A P0138 code on the Honda Civic indicates the downstream oxygen sensor (Bank 1, Sensor 2, located after the catalytic converter) is producing a voltage reading that remains continuously above 0.9 volts, and the most common cause on the Civic is a failed sensor whose internal element has degraded and is biased toward the rich-indicating high-voltage range, though a genuinely rich exhaust condition passing through a deteriorating catalytic converter and a short to voltage in the sensor wiring are both capable of producing the same code.
Correctly diagnosing P0138 requires understanding both what the downstream sensor is supposed to do and what it means when it reads high voltage rather than the normal operating pattern.
What the Downstream O2 Sensor Does and What High Voltage Means
On the Honda Civic 4-cylinder engine (all current generations), Bank 1 is the only bank. Sensor 1 (upstream) is the primary air-fuel ratio sensor located before the catalytic converter, which the ECM uses for closed-loop fuel control. Sensor 2 (downstream) is located after the catalytic converter and is used to monitor converter efficiency and to detect gross fuel control errors.
A healthy catalytic converter oxidizes unburned hydrocarbons and carbon monoxide in the exhaust. After the converter, the exhaust contains more oxygen and fewer unburned compounds than before it. The downstream sensor reads this post-converter exhaust and produces a voltage that oscillates slowly between 0.2 and 0.7 volts, reflecting the low-hydrocarbon, relatively oxygen-rich post-converter exhaust.
When the downstream sensor produces continuously high voltage (above 0.9 volts, which indicates a rich, low-oxygen exhaust condition), one of three things is happening: the sensor itself is malfunctioning and reading high regardless of actual exhaust composition, the catalytic converter is not converting hydrocarbons and the downstream exhaust is genuinely rich, or unburned fuel from an engine running very rich is overwhelming the converter and appearing at the downstream sensor.
P0138 specifically requires the sensor voltage to remain above approximately 0.9 volts for a defined period. A downstream sensor that simply oscillates in the 0.5 to 0.9 volt range (indicating a degraded but not failed converter) would set P0420 (catalyst efficiency below threshold), not P0138. The P0138 is a circuit voltage fault, not a converter efficiency fault.
Cause 1: Failed Downstream Oxygen Sensor (Most Common)
An oxygen sensor’s sensing element is a zirconia ceramic thimble coated with platinum that produces voltage based on the oxygen differential between the exhaust gas and the air reference inside the sensor body. When this element degrades, cracks, or becomes contaminated, it can lose its accurate response and default to a biased output. A sensor that has been exposed to coolant, oil, or excessive fuel over its lifetime is prone to element contamination that causes it to read artificially high.
Honda Civic downstream sensors fail at higher rates in the 100,000 to 150,000 mile range. The downstream sensor experiences lower exhaust temperatures than the upstream sensor (the converter has already reacted with much of the hydrocarbons, reducing the exotherm temperature), but the sensor is still exposed to years of thermal cycling that degrades the element and the heater circuit inside the sensor body.
Diagnosis: with a scan tool displaying live downstream O2 sensor voltage, start the engine cold and allow it to warm fully. A healthy downstream sensor on a Civic with a working converter should show voltage oscillating slowly between 0.2 and 0.7 volts after reaching operating temperature. A voltage that remains at or above 0.9 volts continuously with no oscillation, or that drops to 0.9 and immediately returns high, indicates a sensor biased toward the rich range.
The heater test: the downstream sensor has a built-in heater circuit that brings the sensing element to operating temperature quickly. Check the heater resistance between the two heater wires (typically 10 to 16 ohms on Honda sensors). An open heater circuit (infinite resistance) means the sensor cannot reach operating temperature and may produce erratic or stuck-high voltage readings until it is replaced.
Replacement cost: aftermarket downstream O2 sensors for Honda Civic cost $30 to $80. OEM Denso or NGK sensors (the suppliers for Honda factory sensors) cost $60 to $120. Labor is minimal: the sensor is accessible without lifting the vehicle on most Civic generations and takes 20 to 40 minutes.
Cause 2: Short to Voltage in the Sensor Signal Wire
The downstream O2 sensor produces a signal that ranges from 0.1 to 1.0 volts. The sensor signal wire runs from the sensor body through the chassis wiring harness to the ECM. If this wire develops a short to a 12-volt source (for example, from contact with the alternator output wire, contact with a bare chassis section that has unintended voltage, or from internal PCM harness bundling contact), the ECM reads a fixed high voltage that does not correlate to actual sensor output.
Distinguishing a wiring short from a failed sensor: disconnect the downstream sensor electrical connector at the sensor end. If the P0138 voltage reading on the scan tool drops to zero or a very low value with the sensor disconnected, the high voltage was coming from the sensor element: replace the sensor. If the voltage remains at 0.9 volts or higher with the sensor completely disconnected, the high voltage is coming from the wiring (a short to voltage in the signal wire), not the sensor itself.
This test takes five minutes and eliminates the possibility of replacing a perfectly good sensor while a wiring fault is actually responsible.
Cause 3: Severely Rich Upstream Running Condition
A Civic running excessively rich upstream (from a failed upstream sensor commanding excessive fuel, severely leaking injectors, or a failing fuel pressure regulator holding excessive rail pressure) can produce a post-converter exhaust so rich in hydrocarbons that even a functioning converter cannot oxidize all of them. In this case, the downstream sensor accurately reads a genuinely rich condition and the P0138 reflects a real exhaust composition rather than a sensor fault.
This cause is typically accompanied by additional codes: P0172 (system too rich Bank 1) from the upstream fuel trim, possible misfire codes from the over-rich mixture fouling plugs, and a clearly evident fuel smell from the exhaust. Upstream fuel trim values on the scan tool will show large negative corrections (LTFT below -10 to -15 percent) as the ECM tries to compensate for the excess fuel.
If P0138 is present alone with no companion fuel trim codes and the Civic runs and idles normally, upstream rich running is unlikely: proceed with sensor and wiring diagnosis first.
Cause 4: Catalytic Converter Allowing Raw Fuel Passage
A catalytic converter whose substrate has been destroyed (from sustained misfires introducing unburned fuel, from physical impact, or from simply being at end of life) may no longer convert hydrocarbons. In this case, unburned fuel from normal combustion that would normally be oxidized in the converter passes through unconverted, and the downstream sensor accurately detects the rich post-converter exhaust.
A converter failure that causes P0138 would also typically cause P0420 (catalyst efficiency below threshold) because the converter is failing its efficiency monitor. A P0138 without a concurrent P0420 makes converter failure less likely as the primary cause, though a catastrophically failed converter (not just degraded) can cause P0138 before P0420 sets.
Civic-Specific Sensor Locations by Generation
Civic Generation | Engine | Downstream Sensor Location | Approximate Mileage for Common Failure |
|---|---|---|---|
7th gen (2001-2005) | D17A, K20A3 | Under floor, downstream of cat on exhaust pipe | 120,000 to 180,000 miles |
8th gen (2006-2011) | R18A1, K20Z3 | Midpipe downstream of underbody cat | 100,000 to 150,000 miles |
9th gen (2012-2015) | R18Z1, K24Z7 | Midpipe downstream of underbody cat | 100,000 to 150,000 miles |
10th gen (2016-2021) | L15B7, K20C2 | Downstream of underbody converter | 90,000 to 130,000 miles |
11th gen (2022 to present) | L15CA hybrid and 2.0L | Downstream position; refer to service manual | Insufficient mileage data |
Diagnostic Sequence
Step 1: check for companion codes. P0172 alongside P0138 suggests a rich running condition causing the high sensor reading: address the fuel trim issue first. P0420 alongside P0138 suggests converter failure: have the converter inspected. P0138 alone points to the sensor or wiring.
Step 2: disconnect the sensor and check whether the scan tool voltage drops to near zero with the sensor unplugged. If it does not, there is a wiring short to voltage independent of the sensor.
Step 3: with the sensor connected and the engine at warm operating temperature, monitor live downstream voltage. Record whether the sensor ever oscillates or is permanently stuck above 0.9 volts.
Step 4: test the downstream sensor heater resistance. An open heater (infinite resistance) requires sensor replacement.
Step 5: if the sensor is confirmed to be the cause, replace it with an OEM-grade sensor (Denso or NGK brand for Honda applications). Cheap universal sensors frequently do not meet the signal response specification for Honda ECMs and can set codes again within a short time.
“The P0138 on my Civic sent me down a rabbit hole for weeks. I replaced the downstream sensor, cleared the code, and it came back in two days. A mechanic finally pulled the wiring diagram and found that the sensor signal wire was bundled next to the alternator output wire under the dashboard. The alternator output wire had a spot of worn insulation and was feeding 14 volts into the O2 signal wire. No wonder every sensor I put in tested bad immediately.” — Lauren Tse, 2009 Honda Civic LX 1.8L
Frequently Asked Questions
Is P0138 the same as a failing catalytic converter?
Not necessarily. P0138 can appear with a perfectly functional converter if the sensor itself is failed or if there is a wiring short. P0420 is the code specifically associated with catalytic converter efficiency failure. Check which codes are stored: P0138 alone usually means sensor or wiring; P0138 plus P0420 together suggest the converter may be involved.
Can I drive my Civic with P0138 active?
Yes. The downstream sensor does not affect closed-loop fuel control (that is the upstream sensor’s job). P0138 affects the converter efficiency monitor and emissions compliance. The Civic will drive normally with P0138 active, but it will fail any OBD emissions test due to the illuminated check engine light.
Will a cheap universal O2 sensor work on the Honda Civic?
Universal O2 sensors work functionally in many applications, but Honda ECMs are known to be sensitive to sensor response time and calibration. Multiple shop experiences report that universal sensors from low-cost brands produce P0138 or P0420 return codes on Honda applications because the signal response characteristics do not match Honda specifications. Use OEM-brand sensors (Denso, NGK, Bosch for some applications) that are vehicle-specific rather than universal for best results.
How long does it take to replace the downstream O2 sensor on the Civic?
On most Civic generations, the downstream sensor is accessible from below without removing any major components. An experienced DIYer can complete the job in 30 to 45 minutes. If the sensor is seized from corrosion (common on higher-mileage vehicles in salt-belt regions), penetrating oil applied 24 hours before removal and an O2 sensor socket with a breaker bar are required. A seized sensor that breaks off in the bung requires an extractor set and may require professional assistance.
The Bottom Line
P0138 on the Honda Civic is most often a failed downstream oxygen sensor, and the sensor disconnect test immediately confirms whether the high voltage is coming from the sensor itself or from a wiring fault. Perform the disconnect test before ordering any part. If the sensor is confirmed as the cause, use an OEM-brand vehicle-specific sensor rather than a universal unit for reliable, code-free operation on Honda ECMs. If companion codes P0172 or P0420 are also stored, address those root causes alongside or before the downstream sensor replacement.