The hard-start code Toyota wrote a fix for

P1604 is the most-scanned manufacturer P-code among the makes we track — more scans in our data than any other manufacturer-specific P-code outside the standard OBD-II set. It flags what the engine control unit logs as a startability malfunction: cranking that takes too long to settle into a stable idle, whatever the underlying cause turns out to be. One manufacturer has already written a maintained diagnostic bulletin for exactly this complaint.

What "startability malfunction" actually covers

The code is not aimed at one sensor or one part. It is a catch-all for the engine control computer deciding that a start attempt did not go the way it should have — cranking ran longer than expected, or the engine caught and then stumbled before settling. That makes P1604 a symptom flag rather than a diagnosis, in the same way a random misfire code tells you combustion failed somewhere without naming the cylinder.

Because the trigger is timing rather than a single failed component, the same code can be set by a weak battery, a starter drawing more current than it should, a fuel system that is slow to build pressure, or — just as often — control logic that is simply too strict about how long a normal cold start is allowed to take. That last cause is the one a bulletin fixes without anyone touching the starter.

What shows up alongside it

Those are other control-module codes from the same diagnostic family, which is a useful sign in itself: when P1604 turns up with several of its neighbours rather than alone, the pattern points at the control logic and its stored memory rather than a single mechanical part failing underneath it.

What the manufacturer's bulletins say

Toyota's bulletins describe hard-start diagnosis on certain large engines through a maintained bulletin family — around eight communications, with later versions superseding earlier ones as the diagnostic procedure was refined. That is a sign the manufacturer kept revisiting the same complaint rather than closing the file after one attempt, which is exactly the kind of documented, current fix worth asking for before a starter or fuel pump gets replaced on a guess.

Bulletins are revised or superseded as manufacturers learn more; verify the current version for your vehicle through NHTSA's public database or your dealer.

The neighbours worth ruling out first

A startability code shares territory with a small group of generic faults that produce the same symptom for more ordinary reasons. P0335 and P0340 cover the crankshaft and camshaft position sensors the engine needs to time a start at all; a marginal signal from either one can make an otherwise healthy engine crank far longer than normal before catching. P0230 covers the fuel pump's primary circuit, which produces a very similar slow, reluctant start when it is intermittent rather than fully failed. All three are worth scanning for alongside P1604, because any one of them is a cheaper fix than guessing at the starter.

What to check first

Start with the battery and starter draw — a start that takes marginally too long is often nothing more than a battery on its way out. Scan for the crank and cam sensor codes above before assuming the fault is inside the control module itself. Then ask whether the current bulletin's diagnostic procedure or software update applies to your VIN; that is a lookup a dealer can do in minutes, and it is the step most likely to close the code for good if the hardware checks out clean.

Can you keep driving?

Usually, yes, if the engine is starting at all, just slowly. An intermittent hard start has a habit of getting less intermittent over time, though, so treat a repeat occurrence as a reason to book the check rather than something to keep tolerating. If a start attempt ever fails outright, that is no longer a "keep driving" situation — have it looked at before you rely on your car again.