Intermittent Faults
“It only does it sometimes” is diagnosable — if you capture the conditions. This page shows how to make intermittent problems repeatable.
Step 1: define the trigger
- Hot vs cold (first start vs after a long run)
- Load vs cruise vs idle
- Road speed (motorway only, low speed only)
- Weather (rain/humidity) and electrical loads (lights/heater)
Step 2: use the clues the ECU already stored
- Pending codes matter. They often appear before the MIL turns on.
- Freeze‑frame is your “snapshot” — always save it.
- Fuel trims + misfire counters tell you what system drifted.
Step 3: make it fail on your terms
- Wiggle test harnesses/connectors while watching live data (carefully).
- Heat/cold test: heat gun or freeze spray on suspect sensors/relays (with caution).
- Voltage drop tests for grounds/power feeds (intermittent electrical faults love bad grounds).
- Log data during a drive if the tool supports it.
For live data sanity checks, see Live Data Basics.