Once you've identified the trouble code(s) in the vehicle, you can proceed with the diagnosis.
IMPORTANT: A CODE BY ITSELF IS NOT A DIAGNOSIS, ONLY A STARTING POINT! For a full comprehensive review of what's going on inside your car, we recommend a professional scan tool like TOAD.
What you should do next is use the code to look up the appropriate diagnostic chart in a service manual or electronic database.
The diagnostic chart will describe the procedures you need to use to identify and isolate the problem. Accurate diagnosis is essential if you're going to solve the problem.
Guessing or replacing parts is diagnosis by trial-and-error. It's not very effective and it gets real expensive.
If you do not have access to a service manual or electronic data base, you can try a common sense approach to diagnosis. If a trouble code indicates a particular circuit, check the connectors and wiring for continuity.
Check to see that voltage is getting through. If a sensor is involved, check to see if the sensor's output signal changes as operating conditions change. No change would indicate a faulty sensor.
For diagnostic checks, you will need a multimeter that can read voltage, resistance and/or frequency.
A scan tool can also be used to read sensor data directly through the onboard computer. For more advanced diagnosis, a digital storage oscilloscope (DSO) or graphing multimeter can be used to observe and compare sensor waveforms.
This can help you detect abnormal signals or problems that occur too quickly for a multimeter or scan tool to detect.
The following tips don't cover all possibilities, but they may help you identify a problem more quickly:
OXYGEN SENSOR CODES
These can be set if the O2 sensor readings remain low (lean), or high (rich), or do not change quickly enough, or do not change at all.
The O2 sensor reads unburned oxygen in the exhaust, and generates a voltage signal that is proportional to the amount of oxygen in the exhaust. The signal can vary from a low of about 0.1 volts up to a high of about 0.9 volts.
A low voltage signal indicates a lean fuel mixture. A high voltage signal indicates a rich fuel mixture. The engine computer uses the O2 sensor's input to balance the fuel mixture during closed loop operation. A bad sensor may prevent the system from going into closed loop, and usually causes the fuel mixture to run rich causing an increase in fuel consumption and emissions.
A low voltage (lean) reading may indicate a bad O2 sensor, a vacuum leak, or a condition that allows unburned oxygen to
enter the exhaust. Check intake vacuum at idle, and inspect vacuum hose connections. If okay, check for a misfiring cylinder, a burned exhaust valve that is leaking compression, or a leaky exhaust manifold gasket.
O2 sensor quick checks include watching the sensor's output voltage as the fuel mixture changes. Momentarily disconnecting a vacuum hose will cause a lean response from the O2 sensor. No change in the reading or a very sluggish response would indicate a bad O2 sensor.
NOTE: If you get an oxygen sensor code plus a random misfire code and a MAP sensor code, the engine probably has a serious vacuum leak.
MANIFOLD ABSOLUTE PRESSURE (MAP) SENSOR CODES
These can be set if the MAP sensor output remains too high or too low (out of range), or the MAP sensor readings do not correspond to the Throttle Position Sensor (TPS) readings.
MAP sensors monitor changes in intake vacuum, which is a way of determining engine load. When engine load goes up, intake vacuum drops. Vacuum is highest at idle and drops during acceleration and wide open throttle.
The computer uses the MAP sensor's input to vary ignition timing and the fuel mixture. So a MAP sensor problem may cause drivability problems such as surging, poor fuel economy and performance.
MAP sensors either generate a voltage or frequency signal as engine vacuum (load) changes. Vacuum leaks can cause problems with the sensor's reading, so check for leaks and the sensor's vacuum connection to the engine.
How to check: a MAP sensor's voltage or frequency output should change when engine vacuum (load) changes. If you don't see a change, the sensor is probably bad and should be replaced.
THROTTLE POSITION SENSOR (TPS) CODES
These can be set if the TPS readings are too high or too low (out of range), if the signal is lost, or if the signal does not correspond to the MAP sensor's readings.
The TPS sensor monitors the position of the throttle so the computer can add more fuel when the engine is accelerating or under load. The computer may also need to know when the throttle is at idle or wide open to control other functions.
A bad TPS can cause driveability problems such as hesitation. The sensor's resistance changes as the throttle moves, causing the return voltage signal to vary. Look for a change in the voltage output as the throttle opens and closes. No change or skips
in the output would indicate a faulty TPS sensor.
Also note: the idle voltage is adjustable on some TPS sensors and must be set within specifications for accurate operation. If the voltage adjustment is not within specifications, it can adversely affect performance and throttle response.
ENGINE COOLANT TEMPERATURE OR COOLANT SENSOR CODES
These can be set if the coolant sensor readings do not change as the engine warms up, if the readings are too high or too low (out of range), if there is no signal from the sensor, or if the engine overheats.
The coolant sensor monitors engine temperature. This is a key function because it allows the fuel management system to go into the closed loop mode of operation when the engine warms up. The computer also uses engine temperature to control other functions, too. A failure here can prevent the system from going into closed loop causing a rich fuel mixture, and an increase in fuel consumption and emissions.
A coolant sensor's resistance changes as the temperature increases. If you don't see a change in the resistance as the engine warms up, or the resistance is out of specifications, the sensor is bad.
Other things that can cause bad sensor readings include a low coolant level in the cooling system, a thermostat that is stuck open or shut, or a thermostat that has the wrong temperature rating for the engine.
CAUTION: Do not open the radiator cap if the coolant is hot! Wait until the engine and radiator have cooled to open the cap.
This type of code can be set on newer vehicles with OBD2 systems if one or more cylinders are misfiring. This can be caused by fouled spark plugs, a weak spark (weak coil), loss of compression, vacuum leaks, anything that causes an unusually lean fuel mixture (lean misfire), an EGR valve that is stuck open, dirty fuel injectors, low fuel pressure, or even a dose of bad gas.
A "random misfire" code usually indicates a vacuum leak or bad gas.
If a misfire is only occurring in one or two cylinders, the code will tell you which cylinder(s) to focus your attention on. Start by removing and inspecting the spark plugs. If oil or carbon fouled, the engine is probably sucking oil past worn valve guides and seals. Replacing the spark plug will only temporarily solve the misfire problem.
If the spark plug is normal, check the plug wire and boot for damage or looseness. Measure resistance end to end. Replace the wire if resistance exceeds specifications (about 50,000 ohms/foot).
Next, check compression. If low, the cylinder may have a burned exhaust valve or leaky head gasket.
If ignition and compression are both okay, focus next on the injectors. Use a test light or voltmeter to see if the injector is receiving voltage when the key is on, and that the injector solenoid clicks when the injector circuit is grounded. If the injector is functioning electrically, it may be clogged with fuel varnish deposits. If cleaning cannot restore normal fuel delivery, the injector must be replaced.
Misfire in two "paired" cylinders in an engine that has a distributorless ignition system (DIS) would usually tell you the DIS
system has a bad coil. If each coil has its own coil, only one cylinder will be affected.
EGR RELATED CODES
These codes can be set if the EGR valve is not opening, or is not flowing enough exhaust gas to keep oxides or nitrogen (NOX) emissions under control.
The exhaust gas recirculation (EGR) system routes a small amount of exhaust back into the intake manifold when the engine is under load or accelerating to prevent combustion temperatures from soaring too high. This keeps the formation of NOX to a minimum. If the valve fails to open, the engine will usually experience detonation (spark knock or ping) under load. If the valve is stuck open, it will create a vacuum leak causing idle roughness, hesitation and lean misfire.
If the valve can be visually inspected, look for movement in the valve stem as the engine is revved (it may be necessary to partially restrict the tailpipe to create sufficient backpressure for this to occur on some vehicles). Applying vacuum directly to the EGR valve itself should also cause it to open, causing a noticeable change in idle quality. No change usually indicates a bad EGR valve.
If the EGR valve itself is working, check to see that the vacuum hose connection is properly routed and is not leaking. Also, check any solenoids in the line for proper operation.
Electronic EGR valves use one or more solenoids to open the valve(s). These can be tested by applying voltage to the
solenoid terminals. No movement would indicate a bad valve.
EGR valve passageways in the manifold are often found to be blocked with carbon deposits. Remove the valve and inspect for deposits.
CRANKSHAFT SENSOR & IGNITION REFERENCE CODES
These can be set if the computer fails to see the proper timing signal from a crankshaft sensor, camshaft sensor or distributor pickup.
The computer needs a timing signal for ignition timing and fuel injector timing. No timing signal will prevent the engine from starting.
A quick check is to see if the engine generates a spark when it is being cranked. No spark typically indicates no pickup or timing signal, a bad coil or a bad ignition module.
Check for a crankshaft sensor output signal when the engine is being cranked. Hall effect sensors will create an on-off voltage signal. Problems with this type of sensor include loss of the 5 volt reference signal from the computer (they need a supply voltage to produce a return signal).
Magnetic crankshaft position sensors produce an alternating current (AC) signal. Problems here include loose or broken wires, metallic debris on the sensor tip, or the sensor tip is too far away from the machined notches or reluctor wheel. If a magnetic sensor's resistance is out of specifications, it is bad and needs to be replaced.
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