Automotive Diagnostics

99 flashcards by MentisLibrary

Diagnosing driveability and electrical faults on modern vehicles: OBD-II codes and monitors, scan data and fuel trims, ignition and compression testing, voltage drop and parasitic draw, sensor circuits, emissions systems and CAN networks.

What are the steps of a structured diagnostic process?
Verify the customer concern, gather codes and freeze frame, research service information and bulletins, inspect visually, form a theory, test to confirm or eliminate it, repair, then verify the repair and confirm the monitors run.
Why verify the customer concern before anything else?
Because the concern you fix has to be the one they brought the car in for. A stored code and a customer complaint are frequently unrelated, and a symptom you cannot reproduce is a repair you cannot verify.
What is lost the moment you clear codes?
Freeze frame data, monitor readiness status, and the fault's history. Freeze frame is the snapshot of operating conditions at the instant the code set, and it is often the most useful data on the vehicle.
Why check service bulletins before testing?
Because manufacturers publish known patterns, updated parts and reflash calibrations for faults that testing alone will send you chasing. A bulletin can turn a two hour diagnosis into a five minute lookup.
What does a proper visual inspection catch that a scan tool never will?
Chafed and rodent damaged wiring, disconnected hoses, cracked intake ducting, leaking gaskets, aftermarket accessory taps, corroded connectors, missing fasteners, and recent work done badly.
What is the parts cannon, and what is wrong with it?
Replacing likely parts in sequence hoping one fixes the fault. It is expensive, it destroys the evidence, and it teaches you nothing when it works by accident.
What is half-splitting a circuit or a system?
Testing at the midpoint to decide which half contains the fault, then repeating. Each test eliminates half the remaining possibilities instead of one component.
How do you verify a repair properly?
Reproduce the original operating conditions from freeze frame, confirm the symptom is gone, confirm no new codes set, and then run the appropriate drive cycle so the relevant monitors complete and report ready.
How do you approach an intermittent fault?
Capture it rather than hunt it. Use freeze frame, record and playback on the scan tool, min and max recording on the meter, and a wiggle test on the suspect harness while watching live data.
Why is a known-good comparison so valuable?
Because specifications for live data are often unpublished. Comparing the same PID, waveform or reading against an identical known-good vehicle or the opposite bank tells you what normal looks like.
What does a wiring diagram tell you that the car cannot?
Which circuits share a fuse, a ground, a splice or a reference voltage. That is how one failure produces symptoms in several unrelated systems.
When did OBD-II become required, and what did it standardize?
On 1996 model year light vehicles sold in the United States. It standardized the connector, the communication protocols, a common set of diagnostic trouble codes, and a required set of emissions monitors.
What is the OBD-II data link connector, and where is it?
A standardized 16 pin trapezoidal connector defined by SAE J1962, located in the passenger compartment within reach of the driver's seat, most commonly under the dash on the driver's side.
Which data link connector pins carry power and ground?
Pin 16 is battery positive. Pin 4 is chassis ground and pin 5 is signal ground.
How is a five character diagnostic trouble code structured?
The first character is a letter for the system: P powertrain, B body, C chassis, U network. The second character identifies whether the code is a standardized generic code or manufacturer specific. The third identifies the subsystem, and the last two identify the specific fault.
What does the second character of a P code tell you?
A zero indicates a code standardized by SAE and shared across manufacturers. A one indicates a manufacturer specific code whose definition comes from that manufacturer. The remaining values are also defined but their meaning depends on the code group.
What do the common third character values in a P code mean?
One and two indicate fuel and air metering, three indicates the ignition system or misfire, four indicates auxiliary emission controls, five indicates vehicle speed and idle control, six indicates computer output circuits, and seven and eight indicate the transmission.
What is the difference between P0300 and P0301?
P0300 is random or multiple cylinder misfire detected. P0301 through P0308 identify a misfire in a specific numbered cylinder.
What are P0171 and P0174 telling you?
System too lean on bank 1 and bank 2 respectively. The module is reporting that it has had to add fuel beyond its normal range to reach the target air fuel ratio.
What does P0420 mean, and what does it not mean?
Catalyst system efficiency below threshold on bank 1. It means the downstream oxygen sensor is following the upstream sensor too closely, which indicates the converter is not storing oxygen as expected. It does not automatically mean the converter is bad.
What do P0442 and P0455 indicate?
Evaporative emission system leaks. P0455 indicates a gross or large leak, and P0442 a smaller one. A loose or failed fuel cap is a classic cause of a large leak code.
What is the difference between pending, stored and permanent codes?
A pending code has failed once and is waiting to see whether it fails again. A stored code has met the criteria to turn on the malfunction indicator lamp. A permanent code cannot be cleared with a scan tool and is erased by the module only after the relevant monitor runs and passes.
What is the difference between a steady and a flashing malfunction indicator lamp?
A steady lamp indicates a stored emissions related fault. A flashing lamp indicates a misfire severe enough to damage the catalytic converter, and the vehicle should be stopped promptly.
What are the OBD-II modes, and which ones do you actually use?
Mode 01 current data, 02 freeze frame, 03 stored codes, 04 clear codes, 05 oxygen sensor test results on older protocols, 06 on-board monitoring test results, 07 pending codes, 08 control of an on-board system, 09 vehicle information, and 0A permanent codes.
What is Mode 06, and why is it worth learning?
The raw on-board monitoring test results: the actual measured value for each monitor test, the limit it is compared against, and whether it passed. It shows a system drifting toward failure long before a code sets.
What are readiness monitors, and why do they matter?
Self tests the powertrain module runs on emissions systems. Continuous monitors include misfire, fuel system and comprehensive components. Non-continuous monitors include catalyst, evaporative system, oxygen sensor, oxygen sensor heater and exhaust gas recirculation, and each runs only under specific conditions.
What is a drive cycle?
The specific sequence of operating conditions, including cold start, idle, steady cruise and deceleration, needed to let the non-continuous monitors run. Each manufacturer publishes its own.
What is a PID, and what should you do with a suspicious one?
A parameter identification, one live data value the module reports. When a PID looks wrong, verify it against a physical measurement, because the module reports what the circuit tells it, not what the world is doing.
What is calculated engine load, and what does it tell you?
A percentage the module derives from actual airflow compared with the peak airflow the engine could take at that speed. It is a normalized way to compare operating points across engines.
Why should you graph scan data rather than read numbers?
Because a fault is usually a relationship between two values over time. Graphing shows a dropout, a lag, a glitch or a mismatch that a scrolling number never reveals.