Toyota P1604 records that the engine did not start or stabilize as expected; it is an event code, not a starter-motor diagnosis. First determine whether the engine cranked normally, cranked slowly, started then died, or never received a start request. Check battery voltage and cranking rpm, immobilizer/authorization and companion codes, then verify fuel pressure, MAF/throttle data, ignition and mechanical compression. For 2014–2018 Highlander and Highlander Hybrid, T-SB-0072-19 identifies a special branch: damaged or poorly sealed aftermarket windshield-deicer harnesses can admit water to specified connectors and cause drivability codes including P1604. T-SB-0009-23 supplies a separate fuel-pressure branch for specifically covered 2017–2019 applications.
Match the symptom
Select the closest observation. The result changes the first test; it does not name a part to replace.
Load-test the 12-volt battery and inspect voltage drop before fuel parts.
What can set P1604—and how to separate it
The order below is evidence-driven, not a universal probability ranking. Vehicle year, repair history, companion codes and the operating condition in freeze frame change where diagnosis should begin.
| Possible cause | Evidence that supports it | First useful test |
|---|---|---|
| Weak 12-volt battery or excessive voltage drop | Low cranking voltage/speed or failed conductance/load test | Battery test and positive/ground voltage drop |
| Fuel-pressure loss | Extended crank with pressure below command/specification | Correct low/high-side pressure test |
| Air metering, purge or throttle problem | Implausible MAF/throttle or post-refuel pattern | Inspect intake and graph start data |
| Ignition, crank signal or mechanical compression | No sync/spark or abnormal relative compression | Cranking waveform and relative compression |
| Connector corrosion from deicer harness water path | Matched 2014–2018 vehicle and installation/connector evidence | Follow T-SB-0072-19 |
Startability diagnosis begins with classification
A silent no-crank is electrically different from normal-speed cranking with no combustion. 'It would not start' is not enough detail to select a test.
Record sound, cranking rpm, security indicators and whether the engine briefly catches. Those observations prevent wasted fuel and ignition work.
Capture fuel pressure during the failure
A vehicle that starts on the second attempt may show normal pressure after the rail has recovered. Graph pressure from key-on or READY request through the failed event.
T-SB-0009-23 has exact model/VDS coverage. Use it to support, not replace, measurement.
Water intrusion creates code combinations
T-SB-0072-19 explains why P1604 may coexist with lean, misfire, fuel-pressure or communication codes. Shared connector corrosion can interrupt several signals.
Repair must address both corrosion and the water path. Replacing a component downstream of a wet connector leaves recurrence likely.
Save these scan-data clues
Freeze frame is a snapshot of conditions when the PCM matured the fault. It can distinguish an idle-only problem from a load, cold-start or monitor-completion problem. Clearing first throws that context away.
- Battery voltage and cranking rpm
- Engine-run time before stall
- Fuel pressure commanded versus actual
- MAF, throttle angle and coolant temperature
- Immobilizer, crank/cam, misfire and communication codes
- Weather, windshield accessory history and fuel level
A practical diagnostic workflow
- 01Define what 'did not start' means
No-crank, slow-crank, crank/no-start and start/stall require different trees.
- 02Test the 12-volt foundation
Measure battery health and voltage drop under the actual start attempt.
- 03Preserve start-event data
Save all modules and freeze frame before repeated attempts alter the symptom.
- 04Verify air, fuel, spark and compression
Use simultaneous data where possible instead of changing parts one system at a time.
- 05Apply exact Highlander bulletins
Inspect the deicer-harness water path and fuel-pump applicability only when year/VIN match.
- 06Cold-start more than once
Confirm a reliable soak restart and no pending startability code.
Do not replace the starter, fuel pump or ECM from P1604 alone. The code describes unsuccessful starting. Prove which requirement disappeared, and remember that connector corrosion can create multiple misleading codes while a weak 12-volt battery can distort every cranking measurement.
Why the model year changes the test
Highlander diagnostics span the first-generation 2.4L 2AZ-FE and 3.0L/3.3L MZ engines, later 2.7L 1AR-FE and 3.5L 2GR-family V6 engines, several hybrid systems, the 2020+ 2.5L A25A four-cylinder hybrid, and newer 2.4L turbo applications. Bank layout, fuel delivery, EVAP hardware, VVT control and safe hybrid procedures differ sharply. Confirm model year, VIN, engine code, hybrid status, drivetrain and emissions label before choosing tests or parts.
- 2014–2018: T-SB-0072-19 includes P1604 among possible connector-corrosion symptoms related to certain aftermarket deicer harnesses.
- 2017–2019: T-SB-0009-23 includes defined Highlander/Highlander Hybrid fuel-pump applications; verify VIN/VDS.
- Hybrid READY behavior differs from conventional cranking; use hybrid-capable scan data and safety procedures.
- One code after battery service should be interpreted with voltage history, not automatically treated as an active engine defect.
P1604 FAQ
Does P1604 mean the starter is bad?+
No. It means the start event failed; battery, authorization, fuel, air, ignition, signals and compression all matter.
Can a weak battery set it?+
Yes. Low cranking voltage or speed can prevent the engine from stabilizing and create companion faults.
Can low fuel pressure cause it?+
Yes. Measure pressure during the failed start, not only after the engine eventually runs.
Why inspect aftermarket windshield wiring?+
Toyota documented a water path on certain 2014–2018 installations that can corrode key connectors.
Is the fuel pump covered?+
Only if the exact vehicle falls within program applicability and diagnosis supports pressure loss.
Can I keep trying to start it?+
Repeated attempts can drain the battery, overheat the starter and erase the original pattern; diagnose instead.
Can P1604 be historical?+
Yes. Review occurrence, companion codes and current starting performance before deciding it is active.
What confirms repair?+
Normal voltage and cranking data plus repeatable cold/hot starts with no pending event code.
Ford diagnostic references
These documents explain Ford monitor operation. Exact pinpoint tests, connector views and specifications must be selected for the truck's VIN and model year in current service information.
01Toyota T-SB-0072-19 — Highlander connector corrosion2014–2018 Highlander/Highlander Hybrid only; damaged or poorly sealed aftermarket windshield-deicer harnesses can admit water and cause drivability faults and DTCs including P0171, P0300 and P1604↗02Toyota T-SB-0009-23 — fuel-pump pressure lossIncludes specifically listed 2017–2019 2GR Highlander and 2017–2019 Highlander Hybrid applications; VIN/VDS coverage must be verified↗03Toyota Highlander manuals and warranty guidesOfficial Toyota model-year document lookup; confirm the VIN, engine, hybrid status and emissions certification↗04Toyota Technical Information SystemToyota's current North American repair manuals, bulletins, wiring diagrams and calibration information↗