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P0138 on the downstream sensor usually means the converter is seeing a rich exhaust stream. Before replacing the downstream sensor, check whether the upstream sensor B1S1 is also showing rich conditions. If fuel trims are negative (rich correction), the converter is receiving rich exhaust - fix the rich-running condition first. Normal fuel trims with P0138 suggests the downstream sensor itself has failed. See downstream O2 sensors on Amazon ↗
🗺️ Where Is the Problem?
Exhaust diagram - P0138 high voltage reading at the downstream 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 P0138 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
50%
#1 - Most Likely
Failing Downstream O2 Sensor Stuck High
The downstream sensor element becomes biased toward the rich end of its range due to age or contamination. A sensor failing stuck-high is identified by seeing high voltage even during deceleration fuel cut, when a healthy sensor would drop to near-zero.
🔩 Part
$30–$150
👨🔧 Labor
$50–$120
⚡ DIY Difficulty
Easy
30%
#2 - Check First
Rich Fuel Condition Affecting Downstream Reading
When the engine runs rich and the converter cannot fully oxidize excess fuel, the downstream sensor correctly detects high oxygen-depleted exhaust. Check fuel trims and look for P0172/P0175 codes alongside P0138 before blaming the sensor.
🔩 Part
$20–$200
👨🔧 Labor
$50–$200
⚡ DIY Difficulty
Medium
15%
#3 - Less Common
Failed Catalytic Converter
A converter that has lost its oxidation ability allows rich exhaust gases to pass through unprocessed. The downstream sensor then reflects the same rich composition as the upstream. A failed converter sets P0138 alongside P0420.
🔩 Part
$200–$1,000
👨🔧 Labor
$200–$400
⚡ DIY Difficulty
Medium
5%
#4 -
Leaking Fuel Injector
A stuck-open injector enriches the combustion mixture. Over time, this can overwhelm the converter and show up at the downstream sensor. Check for misfire codes and rough idle alongside P0138 if an injector is suspected.
🔩 Part
$50–$200
👨🔧 Labor
$100–$250
⚡ DIY Difficulty
Medium
🚗 Most Affected Vehicles
🔧 Step-by-Step Diagnosis
- Check Fuel Trims with Scan Tool - Review short-term and long-term fuel trim values for Bank 1. Negative values (below -5%) confirm the engine is running rich - the P0138 reading reflects actual rich exhaust, not a failed sensor. Positive or normal fuel trims with P0138 suggest the downstream sensor itself has failed high.
- Perform Deceleration Fuel Cut Test - With the engine warm, accelerate to approximately 3,000 rpm and immediately release the throttle completely. During decel fuel cut, the downstream sensor should drop to low voltage (below 0.3V). A sensor that stays high above 0.6V during decel fuel cut has definitively failed.
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Tips for Choosing a Shop
- Ask if they charge a diagnostic fee and whether it applies toward the repair
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- 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