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Computer Diagnostic Trouble Codes (DTC's)

A newer computer has a set of trouble codes which can be recalled even after the engine is turned off. These computer codes tell which sensor has an "out of specs" reading.

Intermittant computer sensor problems

Sometimes a check engine light will go off after awhile: this means the computer sensor is reading "OK" again and the computer is happy. An intermittant sensor problem on most vehicles still causes a trouble code to be stored in the computer. The trouble codes can be erased from an older computer using a scan tool, or by disconnecting the battery or pulling the computer fuse for a few minutes. Some newer computer systems keep the codes until they are reset by a scan tool.

Limp Home Mode

When the MIL (Check Engine Light) is on the computer will assume a set of values for the bogus values it's getting from the sensor. Your car may go into a "limp home mode" where the computer "assumed" sensor values make it run differently, even poorly, but it won't leave you on the side of the road. The car can get bad gas mileage and run poorly when in limp home mode, depending on which sensor is defective.

Reading Computer Trouble Codes

Older Vehicles

Older vehicles had different ways of reading trouble codes from their computer. The computer interfaces were different, and the computer diagnostic capabilities varied widely. Some Japanese vehicles had LED lights that you viewed through a clear window in the side of the computer, which was under the seat. The computer blinked the codes to you... blink blink........................blink..... That's a code 21.
Some American vehicles would blink their MIL (Check Engine Light) to communicate computer codes: Chrysler/Mopar would blink codes to you if you turned the ignition switch on and off 3 times, leaving it on after the third time (This may still work!) GM would blink codes if you shorted between 2 terminals of the computer connector. A paper clip worked nicely.

Of course if you owned a shop you had a reader for each manufacturer's computer, or for the cars you worked on the most. Do it yourself-ers usually didn't have computer code readers because they cost hundreds of dollars. Also: the cars changed and computer software updates were required: manufacturers used the same computer trouble code number for different things on different years.

Newer Vehicle Computer

A newer vehicle computer (after 1996) has a standardized interface to access the computer trouble codes. Manufacturers agreed (under some government pressure) to develop a common interface to automotive computer functions. They called this interface "On Board Data", or "OBD". The latest version used is OBD 2 or "OBD II"
They even agreed on SOME terms: the computer that controls the engine is called the ECU, or Engine Control Unit, and the "Check Engine" light is called the MIL, or Malfunction Indicator Light.

Manufacturers can still have their own individual interfaces with the computer, but the OBD II computer (ECM) interface has to be there to sell a car in the USA.

Reading Trouble Codes with OBD II

You need a scan tool to read OBD computer trouble codes, but they are fairly cheap ($40 or so basic, full featured $100-$150) and available at any parts store. The readers read out the computer trouble code number and give a brief description of the code's meaning, like "MAP sensor voltage low" and so on. The scan tool will also erase the codes from the computer and turn the MIL off.

Most computer sensor problems are diagnosed by checking resistance or voltage at the sensor or at the computer wiring harness connector. A digital multimeter is all you need for most sensor tests.

Computer Sensor List

Manufacturers call their computer components by different names, but all the systems are similar in function. Here is a list of the most common sensors and what they do. Most all sensors povide a variable resistance to the computer to let it know what's happening. 5 volts are fed to the sensor, and it returns 0 to 5 volts back to the computer. Although later systems can do without certain sensors and still run, many systems will run poorly or not at all because of one defective sensor.


This stands for manifold Absolute Pressure Sensor. It's nothing but an electronic vacuum gage. As the manifold vacuum changes, the MAP sensor supplies a variable voltage to the computer. FAILURE SYMPTOMS: Poor running, stalling, light on dash NOTE: A MAP sensor can be inaccurate, sending an incorrect voltage to the computer. If this voltage is still within the range of voltage the computer expects, a light or trouble code for the MAP sensor may not be set. Fords are particularly susceptable to this. problem. Thus, if the MAP is old, it might be a good maintenance item.


Often a car will have several of these. All of them have a coolant/engine block temperature sensor. Some also have a manifold temp sensor and an intake air temperature sensor. FAILURE SYMPTOMS: Rich or lean mixture, black smoke (rich), light on dash


This lets the computer know how far you press the accelerator pedal down. It often has a wide open throttle and a closed throttle switch either as part of it or as separate components. FAILURE SYMPTOMS: Hesitation on quick acceleration (passing, "floorboarding it") , sometimes bad idle. May not illuminate light or set codes.


The latest thing: this actually measures how much air enters the engine and adjusts the fuel/air mixture accordingly. FAILURE SYMPTOMS: Poor running, stalling: all conditions. May not illuminate light or set codes.


This measures the exhaust oxygen content. The computer fine tunes the mixture using data from this sensor. OBD II has 2 levels of oxygen sensors: an upstream and a downstream oxygen sensor. The upstream sensor(s) (2 sensors on a "V" motor) give constant feedback to the computer and are used to fine tune the fuel air mixture. The downstream oxygen sensor is placed after the catalytic converter. Its ONLY purpose is to make sure the catalytic converter is working properly and cleaning the exhaust up. FAILURE SYMPTOMS: Poor fuel economy, driveability, light on dash.


(Heated Oxygen Sensor): First seen on Fords, this is just an Oxygen sensor, except it's HEATED! Heated Exhaust Gas Oxygen sensor. It is electrically heated so it works immediately on engine startup: the other type must be heated by the exhaust before it starts to work. A computer using a HEGO sensor places a lot more emphasis on the data from the Oxygen sensor, and uses this data sooner while the engine is still cold.


Used on all engines with computer controlled timing advance, this tells the computer which cylinder should be firing. Some engines use a sensor on the front pulley of the engine (Harmonic balancer or vibration dampener), another sensor in the engine block on a special toothed wheel on the crankshaft inside the engine block, and a third sensor on the camshaft gear. Earlier systems have a pickup inside the distributor as the only input as to crank position. FAILURE SYMPTOMS: No start, hesitation, misfire


This adjusts the mixture according to altitude: the same settings won't work both in Denver and Death Valley! FAILURE SYMPTOMS: Poor fuel economy, light on dash In addition, the computer often monitors things like vehicle speed, brake pedal on or off, power steering pressure, A/C on or off, and transmission shifter position (tells it what gear you're in). FAILURE SYMPTOMS: Poor fuel economy, light on dash, stalling while parking or at traffic lights All vehicles do not have all of these sensors.


Although all computer sensors must function properly to meet emission standards and obtain maximum fuel economy, only a few of them are crucial for proper engine operation. This is important because an engine which runs really badly or not at all can baffle you with dozens of trouble codes. For most cars the "essential" or "major" computer sensors are: MAP sensor, Mass Airflow Sensor (if so equipped), Engine Block Temperature Sensor, and the Crankshaft/Distributor Position Sensor. These sensors can make an engine barely run or not run at all. The other sensors make very fine adjustments to the fuel/air mix and timing and will not cause a gross poor running condition. This is important, because often a "major" sensor will cause a "minor" sensor to read out of specs. If the car barely runs, or doesn't run at all, suspect one of these "major" sensors (and other non-computer stuff, like wiring, coil, fuel supply, or engine mechanical problems) before trying to trace down a code generated by a "minor" sensor.


Your car smokes black, smells like gas, and barely runs. The computer gives a "rich oxygen sensor" trouble code. The Oxygen sensor is replaced. The car still smokes black, still has a "rich oxygen sensor" trouble code. The codes said the Oxygen Sensor reads rich. This just confirms what any good mechanic could tell by smelling the exhaust: it's running rich!! The car really needs a temperature, MAP, or Mass Airflow sensor. Possibly a HEGO sensor if it's not THAT rich. Or it could have a burned intake valve, a leaky injector, a lot of things.


Computer Diagnostic Trees in Repair Manuals

At one time the company who remanufactures GM computers was running an 80% "NO DEFECT" rate on their rebuildable "cores". This meant that only 20% of the computers being replaced were bad!

Watch out for diagnostic "trees" in repair manuals. Don't skip steps! When you get to the bottom of a diagnostic tree and it says "replace computer" DON'T DO IT unless the thing is completely "brain dead" (that is the computer gets power but fails to operate ANYTHING) , or the computer does some things but not everything it should. This should be tested at the back of the computer: wires fail! Intermittants can happen with computers too, but make sure it's THE LAST THING you replace! Every time I've been "fooled" by a computer control issue it's been a wire rubbing on a rough place on the vehicle making an intermittant short to ground.

Computers are voltage sensitive: an undercharging or overcharging alternator, even an alternator with a weak diode can drive the computer crazy! Check system voltage and voltage to the computer before you replace it!

Good Grounds are essential

Each sensor and every device controlled by the computer requires a good ground. There are a bunch of grounds in the fuel injection and computer wiring harnesses for this reason. A broken or loose ground wire can cause all kinds of problems and failures.

How important is a code?

OBD II has so many things it monitors that some of it's codes can be ignored in a pinch, or can be caused by "driver error". If you leave your gas cap a little loose an "Evaporative Emission Control Problem" code can be set. Tighten the cap and the light will go off after awhile, but the code will still be stored.
A downstream Oxygen sensor code will keep your MIL on, but won't affect your fuel economy. Often when you replace this sensor it will stay off for awhile and come back on. This means you wasted time replacing the sensor: it was probably OK and your catalytic converter(s) are inefficient or about to go bad. I still recommend replacing the sensor: it's easy to put in, it does fail sometimes, and the converters are very expensive (hundreds of $$$)

(Of course if the Catalytic Converter clogs it CAN do engine damnage!)
But if you have bald tires, bad brakes, and you also have a converter that's NOT clogged up but is setting a trouble code, for sure put up with the light and fix the brakes and tires first!!!

Other Vehicle Computers

Computer controlled transmissions

Many vehicles have computer shifted transmissions. Some of these transmissions (Chrysler notably) will stay in second gear when in limp home mode. The transmission will have reverse and second gear only. There are several sensors and a solenoid pack that can fail and cause this.

Computer Climate Control

A climate control computer operates air control doors and the A/C compressor. It can shut the compressor off and not let it be turned on again until the trouble codes are reset.

Body Control Computer

Seat position, mirrors, radio stations, some cars set these for a driver, the car recognizes drivers by weight . Some body control computers do the climate control too.

Antilock Brakes

The antilock brake computer prevents wheel lockup using inputs from wheel sensors to control solenoids and sometimes high pressure pumps.

Vehicle Anti-theft and Keyless Entry

A separate computer often controls anti-theft and keyless entry. It's sometimes part of the body control computer, or radio.

Radio anti-theft systems

As an anti-theft measure some radios will stop working if the battery power is lost for more than a few minutes. A code is needed to make the radio work again. Sometimes other electronic systems on the car can be affected, like keyless unlock, etc..
The radio reset code should be on a card with the vehicle owner's manual. If you can't find it, check out for reset instructions.

Air Bags

Air bags are to the best of my knowledge always deployed by their own separate computer. This computer has one or more deceleration sensors that deploy the air bags in a collision. The latest airbag systems sense when someone is in a seat and deploy the airbags only where someone is in the seat. They also have several deceleration sensors, so they will deploy different airbags depending on the direction of the collision.

Reading other computers

Unfortunately there is no "OBD like" computer interface standard for these other computers. Many (but not all) ABS computers have the same interface, but there isn't a standard yet.

Often a modern computer will read out through the radio display when you know what buttons to press (security systems especially), and a climate control computer will often read out through its own display. Once again you need a good manual to tell you the computer readout procedures. Many of these computers "talk" to each other, so maybe someday there'll be a standard way for us to talk to them: but not today!

The Ford Powerstroke Diesel

The Ford Powerstroke Diesel (used in trucks) is computer controlled, and is very fuel efficient. Because the computer controls the fuel so precisely the powerstroke uses no glow plugs or pre-chambers. It has no injection pump per-se.
There is a downside to this computer control: there's a camshaft sensor on the powerstroke which fails sometimes and can leave you on the side of the road. Once they were started the old diesels would keep running even if the electrical system burned up.

That's the downside to all the computer controls. a computer gives us amazing performance and economy, but computer components can fail suddenly, and there's no way to easily test them to see if they are about to fail. With a point ignition you could look at your points to make sure they were gapped right, you could even file them down and adjust them. Of course they wore out and needed to be replaced yearly, and they could only handle so much voltage. An electronic ignition module is used on new vehicles instead of points and condensor, but it can suddenly burn out without warning and strand you on the side of the road. Even when points failed you could re-gap them or file them down and "limp home" to replace them. No fixing a module on the side of the road. Some people carry a spare module (Good idea for some Ford stuff) for just this reason. I think if I drove a Ford powerstroke diesel I'd carry a spare cam sensor and the tools to install it. Same with the Ford distributors with the module on the side. (It's not that I'm picking on Ford, it's just that when their stuff fails it's usually because of those 2 things. These failures are normally after many miles on the vehicles: it's not a recall issue. You may drive a vehicle with that module or a Powerstroke truck and never have a problem.)

High end autos and trucks, along with space and militarty stuff have multiple backup systems to overcome individual electronic and computer component failures.

Even with all that, I think the REALISTIC "Nuke Apocalypse" movies get it right with old 50's and 60's vehicles being the only thing running!
(The EMP (Electromagnetic Pulse) from a nuclear war would fry most electronic and computer components).

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