P0135 Toyota 1.8L – Test a Corolla 02 Sensor

When you have a P0135 Toyota trouble code, it can cause serious problems with drivability and gas mileage. This OBDII check engine code signals an issue with your front oxygen sensor heater wire and the specific wording to the code is Oxygen O2 Sensor Heater Circuit Malfunction. The front oxygen sensor or air fuel sensor in your Toyota is designed to inform the ECU of how much fuel is present in your exhaust stream.

This information is required by your 1.8 liter 1ZZFE ECU to properly operate your engine by way of your Toyota fuel injectors and spark plugs to orchestrate combustion. Today I’ll be showing you how to test your primary Corolla oxygen sensor or the upstream 02 sensor and see if it has failed. This is otherwise known as the Toyota air fuel ratio sensor, EFI, fuel injection, or the Bank 1 sensor. Because the nature of the P0135 Toyota trouble code indicates, the heater circuit is what we will directly be testing.

The front Toyota oxygen sensor is typically a Denso from the factory, and the part number for this air fuel ratio sensor is 234-4207. If you are having a hard time locating the exact Denso part, try these interchange numbers for any number of replacements; ES10656, 213-1355, or ES10716. When this sensor begins to fail or becomes fouled over time, your fuel economy suffers and your 1ZZFE does not run as smoothly as it could.

To perform this test, you will need to use a multimeter to measure the heater wires on your Toyota oxygen sensor. Today I’ll be showing you how to do this in a 2000 Toyota Corolla with a 1.8 liter 1ZZFE engine in it.


Because your Toyota ECU will not be able to properly operate closed loop operation, the way in which your ECU delivers fuel can become counterintuitive. Keep your oxygen sensors in working order to prevent this from happening. If you think your front oxygen sensor is going out, use this guide to test the internal resistance of your primary 02 sensor as well as test the heater wires.

Where is my Toyota Corolla Upstream 02 Sensor?


The front or primary oxygen sensor is screwed into the lower section of your exhaust header assembly. The wiring harness from this four wire 02 sensor leads up near the water neck, and you can simply unplug the sensor to test it.

How do I test my Toyota Corolla Oxygen Sensor Heater Wires?


The primary oxygen sensor has two wires that you should test to correct the P0135 Toyota check engine code, and they are labeled below as A and B. These wires from your 2000 Toyota Corolla should exit as they are shown in the image below. PIN A is a pink colored wire, and this wire represents the heater wire low reference ground. The other wire is PIN B and this wire is a black wire that is the Corolla’s heater circuit.


To test the heater wires connect the black lead to a battery ground and gently touch the front of the engine harness at PIN B. Never force the probe into the metal leads of your electrical connectors, and use extreme care when working around your vehicle. You must have the key turned to the “ON” position, so make sure you are completely clear of your cooling fans in the event your engine is warm.

There should be switched power at this wire, and if there is move on to PIN A which is the pink wire coming into your Corolla engine harness. You must measure this wire for the ground signal to ensure that your oxygen sensor is receiving the right power and low reference ground signal.


If there is power and ground at these two wires, your oxygen sensor has either failed or there is some damage to your pigtail. Remove your oxygen sensor and replace it to clear your P0135 Toyota trouble code by using a scan tool. Have any questions about this OBDII guide? Leave them for me below!

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