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Sordid Tale of a "Service Engine" Light

Those scan tools are ok. They will: 1. read, and erase, the DTC (Diagnostic Trouble Code); 2. indicate how many “monitors have been set” (how many of the monitoring systems the computer has tested and decided that they are monitoring ok, or not monitoring ok); 3. display Freeze Frame data, for the moment the primary DTC code came on (such as: engine temp, rpm, engine load, car’s speed, fuel trim). That’s it.

To see what the sensors, and actuators, are doing, you need to go to a scan tool with a screen display (its own display screen, or uses your personal computer (pc)). These start at $250.

Go to: for some articles on using scan tool data. They have a symptom and cause booklet.

Now, the last time I had my codes read was at an inspection station. They hooked it up to a computer (with me watching) and showed me the numbers displayed for the individual O2 sensors. I think one of them was zero. This is when they inspected my hoses and showed me one that had an obvious leak. So, they were not able to suggest anything better than that from the data they were looking at. Can I use something like the scan tools or the autotap hardware/software to show me WHY a particular sensor is probably giving a bad reading, or does it just take a thorough investigation on my part? Maybe the repair book will help in that regard?

The Haynes, or Chilton’s, repair manual has the instructions for testing sensors and actuators (egr valve, idle air control valve, fuel injectors, spark production). The key is that the sensors are an integral part of their circuit. They are a part of the whole…not separate. You need a digital electrical multimeter for these checks. You check for voltage, or ohms, in a component and its circuit.

You may have at your public library (ask a librarian). Alldata has the charts to follow for a particular DTC (“code”). Or, you can go online and buy a year’s subscription, for one car/truck, for $25, to alldata.

The best way to answer your questions about is to go there and browse.

Bah, I swear, sometimes my posts get lost to the ether on this forum…

What I HAD posted was that I may try getting a manual and subscribing to alldata. Before that, however, I I’m going to do a more thourough vacuum leak investigation because when I google my three codes, the most common response is that there is a leak somewhere. I’m going to try the WD40 method because that looks to be the simplest and safest method for me.

Lastly, a question for you hellokit, I JUST happened to run across a neat looking scan tool I hadn’t found before by CEN-TECH: and it has these features, which look great for the money:

Reads Live PCM datastream

Graphs data

Reads and clears trouble codes

Views freeze frame data

Displays live O2 sensor data

Easily determines the cause of the “Check Engine Light (MIL)”

Performs modules present test

Prints data via PC

What do you think about that for a $100? $200 is a bit steep for me at the moment…

That scan tool seems to be a pretty good deal. If you scroll down on that page, you can download the 73 page manual that goes with the scan tool. I hope that it’s as easy to use as the manual implies.

For more knowledge on emissions controls, you can go to,, and get books from
The site shows you what the graphs of data, such as the oxygen sensors, look like. The more you know, the better you can use your scan tool.

how many people know what fuel trims actually do? (very great info if you know what you are seeing)and comprehend.

and system monitors, although very important.

much less freeze frame data,when most who fix their own car never clear the codes and leave all the old trash in there,to confuse themselves the next time the light comes on,just a thought.

way too much info fot Harry Homeowner.

Yes, I flipped through it. My only concern is that I couldn’t find much info on that brand… but that includes negative reviews, so it won’t stop me. I have an analog multimeter, nothing fancy, would that be enough, or do I need something digital and made for autos?
I wont’ be able to do much to the car tonight, on account of a thunderstorm, but I will take the hose to it between showers. Again, thank you so much, I will keep posting until this is resolved.

Don’t use an analog voltmeter. It can damage electronics, especially that expensive engine computer and other electronic modules. Analog voltmeter have battery power, up to 9 volts, which can harm electronics which use much less voltage to operate.

Walmart has a digital, high impedance, multimeter for about $30. That’s a low price, isn’t it?

Alrighty, I’ll spring for something like that then, thank you. I hosed down everything under the hood pretty well and tried spraying with WD40 while the engine was running to find a stutter, but didn’t. I could have missed a tube or two though, it got dark before I finished. I probably should have said in the beginning that the car IS actually running great, and idles just fine. I’ve had the car for some 5 years with no real problems.

EDIT: Oh, and I looked inside the MAF assembly and everything was clean as a whistle.

How about an update? Short update is that I’m still working on it…

I did end up buying a pretty nice scan tool ( ). One of the things it can do is tell the car computer to run a leak test on the EVAP system, but the manual warns me that I need to figure out how to stop the test once it is started… which is weird… so I’m hunting that down. Next I will clear the codes and try a bunch of things all over again. Here is kind-of a summary of data collected by the scanner. I also had it record live data during town and highway driving conditions (in a mode where it would start recording when an error was thrown), I’ve attached it to this post.

? Control Module
? $10 SAE J1850 PWM
? Vehicle Info.
? Not Supported Or Sto-red No Data.
? Since DTCs Cleared
? MIL Status ON
? Misfire Monitor OK
? Fuel System Mon OK
? Comp. Component OK
? Catalyst Mon INC
? Htd Catalyst N/A
? Evap System Mon INC
? Sec Air System N/A
? A/C Refrig Mon N/A
? Oxygen Sens Mon OK
? Oxygen Sens Htr OK
? EGR System OK
? Stored Codes
? P0155 O2 Sensor Heater Circuit Bank 2 Sensor 1
? P1131 Lack Of HO2S11 Switch - Sens Indicates Lean
? Pending Codes
? No pending codes are stored in the module!
? View Freeze Frame
? ETC(?C) 181
? LONGFT1(%) 0.0
? SHRTFT2(%) -6.3
? LONGFT2(%) 0.0
? RPM(/min) 1834
? VSS(km/h) 17
? O2 Monitor Test
? O2 Bank1 Sensor1
? Not Supported Or Sto-red No Data.
? O2 Bank1 Sensor2
? Not Supported Or Sto-red No Data.
? O2 Bank2 Sensor1
? Not Supported Or Sto-red No Data.
? O2 Bank2 Sensor2
? Not Supported Or Sto-red No Data.
? On-Board Mon. Test
? Not Supported Or Sto-red No Data.

The ECT sensor is running very hot… supposedly… maybe it is bad. I’m looking into it.

What temperature are you expecting on the ECT (Engine Coolant Temperature) sensor? I think you are confusing normal engine temperature with the temperature the thermostat opens. Thermostats open from 180F to 195F degrees. Engines run, normal operating temperature, up to 225F degrees. At idle, your engine may have cooled down to 180F degrees.

The temperature indication is from the voltage return from the ECT, to the engine computer; and, the engine computer’s interpretation of that voltage (say, x volts equals y temperature). Voltage to the ECT, reduced by the ECT’s resistance at a particular temperature, equals a pre-calculated coolant temperature value.

Your digital multimeter can be used to read the voltage in, and voltage out, of the ECT sensor, by back-probe (using straight pins, or back-probe test lead terminals). Then, you need the instructions to read the temperature dependent resistances (ohms) of the ECT sensor.

Next, use the digital multimeter, and wiring diagram, to check the wiring, and voltage, to the oxygen sensors. Go here, and click on the particular wiring diagram for your car: The oxygen sensor heaters are provided power for only a couple of minuets after the ignition key is turned ON. The ECM probably grounds the O2 heater element wires to allow current to flow in the heaters.

I was referring to the scanner freeze frame showing “ETC(?C) 181”, which is like 358?F…

An engine coolant temperature of 358F degrees is, of course, impossible. The wire from the engine to the ECT sensor (possibly, the ECT, itself) is shorted to ground (maybe through another wire, or the engine body). Here are the instructions on the ECT sensor: Go to Fig. 3. Enlarge. See the temperature (of the sensor) and the corresponding resistance (ohms) values? Now, is your chance to use that super, duper, new digital multimeter to earn its keep.

Yes! My thoughts exactly, and I used that webpage as well to help me get to the sensor, but I could not physically get my meter’s leads to the sensor because there are a bunch of parts blocking it. I will try again though. Before I did that, I was going to try clearing the codes, just in case.

You need the PCM pin-out chart for the PCM terminal identification: Click on Fig. 4, or Fig. 5 (2 valves per cylinder, or 4 valves per cylinder). Click the maximise button to enlarge.

The PCM terminal (“pin”) 91 supplies a ground for the gray/red wire, to the O2 sensor elements. The red wires are a common power (12 volts) for the O2 heaters. You can safely back-probe with a very thin (0.040", or, 1 mm) probe tip on your multimeter.

Turn the ignition key OFF. Remove fuse #19 (15A). Very important that you do!. Disconnect the electrical connector from the PCM. Check the resistance (ohms) from pin #38 (light green/red wire) to the gray/red wire on the ECTS. If that ohms is ok, a power wire, maybe a red wire, carrying 12 volts, is shorted to the ECTS LG/R wire.

In the O2 symbol, the hash lines, with an arrow through them, are the O2 elements. An O2 element generates its own voltage (from 0.1 to 1.0 volts) in a scratchy pattern. So, the O2 element isn’t connected to a voltage (it’s got its own, thank you!).

OOPS! I miss-ID’d some wires and pins. Corrections made above.

All of the images are broken at the moment… I sent them an email and will keep searching for another diagram.

I did clear all of the car’s codes and left the scanner hooked up until it threw them again. Again, the ECT was recorded as being over 300?F…

You say, “All the images are broken…”? Do you mean that when you click on a Fig. only 1/4 th of the figure comes on? Click on the maximise button in the upper right corner of the 1/4 image. The maximise button looks like the outline of one, or two, little boxes. (I doubt that AutoZoneWeb will ever respond to you. Tell me if I’m wrong. Anyway, thank Auto Zone for making wiring diagrams, and other information available. I use the hech out of it!)

Erratum, again.

Thanks hellokit, for some reason the images do not come up at my office… on Firefox or IE… But they do at home, so I guess that’s that. And yes, that autozone website is awesome! Now I’m doing a bit more research before I start poking around in there with the meter.

Before I dive into the PCM, I wanted to test the resistance of the ECT sensor. After an hour of parking the car, while the engine was still hot, I set my multimeter to (auto) ohms and put the negative lead on the engine and the positive on the prong of the sensor (I have not taken off the sensor). It read… 10 mega-ohms… Obviously it should have read something in the kilo range, so what do you make of that? I tried it several times until I finally got into a (contorted) position where I could see the prong to put the positive lead on it. Does it sound like I’m doing something wrong?