MIL Light Mayhem

Mike writes:

“The following report is abased on a 1994 Toyota Camry with a 3.0 Litre V-6 engine. Mrs. Murphy, and yes, that is her real name, telephoned me and asked if I might be able to help her solve a problem with her car. She informed me that the MIL light had come on and that she was afraid to drive the car. A few words of assurance and she soon arrived at my shop (Duguay’s Auto Repair, Surrey, B.C.-ed) .

The first thing I did was to hook up the scanner and identify the fault code or codes. The only code was:

PO135 Heated Oxygen Sensor Heater Circuit Malfunction (B-1 S-1)

The next step was to consult my Mitchell Repair Manual (1994), page 108, and then follow the suggested diagnostic checks, as outlined in Section #1. The flow chart asks you to check the voltage readings of the three Oxygen Sensors (Heater Element Circuits) at the ECM and record the results. The readings were :

Terminal HTR to bank 1 sensor 1Voltage: 9.5V

Terminal HTS to bank 1 sensor 2Voltage: 13.8V

Terminal HTL to bank 2 sensor 1Voltage: 13.8V

The specifications call for a voltage reading of 9-14 volts, and since the readings I got were within the required specifications, I decided to check for the possibility that there might be a loose connection or some other abnormality. I was surprised, and somewhat relieved, to find a loose connection at the oxygen sensor at the bank 1 sensor 1 position. The repair was completed, the fault code was cleared and the vehicle was road tested. And since the MIL light did not come back on, I sent the customer on her way.

But before Mrs. Murphy left my shop, I mentioned that the repair manual recommended as the next step to repair the problem, that the ECM should be replaced! I was not convinced that this was necessary…something else was wrong. But what? I expressed my thoughts to her and she agreed to drive the car. She would call if the MIL light came back on.

A few days later, the phone rang. Mrs. Murphy was worried all over again: “Should I drive the car back to you or have it towed to your shop?”, she said, adding, “I’m afraid that I might damage the vehicle’s computer or something else!”

“Not to worry”, I assured her and within thirty minutes, she arrived.

Again the scanner was hooked up to the ECM and the same fault code was identified. This time, however, I chose to move on to the next step (Part 2) in the Repair Manual:

Check Resistance of Heated Oxygen Sensor Heater

The resistances of each of the oxygen sensors were checked and the following readings were recorded:

HTRbank 2 sensor 1Resistance:14.3 Ohms

HTRbank 1 sensor 2Resistance:14.3 Ohms

HTRbank 1sensor 1Resistance: OPEN GOT YA!

The source of the problem was finally found! The sensor was quickly replaced, fault code cleared and a thorough road test was completed. The vehicle was returned to Mrs. Murphy and she had no further problems.

I’m happy that I did NOT replace the ECM as the flowchart suggested. Although the voltage readings for the sensors were within specifications, we need to, as technicians, check and then double check circuits, connections, etc. before replacing an expensive component like an ECM.