Malfunction Indicator Lamp (MIL) (or Check Engine Light)

Normally, you should see the malfunction indicator lamp (MIL) only momentarily at engine start-up.

If this light comes on while you're driving, something is wrong. Many fault conditions can trigger this light. First, I need to find out what specific DTC has caused the MIL to come on.

The automotive specification On-board Diagnostics II (OBD2 or OBDII). Among other things, OBD II specifies these fault conditions, or diagnostic trouble codes (DTCs), and the interface necessary to retrieve them.

Actron OBD II (OBD2) PocketScan Reader

First, I need an OBD2 reader or scanner is required to retrieve the diagnostic trouble codes (DTCs). There is a wide range of OBD2 products available.

The features I need are simple:

I want to retrieve the diagnostic trouble code (DTC).

I want to reset, or turn off, the malfunction indicator lamp (MIL), or Check Engine Light.

A low-end reader is all that I really need. Hence, I bought the Actron PocketScan Reader CP9125 for $46.99 (Spring 2008) from www.amazon.com.

OBD2 Reader and Data Link Connector (DLC) Hook-up

The OBD2 data link connector (DLC) is located underneath the dashboard on the driver's side. It is the only connector that can mate with the OBD2's DLC cable.

Reading OBD2 Diagnostic Trouble Codes (DTC)

After the OBD2 reader is connected, I can now read the pesky diagnostic trouble code (DTC) that has lit up my malfunction indicator lamp (MIL):

1. I start the engine and press the "READ" button. My OBD2 reader reports 2 error codes.
2. I press the "READ" button again and read the first DTC: P0155. Referring to the booklet that came with my OBD2 reader, I learn that P0155= "O2 Sensor Heater Circuit Malfunction (Bank 2 Sensor 1)".
3. Pressing the "READ" button again, yields a second pending P0155 diagnostic trouble code (DTC).

So, now I know what caused the MIL. Next, I had to decide how to fix it.

Oxygen Sensor Replacement Cost Options

First, some background. There are a total of four oxygen sensors. Two oxygen sensors are before catalytic converter oxygen, one on bank 1 (the left side of that car if you are sitting in the driver's seat) and one on bank 2 (the right side). Two oxygen sensors are after the catalytic converter. The pre-catalytic converters are subject to more heat and stress and fail earlier than the two post-catalytic oxygen sensors at about 90,000 to 100,000 miles.

Oxygen sensors allow an automobile's fuel injection system to measure and fine-tune the engine's air-fuel mixture. On newer cars, there are 2 oxygen sensors: one before the catalytic converter and one after. The one before the catalytic converter is subject to higher temperatures and wears out faster.

On the LX470, there are a total of four oxygen sensors. Relative to the driver, on the left manifold (bank 1), there is one before the catalytic converter (bank 1, sensor 1) and one after (bank 1, sensor 2). On the right manifold (bank 2), there is one before the catalytic converter (bank 2, sensor 1) and one after (bank 2, sensor 2).

Given that I have 90,000+ miles on my LX470 and the DTC P0155, it is safe to assume that my bank 1, pre-catalytic oxygen sensor has reached its service life. Further, if I replace one pre-catalytic oxygen sensor, it only makes sense to replace the other pre-catalytic oxygen sensor since the second will fail soon as well.

The bad news: cha-ching! Means some hard-earned money is going to have to be spent. Let's go over some of the costlier options for the repair:

$768 for labor and parts to replace both pre-catalytic converters at $384/each. This is how much my local Lexus dealership wanted in Summer 2008.

$312 for 2 new Toyota/Lexus oxygen sensors PN#89465-50150 at $156/each. This is how much my local Lexus dealership wanted in Summer 2008.

$278 for 2 new Toyota/Lexus oxygen sensors PN#89465-50150 at $139/each. This is how much my local Toyota dealership wanted in Summer 2008.

$194 for 2 new Denso PN#234-4169 at $97/each. This is online at www.oxygensensors.com, including shipping and handling. Denso supplies many OEM parts for Lexus/Toyota like Bosch does for BMW.