2008 Toyota Yaris – Cranks but Won’t Start After ECM Replacement

[Ghull]

Background:
Vehicle originally ran but had a rich fuel mixture issue after completing a head gasket and timing chain/guides/gears replacement. Over the course of troubleshooting that problem, the following parts and components have been replaced or tested:

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Parts Replaced / Work Done

Head gasket

Timing chains, guides, and gears

Spark plugs

Ignition coils

Fuel injectors

Fuel pump

Oxygen sensor (upstream)

Mass Air Flow (MAF) sensor

ECM (replacement unit from Flagship One)

Relay/fuse block on driver-side fender (replacement made, but no change)

Battery terminals and grounds cleaned (including G101/G102 at radiator support)


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Electrical & Diagnostic Testing

Verified 12V power and ground at OBD2 connector

Verified ~2.5V on CAN high/low lines (pins 6 and 14)

Tested resistance between MAF connector and ECM connector for continuity – good

Checked all underhood fuses and relays for power on both sides – all good

Check engine light illuminates with key ON (indicating ECM is powered)

Verified power at ignition fuses in dash and underhood fuse panels

Cleaned and re-secured multiple ground connections (battery, chassis, radiator support)


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Symptoms

Engine cranks but does not start

No fuel or spark output detected

OBD2 scanner fails to auto-detect VIN and cannot connect to ECM directly

Some other modules (like ABS/body) show partial communication

Check engine light still turns on with ignition (suggesting ECM has power but may not be communicating)

Tried both ECMs (original and replacement) — same result