📟
P0238 = signal voltage pegged near 5V. The sensor is reporting impossibly high boost. Usually the signal wire is shorted to a 12V or 5V circuit, or the sensor ground is open. A multimeter at the connector tells you in two minutes which one. See top-rated multimeters on Amazon ↗
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 P0238 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
40%
#1 - Most Likely
Open Sensor Ground Wire
When the sensor ground circuit is open, the signal floats up to reference voltage and the ECM reads it as maximum. Check ground continuity from the sensor connector ground pin to chassis ground - should be under 0.5 ohm. A broken or corroded ground is the most common cause.
🔩 Part
$5–$40
👨🔧 Labor
$50–$120
⚡ DIY Difficulty
Medium
30%
#2 - Check First
Faulty Boost / MAP Sensor
An internal sensor failure can drive the output to rail at 5V regardless of actual pressure. Confirm by unplugging the sensor - if voltage at the signal pin (key on) sits at 5V on its own, suspect wiring; if the sensor outputs 5V when connected and powered, it's the sensor.
🔩 Part
$25–$150
👨🔧 Labor
$30–$80
⚡ DIY Difficulty
Easy
20%
#3 - Less Common
Signal Wire Shorted to Voltage
A signal wire chafed against a 12V or 5V circuit feeds high voltage straight to the ECM input. Inspect the harness between the sensor and the ECM for rub-throughs, especially where the harness passes through grommets or near the firewall.
🔩 Part
$5–$40
👨🔧 Labor
$50–$150
⚡ DIY Difficulty
Medium
10%
#4
Corroded / Water-Intruded Connector
Moisture in the connector creates a parallel conductive path between pins, raising the signal voltage falsely. Pull the connector, dry thoroughly, look for green or white corrosion, and re-pin or replace. Dielectric grease helps long-term.
🔩 Part
$5–$40
👨🔧 Labor
$0–$60
⚡ DIY Difficulty
Easy
🚗 Most Affected Vehicles
🔧 Step-by-Step Diagnosis
- Verify Sensor Ground Continuity - Unplug the boost sensor connector and measure resistance from the ground pin to chassis ground. Anything over 1 ohm indicates a broken or corroded ground - trace and repair it before replacing parts.
- Check Signal Voltage with Sensor Disconnected - Key on, engine off, unplug the sensor and read voltage at the signal pin. It should sit near 0V or 5V depending on the system. A constant 5V or 12V reading means the signal wire is shorted to power upstream.
📍 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