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Passing inspection

Tuesday, June 2, 2015 - 07:00
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Working as a mobile tech for the last 20 years, I have had a lot of strange but exciting repairs. Using the word exciting might be a little strange to some of you, but along with this job comes a lot of challenges. The challenge to be able to quickly and efficiently solve the problem is what makes it exciting. It’s also exciting that when a job is a tough one it usually is a job that I learn from. In a lot of ways, these jobs are how we as techs get our training. You really get to know the system and how it works when you spend a good amount of time analyzing, reading and testing the circuit or circuits at hand. Fortunately, making those sacrifices and spending extra time on jobs of this type is what will enhance your skills and knowledge for the next tough job.

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Vehicle Details
Year: 2003
Make: Chevrolet
Model: Impala LS
Mileage: 120,639
Engine: 3.8L V6
Transmission: Automatic
Mileage: 129,025
Customer Complaint: Will not pass state inspection

A failed inspection
This month’s story is about a 2003 Chevy Impala that needs to have a state inspection performed to be in compliance with the states motor vehicle law. Here is a quick rundown on how my state does its test. The vehicle has to pass a safety test and an enhanced emission test. On the safety test side of the inspection, the mechanical components have to be in good working order, as well as other parts of the vehicle like the lights, tires, etc. The emission part of the test has to do with the engine and computer operating system. Since I live in upstate New York, we don’t have to do a smog test or tailpipe test like the folks down by New York City. On this particular vehicle, the check engine light was not on. If that indicator light is on at the time of inspection it automatically fails.

Although the check engine light was not on, the customer’s car still failed because not all of the monitors had completed. Monitors are tests that the Engine Control Module (ECM) runs on the system and/or components it controls. The state-run inspection program, a software program that looks at all the monitors in the vehicle's ECM, checks to see what monitors have run to completion and passed. In New York State, a vehicle older than model year 2000 can have up to two monitors not completed and still pass the emission test. A vehicle newer than model year 2000 can only have one monitor not completed. 

This Impala came in with the check engine light off, but with codes P0135 and P0141 stored in the ECM memory. The description for these codes is heated oxygen sensor (O2) performance bank 1 and bank 2, respectively. Along with the codes that were stored, the monitors for the heated oxygen sensor, the catalyst monitor and the evaporative emission monitor were not complete. Most of the time these codes are set due to the fact the O2 sensors’ internal heater isn’t working correctly. The shop replaced both of the sensors twice thinking they received faulty parts. After reviewing the freeze frame data set by these codes, I didn’t notice anything that stood out to alert me of a problem.

The next step was to grab a wiring diagram and look at the circuits of these two O2 sensors. These sensors each have a four-terminal connector. Terminal A is the sensor low signal circuit, terminal B is the sensor high signal circuit, terminal C is the ground circuit and terminal D is the power feed circuit. Looking at the sensor schematic, I first checked the fuse that powers this circuit. The fuse was actually labeled oxygen fuse. I hooked up my meter to the negative battery terminal and checked both sides of the fuse. I didn’t use a test light as I personally like to see the voltage being read. Remember, it doesn’t take a lot of voltage to light a test light. With today’s computer-controlled vehicles, using a Digital Volt Ohmmeter (DVOM) is much more accurate. So at the fuse I had 12 volts on the meter. Next I unplugged the O2 sensor connector and put my meter on terminal D, which is the 12-volt feed from the oxygen fuse. I had a solid 12 volts. It looks like my sensors are getting the power to turn on, right?

The check engine light wasn't on, but there were codes stored leading to a failed emission test for this Chevy. Restoriing the ground brought the oxygen sensors back online, but now they were pointing me to a problem with the cat. The EScan test verified a damaged converter.
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