80s–00s Lexus/Toyota ECU/ECM

80s–00s Lexus/Toyota ECU/ECM Post-Repair Troubleshooting Guide

Important: Use this guide if you reinstalled your repaired 80s–00s Lexus/Toyota ECU/ECM and the problem is still happening.
Important: Please do not open/disassemble the ECU. Opening it can damage the unit and may void warranty coverage.

Common concerns this guide helps with:

• No start / no spark / no injector pulse
• Hard shift or shifting issues (ECU-controlled transmissions)
• Incorrect voltage outputs / sensor reference issues
• Intermittent stalling / rough running

Step 1: Confirm the correct ECU and installation

• Confirm the ECU part number matches the original ECU removed from the vehicle
• Confirm all ECU connectors are fully seated and latched (no “half-click”)
• Verify no pins are bent, pushed back, or corroded in the ECU plugs

Step 2: Battery voltage and grounds (do this first)

• Battery should be fully charged
• Inspect and clean battery terminals
• Verify main engine ground straps are intact and tight (battery-to-body and body-to-engine)
• If the issue began after recent battery/alternator work, re-check what was touched

Step 3: Check ECU power and ECU fuses/relays

• Check all ECM/EFI/IGN fuses (engine bay and interior fuse panels)
• Check the EFI main relay / ECM relay (swap with a known-good similar relay if applicable)
• If any fuse is blown, do not keep replacing it—find the short first

Step 4: Verify ignition spark (to separate ignition vs fuel)

• Check for spark at the coil/igniter output (or at a plug wire depending on model)
• If NO spark:
  • Confirm power to ignition coil/igniter
  • Inspect crank/cam sensor connectors and wiring
  • Check ignition-related fuses/relay again
• If spark IS present, move to Step 5

Step 5: Verify injector pulse (noid light) and fuel delivery

• Use a noid light (or equivalent) to confirm injector pulse while cranking
• If NO injector pulse:
  • Confirm ECU is powering up (Step 3)
  • Check crank/cam sensor signal source and wiring (these commonly disable injector pulse)
  • Verify immobilizer/security system is not active (if equipped)
• If injector pulse IS present:
  • Confirm fuel pressure and pump operation
  • Confirm injectors are not clogged/stuck (especially on vehicles that sat)

Step 6: Scan for codes (if your vehicle supports OBD scanning)

• Pull stored and pending codes
• Focus first on:
  • Crank/cam sensor codes
  • Immobilizer/security-related codes
  • Power supply / main relay codes
  • Communication-related codes (if applicable)
• Clear codes, key cycle, and re-check what returns immediately

Step 7: Check for vacuum/air intake issues (rough idle / stalling)

• Inspect for disconnected vacuum lines
• Check intake ducting for cracks or loose clamps
• Verify MAF connector is fully seated
• If the issue began after recent intake/throttle body work, re-check those areas first

Step 8: Hard shift / shifting issues (ECU-controlled)

• Verify transmission fluid level and condition
• Check transmission fuse(s) and any “ECT/AT” related fuses
• Inspect the transmission range selector / inhibitor switch connector (if equipped)
• If you have codes, note any shift solenoid or speed sensor codes—these can mimic ECU faults

Step 9: If the ECU communicates but the problem persists

• Intermittent or vehicle-side electrical faults are common on older platforms:
  • Corroded grounds
  • Loose fuse box terminals
  • Failing main relay
  • Damaged wiring near the battery, ECU, or engine harness bends
• Perform a careful harness/connector inspection before assuming the ECU is still at fault

What to send us if the issue continues

• VIN
• ECU part number (from the label)
• Exact symptoms (no start, no spark, no injector pulse, hard shift, etc.)
• Any codes pulled (even if they seem unrelated)
• Battery voltage readings (key off + while cranking)
• Clear photos of ECU connectors/pins and any corrosion found