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Stuck-high voltage = rich. Healthy post-cat readings hover around 0.6-0.8V. If P0158 sets at over 1.0V steady, the engine is dumping fuel or coolant into the exhaust. Long-term high voltage burns up cats fast - chase this within a few hundred miles. See top-rated scanners on Amazon ↑
🗺️ Where Is the Problem?
Blueprint view - P0158 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 P0158 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.
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🎯 Top Causes & Probability
45%
#1 - Most Likely
Engine Running Rich (Fuel Trim Issue)
A leaking injector, stuck-high fuel pressure regulator, or contaminated upstream sensor on Bank 2 can flood the exhaust with unburned fuel. The downstream sensor reads it as a permanent rich condition - check Bank 2 long-term fuel trim first.
🔩 Part
$30–$300
👨🔧 Labor
$80–$300
⚡ DIY Difficulty
Medium
35%
#2 - Check First
Sensor Contaminated by Coolant or Oil
A small head-gasket leak or burning oil coats the post-cat sensor element. The contaminated sensor reads stuck-high regardless of true exhaust composition. Replacing the sensor is only a temporary fix until the source leak is addressed.
🔩 Part
$40–$160
👨🔧 Labor
$50–$120
⚡ DIY Difficulty
Easy
20%
#3 - Less Common
Sensor Signal Wire Shorted to Power
If the downstream signal wire chafes against a 12V circuit (often near a melted connector), the input pegs above 1.0V regardless of the sensor itself. A continuity check between signal and ground vs. battery+ confirms.
🔩 Part
$5–$60
👨🔧 Labor
$80–$200
⚡ DIY Difficulty
Medium
🚗 Most Affected Vehicles
⚠️ Is It Safe to Drive With P0158?
Drive with caution. P0158 usually means the engine is running rich - which can overheat and damage the catalytic converter over a few hundred miles. Short trips to a shop are fine; long highway runs at full throttle are not. Don't ignore for more than a couple of weeks.
🔧 Step-by-Step Diagnosis
- Check Bank 2 Long-Term Fuel Trim - At warm idle, read LTFT for Bank 2. Anything below -10% means the engine is running genuinely rich and the sensor is reporting truthfully. Hunt the fuel-side problem before replacing the sensor.
- Inspect the Sensor for Oil/Coolant Soot - Remove the Bank 2 downstream sensor. White or sweet-smelling residue indicates coolant. Black, oily soot means oil consumption. Both demand engine repair, not just a sensor swap.
📍 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