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All codes › P0138 › 2017 Honda Civic

P0138 on a 2017 Honda Civic

Post-Cat O2 Sensor High Voltage (Bank 1 Downstream)

Severity: low Safe to drive (short term) Compact Sedan 2015-2019 Honda Civic

What does P0138 mean on a 2017 Honda Civic?

P0138 is set when the downstream (post-catalyst) oxygen sensor on Bank 1 reports a voltage above the calibrated maximum (typically above 1.0 V) for an extended period. The downstream sensor normally sits at a relatively steady 0.6–0.8 V because the catalyst smooths the rich/lean oscillation it sees coming in. A reading stuck above 1.0 V is either a genuine very-rich condition reaching the catalyst, a shorted signal wire, or a failed sensor biased high.

Symptoms on a 2017 Honda Civic

Likely causes on a 2017 Honda Civic

  1. Failed Bank 1 downstream O2 sensor (stuck high) Most common
    Estimated repair: $150– $450
  2. Shorted-to-power O2 signal wire (sensor reads max) Common
    Estimated repair: $80– $350
  3. Genuine rich condition reaching the catalyst (leaking injector, FPR) Common
    Estimated repair: $150– $1,200

    Address the rich condition; the sensor is correctly reporting it.

  4. Damaged sensor connector — corrosion biasing the signal Common
    Estimated repair: $30– $200
  5. Internal sensor short due to contamination Occasional
    Estimated repair: $150– $450
  6. Coolant or oil in the exhaust (head gasket / valve seals) Rare
    Estimated repair: $1,200– $3,500

How to diagnose this on a 2017 Honda Civic

  1. Determine whether the reading is real or sensor-reported

    Read fuel trims at idle and 2500 RPM. If Bank 1 LTFT is around −10 % or worse (the ECM pulling fuel out), the engine is genuinely running rich and the downstream sensor is correctly reporting it. If LTFT is near zero, the engine is fine and the sensor or its wiring is the problem.

    Tools: Scan tool with fuel trim PIDs

  2. Inspect for shorted signal wire

    Disconnect the Bank 1 downstream O2 sensor. With the connector unplugged and the key on, scan-tool voltage on the signal wire should fall to bias voltage (0.4–0.5 V). If voltage stays high (above 1 V), the signal wire is shorted to power somewhere between the sensor and the PCM.

    Tools: Scan tool, Multimeter

  3. Verify with a known-good sensor

    The downstream sensor is essentially a slow O2 sensor that should not switch rapidly. If a known-good sensor (or the Bank 2 downstream sensor if equipped) reads correctly while the Bank 1 downstream sticks high, the Bank 1 sensor is bad.

    Tools: Scan tool with sensor swap capability

  4. Inspect for fuel contamination of the catalyst

    If a separate rich-running fault (P0172, stuck injector) has been present for months, fuel may have entered the catalyst and altered its oxygen-storage behavior — which the downstream sensor sees as a chronic rich signal. This requires the rich cause fixed AND likely catalyst replacement.

    Tools: Diagnostic history review

  5. Check for exhaust system damage

    Look for any damage to the exhaust between the catalyst and the downstream sensor that could let oil mist or coolant into the exhaust stream. Coolant entering the exhaust (head gasket, cracked head) creates rich-mimicking sensor readings.

    Tools: Visual inspection, Coolant pressure tester

Known Technical Service Bulletins for the 2015-2019 Honda Civic

Manufacturers publish Technical Service Bulletins (TSBs) when a known issue affects a specific vehicle. These bulletins come from the NHTSA database for your Honda Civic.

+14 more TSBs available in MECH AI's TSB explorer for this vehicle.

Common fixes

About the 2015-2019 Honda Civic

The 2015-2019 Honda Civic was commonly sold with the following powertrains: 2.0L I4, 1.5L Turbo I4, 1.8L I4. Common trims include LX, Sport, EX, Touring.

What the downstream O2 sensor actually does

Unlike the upstream sensor, which feeds closed-loop fuel control, the downstream sensor’s main job is to monitor catalyst efficiency. It also provides a slow secondary fuel-trim correction. A properly functioning catalyst smooths the rich/lean oscillation coming from the engine — so the downstream voltage normally hovers steadily around 0.6–0.8 V instead of switching.

P0138 means that smoothing is gone and the post-cat reading is biased high — either the cat is gone, the engine is running rich, or the sensor itself is misreading.

P0138 with no other rich codes

This pattern usually points at the sensor or its wiring rather than a fuel system problem. A genuine rich condition severe enough to peg the downstream sensor at >1 V would also set P0172. P0138 alone with normal fuel trims is the textbook “downstream sensor has failed” pattern.

When P0138 reflects a real fuel problem

If P0138 sets alongside P0172 (system too rich), P0171 (lean) with heavy intake leak, or recent fuel-injector work, the sensor is likely correct and the fueling is at fault. Replacing the sensor without fixing the upstream condition will result in P0138 returning within days.

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