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Stuck-low downstream voltage = "lean" reading. A healthy post-cat sensor on Bank 2 should hover near 0.6-0.8V at steady cruise. If the PCM sees less than ~0.1V for over 2 minutes, it sets P0157. Check for an exhaust leak before condemning the sensor. See top-rated scanners on Amazon ↑
🗺️ Where Is the Problem?
Blueprint view - P0157 fault location in the exhaust/intake circuit
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 P0157 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
40%
#1 - Most Likely
Exhaust Leak Near or Before the Sensor
A leak in the Bank 2 exhaust pipe, manifold, or a loose sensor lets atmospheric oxygen enter the stream, pegging the sensor low. Listen for ticking near the manifold at cold-start; visually inspect for soot trails.
🔩 Part
$10–$120
👨🔧 Labor
$80–$300
⚡ DIY Difficulty
Medium
35%
#2 - Check First
Failed Bank 2 Downstream Sensor
Internal short to ground, contamination, or end-of-life zirconia degradation can lock the signal at 0V. Replacing the sensor with an OE-grade unit is usually the cure when wiring checks out clean.
🔩 Part
$40–$160
👨🔧 Labor
$50–$120
⚡ DIY Difficulty
Easy
25%
#3 - Less Common
Signal Wire Shorted to Ground
A pinched, melted, or chafed signal wire grounded against the chassis or exhaust shielding will hold the sensor reading at 0V. Trace the harness with a multimeter from sensor to PCM, looking for less than 100k ohms to ground.
🔩 Part
$5–$60
👨🔧 Labor
$80–$200
⚡ DIY Difficulty
Medium
🚗 Most Affected Vehicles
⚠️ Is It Safe to Drive With P0157?
Yes, P0157 is generally safe to drive. The downstream/post-cat sensor mostly affects emissions reporting, not engine operation. You'll fail an emissions test until repaired, but drivability should remain normal. Plan a fix within the next month to keep your monitor readiness up.
🔧 Step-by-Step Diagnosis
- Live-Data Voltage on B2S2 - At idle, watch Bank 2 Sensor 2 voltage. A code-worthy stuck-low signal will read under 0.1V steady. Compare to Bank 1 Sensor 2, which should read 0.5-0.8V steady.
- Pressurize the Bank 2 Exhaust - With the engine off, listen and feel along the Bank 2 exhaust for leaks while a helper plugs the tailpipe. A soapy-water spray reveals tiny leaks immediately.
📍 Find a Trusted Shop Near You
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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