When trouble code P0135: Heated Oxygen Sensor Circuit (Sensor #1) sets, it usually signals a problem with the internal heater of the upstream oxygen sensor.
Thankfully, you can easily diagnose this trouble code without any expensive diagnostic test equipment... all you need is a multimeter. In this tutorial, I'll show you the 3 basic tests you'll need to perform to diagnose the upstream sensor as good or bad.
NOTE: This oxygen sensor is known by several different names:
- Front Heated Oxygen (O2) Sensor.
- HO2S 11.
- Upstream Oxygen (O2) Sensor.
- Oxygen Sensor Bank 1 Sensor 1.
- Pre-Catalytic Converter O2 Sensor.
Here are the contents of this tutorial:
- Circuit Descriptions of the Upstream Oxygen Sensor.
- TEST 1: Verifying the Heater Element is Getting Power.
- TEST 2: Verifying the Heater Element is Getting Ground.
- TEST 3: Testing the Heater Element's Resistance.
- Where to Buy the Oxygen Sensor and Save Some $$$.
- More 1.8L Toyota Diagnostic Tutorials.
NOTE: If you need to test the rear oxygen sensor's heater (or trouble code: P0141), see this tutorial: How to Test the Rear O2 Heater -P0141 (1998-2002 1.8L Corolla).
Circuit Descriptions of the Upstream Oxygen Sensor
As you're already aware, your 1.8L Toyota Corolla uses 2 oxygen sensors. One is located located before the catalytic converter and the second one is located after it.
Both sensor's are equipped with an internal heater and thus have 4 wires sticking out of them.
2 wires are for actual oxygen sensing part of the sensor assembly. The other 2 are to supply the heater with power and ground.
Below, you'll find the color of the wires of the engine wiring harness oxygen sensor connector for sensor HO2S 11:
|Upstream Oxygen Sensor (HO2S 11) Pinout
(1998-2002 1.8L Toyota Corolla)
|1||PNK||Heater Ground (-)|
|2||BLK||Heater Power (+)|
|4||BRN||O2 Signal Ground|
TEST 1: Verifying the Heater Element is Getting Power
To get our P0135 trouble code diagnostic under way, we're gonna' start by making sure that the upstream O2 sensor's heater is getting power. Then, after confirming power, we're gonna' make sure it's getting ground in TEST 2.
The black (BLK) wire, of the O2 sensor engine wiring harness connector, is the one that feeds power to the front O2 sensor's heater element.
CAUTION: Perform all of the oxygen sensor tests with a completely cold engine. The O2 sensor gets extremely hot and stays hot long after the engine has been turned off. Be careful and take all necessary safety precautions! Also, if you raise your vehicle with a jack, place it on jack stands.
IMPORTANT: The illustration of the connector above is of the connector on the oxygen sensor itself. To check for power, you need to test the BLK wire of the engine wiring harness sensor connector.
OK, this is what you'll need to do:
Locate the right front oxygen sensor and disconnect it from its engine wiring harness connector.
Find the BLK wire of the engine wiring harness oxygen sensor connector.
With your multimeter in Volts DC mode, probe the BLK wire with the red multimeter lead.
Ground the black multimeter lead directly on the battery's negative terminal.
If you don't own a multimeter or need to upgrade yours, take a look at my recommendation here: Buying a Digital Multimeter for Automotive Diagnostic Testing (at: easyautodiagnostics.com).
With the Key On Engine Off (KOEO), the BLK wire should have 10 to 12 Volts DC, if all is OK with this power circuit.
Let's take a look at your test results:
CASE 1: Your multimeter confirms that the BLK wire has 10 to 12 Volts DC- This confirms that the upstream oxygen sensor's heater element is getting power.
The next step is to make check that the BLK wire, of HO2S 11 engine wiring 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 confirms that the BLK wire DOES NOT have 10 to 12 Volts DC- Re-check that you're testing the correct wire and that the Key is in the RUN position (but don't crank or start the engine) and re-test.
If your multimeter still does not register 10 to 12 Volts DC... then you can conclude that HO2S 11 itself IS NOT BAD... since without power, the heater element 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.