Testing Trouble Code P0135. Front Oxygen Sensor Heater Tests. 2.0L (SOHC, DOHC) Neon

A trouble code P0135: O2 Bank 1 Sensor 1 Heater Circuit Performance is not hard to diagnose when it pops up and lights up the check engine light (CEL) on your 97, 98, or 99 Dodge or Plymouth Neon.

In this tutorial, I'll show you how to test the upstream oxygen (O2) sensor's internal heater element to see if it's fried (and causing the P0135 DTC) or not.

Here are the contents of this tutorial:

  1. Circuit Descriptions of the Upstream Oxygen Sensor.
  2. TEST 1: Checking Bank 1 Sensor 1 Power Circuit.
  3. TEST 2: Checking the Heater Ground.
  4. TEST 3: O2 Heater Resistance Test.
  5. Where to Buy the Oxygen Sensor and Save Some $$$.

NOTE: This upstream oxygen sensor tutorial covers the 1997, 1998, 1999 Dodge Neon (Plymouth Neon) with a 2.0L SOHC or DOHC engine. You can find the 1995, 1996 tutorial here: Trouble Code: P0135 Test 2.0L Dodge Neon (1995-1996) .

Circuit Descriptions of the Upstream Oxygen Sensor

Circuit Descriptions of the Front O2 Sensor. Testing Trouble Code P0135. Front Oxygen Sensor Heater Tests. 2.0L (SOHC, DOHC) Neon

If you've done any type of research... you've found out by now that the Bank 1 Sensor 1 oxygen sensor is known by several different names:

  1. O2 sensor before the catalytic converter.
  2. Front oxygen (O2) sensor.
  3. Bank 1 Sensor 1.
  4. Pre-catalytic converter oxygen (O2) sensor.

It doesn't matter what the upstream 02 sensor is called... the fact of the matter is that it has 4 wires coming out of it.

2 of these 4 wires belong the the heater element inside the oxygen sensor. The other 2 are the ones that provide the oxygen content info of the exhaust, to the PCM (Powertrain Control Module = Fuel Injection Computer).

To get to the bottom of the P0135 Trouble Code, we don't have to test all 4 circuits (wires).. we only have to worry about the 2 wires that supply the heater element with power and ground.

The chart below will help us to identify all of the upstream oxygen sensor wires on your 97, 98, or 99 Dodge or Plymouth Neon with a 2.0L engine (SOCH or DOHC).

O2 Bank 1 Sensor 1 Circuits
Pin Wire Color Description
1 Dark Green w/ Orange stripe Heater Element 12 Volts (from Auto Shutdown Relay)
2 Black Heater Element Ground
3 Black w/ Light Blue stripe HO2S Low Signal
4 <--> Tan w/ White stripe HO2S High Signal (1997, 1998 Neon)
4 <--> Black w/ Dark Green stripe HO2S High Signal (1999 Neon)

<--> Connector may have more than color depending on the build year.

TEST 1: Checking Bank 1 Sensor 1 Power Circuit

Checking Bank 1 Sensor 1 Power Circuit. Testing Trouble Code P0135. Front Oxygen Sensor Heater Tests. 2.0L (SOHC, DOHC) Neon

Alright, to get our P0135 diagnostic on its way...

... The very first thing we'll check is if terminal number 1 of the O2 sensor's harness connector is feeding the O2 sensor's heater element with power.

This power is in the form of battery voltage (10 to 12 volts DC) and in we'll check for it with the Key On Engine Off (KOEO).

Important: All of the O2 sensor tests should be done with a completely cold engine to avoid the risk of getting severely burned by the O2 sensor! Be careful and take all necessary safety precaution!

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

  1. Locate the upstream oxygen sensor and disconnect it from its electrical connector.
  2. Locate the wire that connects to terminal number 1 of the engine wiring harness oxygen sensor connector.
    1. This is usually a Dark Green w/ Orange stripe wire.
  3. With your multimeter in Volts DC mode:
    1. With the red multimeter lead, probe the wire that connects to terminal number 1 of the O2 harness connector (see illustration in the image viewer).
    2. Ground the black multimeter lead.
  4. With the Key On, engine Off, this 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 is good and let's you know that the oxygen sensor before the catalytic converter on your Dodge Neon (or Plymouth Neon) is getting power from the Auto Shut Down Relay.

The next step is to make sure that the oxygen sensor's heater element is getting fed ground on terminal number 2 of the O2 sensor's harness connector. For this test, go to TEST 2: Checking the Heater Ground.

CASE 2: Your multimeter DID NOT record the indicated voltage- 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 you still don't see 10 to 12 Volts DC... then this test result exonerates the upstream oxygen (O2) sensor on your Dodge Neon (Plymouth Neon) as BAD....

Although it's beyond the scope of this article... the next step is to check the continuity between the terminal number 1 wire of the O2 sensor harness connector and the Auto Shutdown Relay.