Downstream Oxygen Sensor Heater Test -P0141 (1998-2002 1.8L Toyota Corolla)

Testing trouble code P0141: Heated Oxygen Sensor Circuit (Sensor #2) on your 1.8L Toyota Corolla involves two basic tests. These two are: making sure the rear O2 sensor's heater is getting power and ground, and then checking the heater's internal resistance.

These two tests can be easily accomplished with only a multimeter. Yes, that's right, you don't need an expensive scan tool to test the rear oxygen sensor's heater!

NOTE: This oxygen sensor is known by several different names:

  1. Rear Heated Oxygen (O2) Sensor.
  2. HO2S 1/2.
  3. Downstream Oxygen (O2) Sensor.
  4. Oxygen Sensor Bank 1 Sensor 2.
  5. Post-Catalytic Converter O2 Sensor.

Here are the contents of this tutorial:

  1. Circuit Descriptions of the Downstream Oxygen Sensor.
  2. TEST 1: Verifying the Heater Element is Getting Power and Ground.
  3. TEST 2: Testing the Heater Element's Resistance.
  4. Where to Buy the Oxygen Sensor and Save Some $$$.
  5. More 1.8L Toyota Diagnostic Tutorials.

NOTE: If you need to test the front oxygen sensor's heater (or trouble code: P0135), see this tutorial: Front Oxygen Sensor Heater Test -P0135 (1998-2002 1.8L Toyota Corolla).

Circuit Descriptions of the Downstream Oxygen Sensor

Rear Oxygen Sensor Heater Test -P0141 (1998-2002 1.8L Toyota Corolla)

The rear oxygen sensor is equipped with an internal heater (just like the front one). The heater's job is to get the O2 sensor to its operating temperature fast and keep it there thru' all engine operation conditions (especially engine idle).

Since the O2 sensor has 4 wires sticking out of it, you and I need to know what wires do what to be able to test it. So, in the table below you'll find the color of the wires of the engine wiring harness oxygen sensor connector for the rear oxygen sensor.

One last thing you need to know: Checking for power and ground is done on the O2 sensor's engine wiring harness connector. This connector has female terminals. Testing the heater's resistance is done on the O2 sensor's connector itself... and this connector has male terminals.

Downstream Oxygen Sensor (HO2S 12) Pinout
(1998-2002 1.8L Toyota Corolla)
Pin Wire Color Description
1 PNK Heater Ground (-)
2 BLK Heater Power (+)
3 RED O2 Signal
4 BRN O2 Signal Ground

TEST 1: Verifying the Heater Element is Getting Power and Ground

Rear Oxygen Sensor Heater Test -P0141 (1998-2002 1.8L Toyota Corolla)

The rear oxygen sensor's heater needs battery power and ground to heat up the O2 sensor. So the first thing we'll do, in our P0141 diagnostic, is verify that they're being fed to the rear O2 sensor's heater.

The 2 wires that we need to check are:

  1. The black (BLK) wire (of the engine wiring harness connector) is the one that feeds the rear O2 sensor's heater with power (12 Volts).
  2. The pink (PNK) wire (of the engine wiring harness connector) is the one that feeds the heater with ground (this ground is provided by your Toyota Corolla's PCM).

CAUTION: Be careful and test the O2 sensor with a completely cold engine! The O2 sensor and the exhaust pipe it's bolted into gets extremely hot and stays hot long after the engine has been turned off. Be careful and take all necessary safety precautions! Also, don't trust the jack to keep your Corolla up in the air -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 and ground you'll test the connector of the engine wiring harness sensor (which has female terminals).

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

  1. 1

    Locate the downstream oxygen sensor and disconnect it from its engine wiring harness connector.

  2. 2

    Set your multimeter to Volts DC mode and turn the key On but don't crank or start the engine (this will power up the O2 sensor's engine wiring harness connector).

  3. 3

    With your multimeter test leads, probe the female terminals that correspond to the black and pink wires of the connector.

    Remember, you're testing the engine wiring harness O2 sensor connector (which has female terminals) and not the connector of the O2 sensor itself.

  4. 4

    With the Key On Engine Off (KOEO), your multimeter should register 10 to 12 Volts DC.

Let's take a look at your test results:

CASE 1: Your multimeter registered 10 to 12 Volts DC- Good, since this confirms that the rear oxygen sensor's heater element is getting power and ground.

The next step is to verify that your Toyota Corolla's rear oxygen sensor heater's resistance is within specification. For this resistance test, go to TEST 2: Testing the Heater Element's Resistance.

CASE 2: Your multimeter DID NOT register 10 to 12 Volts DC- The most likely cause is that power is missing due to a blown fuse or a short (or open) in the wiring. You'll need to check the fuse in the fuse box and make sure it's not blown.

If the fuse is OK, your next step is to find out why this battery power (or ground) is missing using a wiring diagram.