Fault Tree: Petrol No Power / Limp Mode

Quick triage & tools

Scan first: read codes (all modules if you can) + live data that matches the symptom.
Power basics: battery voltage (resting + while cranking) and quick fuse checks before deep dives.
Have ready: basic scan tool, multimeter, and a way to confirm the symptom (road test / idle test).

Tip: if a step says “check wiring”, use Wiring diagrams basics → and Voltage drop testing →

Before you start

Scan for codes + freeze frame. Note engine load, RPM, coolant temp, and whether the fault appears under boost or cruise.
If it’s in limp right now, cycle the ignition and see if full power returns briefly — that helps separate hard faults from intermittent ones.

Decision flow

Is the engine overheating or pulling timing heavily?
- Coolant temp high or fan running constantly → solve cooling first (limp is often protective).
- Normal temps → proceed.
Air/boost quick checks (most common)
- Check intake pipework, intercooler hoses, clamps, PCV lines for splits or oil mist trails.
- If you can, do a smoke test before replacing sensors.
- Codes like P0299 / underboost, P0101 often point here.
Throttle/ETC plausibility
- Any ETC codes (P2101, P2111, P2112, P2135)? → inspect throttle body connector, pedal sensor, wiring and grounds.
- Check live data: pedal % should track throttle % smoothly (no jumps).
Fuel trims: lean vs rich direction
- Positive trims (+) under load → suspect fuel pressure/flow limitation or unmetered air.
- Negative trims (-) → suspect leaking injector, high fuel pressure, MAF bias, or EVAP purge stuck open.
Fuel pressure reality check
- GDI engines: compare requested vs actual rail pressure in live data during a pull.
- If actual drops below request → filter/low-pressure supply, HP pump, or pressure control issues.
MAF/MAP plausibility
- Unplugging the MAF is not a definitive test, but if it instantly improves driveability, suspect MAF bias or intake leaks.
- Compare MAP at key-on engine-off to barometric pressure (should be close).
If still stuck
- Do a voltage drop check on engine grounds (bad grounds cause sensor plausibility faults).
- Check exhaust restriction (collapsed cat) if it won’t rev freely even in neutral.

← Back to fault trees

Print / save checklist

Tick these off as you work. If you need to hand this to a mechanic, print it as a short job card.

Freeze-frame captured / conditions noted
Battery voltage checked (resting + cranking)
Basic visual checks (hoses, connectors, grounds, fuses)
One test at a time (don’t change multiple variables)
Confirm fix by reproducing the original condition

Print / Save as PDF Back to symptom selector

What to do next

Use the links below to deepen the test you’re about to perform, cross-check related codes, or jump to a faster symptom-led flow.

Find another symptom flow Jump to the symptom selector to locate the closest decision tree. Workshop Guides Deep-dive how-to tests: voltage drop, wiring diagrams, smoke testing, fuel pressure and more. Diagnostic Codes Look up DTC meanings, common causes, and related checks. AI Tools Use AI assistance to summarise symptoms, plan tests, or sanity-check a diagnosis.