inspect Downstream Sensor and Wiring - Locate the B1S2 sensor on the exhaust pipe downstream of the catalytic converter. Inspect the sensor body for physical damage or corrosion. Trace the wiring harness toward the vehicle for heat damage, chafing, or connector corrosion.
- Check Sensor Signal with Scan Tool - Monitor the B1S2 voltage PID with the engine warm. The downstream sensor on a healthy system should hold a relatively stable high voltage (0.6–0.9V) with slow fluctuations. A flat 0.45V reading indicates an open circuit.
📍 Find a Trusted Shop Near You
Please enter a valid 5-digit ZIP code.
Tips for Choosing a Shop
- Ask if they charge a diagnostic fee and whether it applies toward the repair
- Request a written estimate before approving any work
- Ask specifically about the part brand - OEM vs. aftermarket matters for this code
- Check Google reviews for recent mentions of the specific repair you need