P0135 Trouble Code Diagnostic Tests (1995-2000 1.6L Honda Civic DX, EX, and LX)

A failed upstream O2 sensor heater element causing a P0135: Primary Heated Oxygen Sensor (Primary HO2S)(Sensor 1) Heater Circuit Malfunction trouble code is a very common problem on the 1.6L Honda Civic.

Thankfully, the upstream O2 sensor's heater element can be easily tested with just a multimeter (no scan tool required) and in this tutorial you'll find out how.

Here are the contents of this tutorial:

  1. Circuit Descriptions Of The Upstream Oxygen Sensors.
  2. TEST 1: Verifying The Heater Element Is Getting Power.
  3. TEST 2: Verifying The Heater Element Is Getting Ground.
  4. TEST 3: Testing The Heater Element's Resistance.
  5. Where To Buy The Oxygen Sensor And Save Some $$$.

En Español You can find this tutorial in Spanish here: Cómo Probar El Código P0135 (1995-2000 1.6L Honda Civic) (at: autotecnico-online.com).

Circuit Descriptions Of The Upstream Oxygen Sensors

P0135 Trouble Code Diagnostic Tests (1995-2000 1.6L Honda Civic DX, EX, and LX)

Your Honda Civic's upstream O2 sensor is a 4 wire sensor.

Each wire has a specific job to do. The two that we're gonna' be concerned about, to diagnose the P0135 trouble code lighting up your Civic's check engine light, are the ones that carry power and ground to the upstream O2 sensor's heater element.

The table below shows a brief description of the front O2 sensor's circuits:

Here are the circuit descriptions of the 4 wires (taking into account the engine code):

Upstream Oxygen Sensor Pinout
(1995-2000 1.6L Honda Civic DX, EX, and LX)
Pin Wire Color Description
1 WHT PHO2S (Sensor Signal)
2 GRN/BLK SG2 (Sensor Ground)
3 BLK/YEL Heater Element Power
4 BLK/WHT Heater Element Ground

TEST 1: Verifying The Heater Element Is Getting Power

P0135 Trouble Code Diagnostic Tests (1995-2000 1.6L Honda Civic DX, EX, and LX)

To get our P0135 diagnostic off the ground, the first thing we'll do is check that the BLK/YEL wire (of the harness connector) is feeding power to the upstream O2 sensor's heater element.


This is the first of 3 multimeter tests that we'll do to diagnose the front oxygen sensor on your Honda Civic (DX, EX, or LX).

IMPORTANT: If your Honda Civic's engine has been running, wait till it cools down completely. The O2 sensor can get and stay very hot! Be careful and take all necessary safety precautions!

OK, this is what you'll need to do:

  1. 1

    Locate the upstream oxygen sensor and disconnect it from its harness connector.

  2. 2

    Locate the BLK/YEL wire of the engine wiring harness oxygen sensor connector.

  3. 3

    With your multimeter in Volts DC mode, probe the BLK/YEL with the red multimeter lead.

    Ground the black multimeter lead directly on the battery's negative terminal.

    NOTE: You should avoid probing the front of the harness connector's female terminals or you run the risk of damaging them permanently with the multimeter's test lead. I recommend back probing the connector or using a wiring harness connector.

  4. 4

    With the Key On Engine Off (KOEO), the BLK/YEL wire should have 10 to 12 Volts DC.

Let's take a look at your test results:


CASE 1: Your multimeter recorded 10 to 12 Volts DC- This voltage test result tells you that the BLK/YEL wire is feeding the upstream oxygen sensor's heater element with power.

The next step is to make check that the BLK/WHT wire, of the O2 sensor harness connector, is feeding ground to the heater element. For this test, go to TEST 2: Verifying The Heater Element Is Getting Ground.

CASE 2: Your multimeter DID NOT record the indicated voltage- Re-check that you're testing the correct wire and re-test.

If you still don't see 10 to 12 Volts DC then you now know that the O2 sensor itself is not causing the P0135 code, since without power the upstream O2 sensor's heater won't work.

Although it's beyond the scope of this article, the next step is to find out why this battery power is missing using a wiring diagram.