Vehicle Sensors and OBD II DTC List

[Shaun_300]

I hear a lot of people come up to me all the time asking me what the abbreviation means or what this sensor and that sensor means or does in their vehicle, and I'm sure most have you wondered the same thing at one point, so I'll explain some of the main sensors that all vehicles have, some are really simple and self explanatory, some aren't. I won't get into the more complex ones I have to deal with in transmissions because if you don't get confused with these, you definitely will with those!

First off, I'm sure you all know the term vehicle's "computer". They're the big brain that controls vehicles nowadays. Some of our Cadillac's have up to 40 modules controlling different things in the vehicle from the engine to a drivers door. The main ones are called a number of things depending how much they do. Some manufacturers, mainly imports refer to them as ECU. The domestics usually go ECM (Engine Control Module), TCM (Transmission Control Module) or PCM (Powertrain Control Module). A PCM controls both the engine and transmission in the same module. An ECM/TCM are two separate modules, one controls the engine only, and TCM is transmission only. They talk back and forth on data lines to share information to eachother. Our (GM) different ways of communicating are serial data, Class 2 data, GM low speed LAN and GM high speed LAN. Now we're getting confusing, I'll stop there. Onto the sensors.

Intake Air Temperature Sensor (IAT Sensor): This small 2-wire sensor is found in your intake pipe between the air filter/box and the throttle body, it measures the temperature of the incoming air going into the engine. The ECM reads this value and plays with things such as fuel timing, idle speed, transmission line pressure, etc.

Mass Air Flow Sensor (MAF Sensor): Also found in your intake pipe between the airbox and throttle body. This measures the amount of air coming through from the air filter. The ECM reads this and varies the amount of fuel to push through the injectors depending how much air is there too feed the cylinders. These sensors are VERY sensitive, the smallest amount of dirt on them can screw them up. When they screw up it'll make the engine run very rough, sometimes hardly at all.

Throttle Position Sensor (TPS): This is connected to your throttle plate on the opposite side the cable hooks to the throttle body. It reads how much the throttle plate in % and volts. They run on a 5V reference to the ECM, 0V = 0% throttle angle, 5V = 100% throttle angle. This also is an input to the ECM for fuel tailoring, it is also used by the transmission as well.

Idle Air Control Valve (IAC Valve/Motor): This is a stepper motor found in your throttle body. When you have you foot off the gas the throttle plate is closed, the engine still needs air to run, this is the IAC's job. It is a small pintle that's in a bore in the throttle body. Depending how much load is on the engine, more or less air has to be metered through by the IAC. It stretches out and pulls in to let so much air through the port around the throttle body into the intake manifold. Sometimes these go, electrically or mechanically. I just had a Cavalier today that was shorted internally. Sometimes it wouldn't idle at all, sometimes it'd rev WAY up. Sometimes you'll get a no idle condition with just a really dirty IAC port, take the valve out, clean it all out with brake clean and you're good to go again.

Manifold Absolute Pressure Sensor (MAP Sensor) This is a 3 wire sensor found in the intake manifold. It also runs off a 5V reference. It measures how much vacuum is in the intake manifold, with the engine off it reads stoichiometric which is right around 100kPa (14.7psi). The most vacuum found on an engine is at idle, the reading gets down to 30kPa or so, at wide open throttle there's no vacuum, so it goes back up to 100kPa. Only way it goes over that is with a turbocharger or supercharger on the engine. Turbo/Supercharged engines can use this sensor to tell how much boost pressure is in the intake, for example at 10psi boost the MAP sensor would read 24.7psi or 165kPa. Or they could have another sensor in the charge air pipe which is between the intercooler and the throttle body to do the same thing, or use for a boost gauge in the dash. Our factory turbocharged vehicles like the Cobalt SS and HHR SS have both sensors.

Oxygen Sensors/Heated Oxygen Sensors (O2S/HO2S sensors): These sensors are one of the most important inputs to an ECM/PCM. They are located in your exhaust, they calculate the exhaust mixture to see what the oxygen content is, this has the greatest effect on your fuel system. Most vehicles have multiple oxygen sensors, for example a V8 Chevy truck has 4, 2 on each bank of cylinders, 1 before and 1 after each catalytic converter. The ones before are the most important ones, tell the ECM/PCM what the mixture is and the module adds or takes away injector pulse rates to keep the air/fuel ratio perfect. The ones after the cats just monitor the efficiency of the cats. The readings are in mV (millivolts) 450mV is stoichiometric. <450mV is lean, which means the PCM will add more fuel, >450mV means rich, and the PCM takes away fuel. I've seen many GM trucks have a pre-cat sensor fail and read 0mV which means the PCM cranks the fuel to the one bank so much all 4 cylinders misfire, and black soot and nearly raw fuel comes out the tailpipe. Sometimes this will kill a converter if the customer doesn't bring it in to be fixed quickly. Heated oxygen sensors simply have a heater in them to warm them up quickly to get the fuel system in closed loop as quickly as possible, to better fuel economy. Open loop is how the vehicle runs on a cold start, it ignores the O2 sensor readings since they are not accurate until they get hot. You'll hear the term "Bank 1 Sensor 1, Bank 1 Sensor 2, Bank 2 Sensor 1, Bank 2 Sensor 2.", this just identifies sensor location. Bank 1 is always the bank where #1 cylinder is which on any GM V8 is on the drivers side. Bank 2 is passenger side bank. Sensor 1 is pre-cat, Sensor 2 is post-cat.

Crankshaft/Camshaft Position Sensors (CKP/CMP sensors): These sensors are located close to the crankshaft and camshaft. There's a reluctor wheel on each with a tooth pattern, these tell the ECM their positions which in turn tells the coils to spark and injectors to pulse. They also detect misfires, and can even tell you which cylinder is misfiring specifically.

Engine Coolant Temperature Sensor (ECT Sensor): This sensor is located in a coolant jacket, usually screwed into the cylinder head. It detects what the temperature of the coolant is. It's used a bit for fuel timing, but also is used to tell the PCM when to turn the electric cooling fans on.

Alright! Those are the main ones, any questions let me know.

Here's a master DTC list for OBD II Diagnostics. If you're wondering why your check engine light is on, it could be one of these kabillion codes.

http://www.myscantool.com/dtc/powertrain.html

[PA_Spike_King]

I bet your fingers are tired after typeing all of that great info. LOL

[Shaun_300]

Nah not really, once I get yapping about car stuff I just keep going and going like the energizer bunny. Couldn't believe it was that long after I posted it.

[redkneck]

Good post bud. To me there is no better tool to own than a scanner and a service manual. I just oredered one for the wife's BMW. It was $100 for the thing, and that was 33% off, but will be worth every penny!

A scanner and Google is a shade tree man's best friends

[Shaun_300]

Yup, also there's some good automotive forums to ask questions and get answers. Just gotta watch who you listen to that's all. Can get some really good information or really bad information.

[TBow]

Sensors and their applicable engiine tuning adjustments are truly a boon to a vehicle's fuel efficiency and performance improvements........when they're working!

BUT man oh man! What a hole they can put in the pocket book when they crap out. I just recently had a couple "service engine" lights come on, and after some checking, it appears it'll take about $2K to remedy the problem. OUCH! And that was just to replace the sensors that were faulty.

This is a good thread Shaun. It came right in time when I needed some understanding of what was going on in my truck's bits and bytes.

TBow

Edited November 15, 2010 by TBow

[LETMGROW]

2k in sensors?

What are the symtoms and codes?

What is the vehicle?

Lynn

[TBow]

2007 Chev "classic" Silverado, 2500HD, Duramax (LBZ) with 6-speed Allison tranny, extended cab, 4X4 (Z71), 8-foot box.

The engine performance didn't seem to be affected, although the MPG (actually L/100km) readout on the dash did seem to have been negatively impacted. The two engine codes that originally popped up were "turbo related" codes. I checked on the pricing of parts first before making an appointment and found out the two parts that were needed, retailed around $1900 cdn (OUCHHHHHHHHHHH!!!!!!!). Friends told me that diesel parts were gon'na be expensive.

Had the parts replaced today under warranty (thank goodness for drive-train warranties). Drove it home and filled her up with diesel. Soon as I started her up at the pumps, the "service engine" light came on again! OH GREAT! She's going into the shop tomorrow morning to have the techs check the latest engine code(s).

Still, I'm averaging about 18 -19 mpg combined, which beats the heck out of the 14 mpg hiway on my old Dodge Ram with a 318 Magnum. If you can believe the dash readings, I'm getting 11.1L/100km (24.5 mpg) hiway. I can live with that.

TBow

Edited November 23, 2010 by TBow

[Shaun_300]

Gotta remember that just because there's a code for a sensor, doesn't mean it's always a sensor. Could be the module or wiring to the sensor. Also if there's 2 or more codes one could be setting the others. Don't be throwing expensive parts at things if you don't know it needs the parts. Get them diagnosed properly. A code for a $1000 part could be as simple as a bad connection.

[TBow]

Don't be throwing expensive parts at things if you don't know it needs the parts. Get them diagnosed properly. A code for a $1000 part could be as simple as a bad connection.

I hear ya Shaun. As long as it's GM throwing the cash at the problem, that's O.K. with me. It's gon'na be another thing once the warranty card plays itself out and 'ol TBow has to foot the bill.

TBow

[TBow]

Had the codes checked today and one of the previous codes is back up and another new one now. They've all been turbo codes, so back into the Chevy dealership she went this morning. Checked with them tonight and they're going to change the entire turbo. Gon'na take about 4 days for the parts to get here, then probably another day to change everything I'm guessing. Couldn't have happened at a worst time what with it being peak rut and all. Oh well! C'est la vie. Better they replace it all under warranty now, than it break down after the warranty expires.

TBow

[rage man]

crank position sensor

What symptoms usually show he crank sensor is going bad?

[Shaun_300]

What symptoms usually show he crank sensor is going bad?

Sorry for the delayed reply. For faster replies make your own thread rather than posting a question to a stickied thread. I usually overlook information threads like this for questions.

Symptoms for a crank sensor going bad is a misfire, the engine may run rough or chuck and buck like crazy, or it simply may not start at all and leave you stranded. Most of the time they'll set a check engine light and set a code for a crankshaft position (CKP) sensor performance. Some of the newer engines will see the failure and the ECM will ignore it and run off the camshaft position sensor(s) and throw the check engine light on.

What kind of vehicle do you have and why do you suspect the crank sensor is going bad?

[rage man]

thanks for reply

It was cutting off randomly. I got a code scanner and have since replaced the crack sensor. It was a pain in the butt.

05 Nissan altima. 2.5.