I found info on this on another forum relating to Toyotas, and it appears to be a generic code. Technically the code doesn't even turn the CEL on, it's probably set by extended cranking and no start. Unfortunately it doesn't directly correlate to an issue with a component. It's meant as a freeze frame data point to check other pieces of information.
I would triple check all connections and make sure everything looks right. It's odd that cylinders 1-4 and 2-3 seem different, seeing as how they're not even on the same head.
Quote:
This DTC is stored when the engine does not start even though the STA signal is input or when the engine takes a long time to start, and when the engine speed is low or the engine stalls just after the engine starts.
Using the Techstream, the conditions present when the DTC was stored can be confirmed by referring to the freeze frame data. Freeze frame data records engine conditions when a malfunction occurs. This information can be useful when troubleshooting.
It is necessary to check if the vehicle ran out of fuel before performing troubleshooting, as this DTC is also stored when there is engine starting trouble due to running out of fuel.
HINT
* In contrast to normal malfunction diagnosis for components, circuits and systems, DTC P1604 is used to determine the malfunctioning area from the problem symptoms and freeze frame data when the user mentions problems such as starting difficulty.
As these DTCs can be stored as a result of certain user actions, even if these DTCs are output, if the customer makes no mention of problems, clear these DTCs without performing any troubleshooting and return the vehicle to the customer.
* If any other DTCs are output, perform troubleshooting for those DTCs first.
* When the Data List item "Immobiliser Fuel Cut" is ON, the engine cannot be started.
* Read freeze frame data using the Techstream. Freeze frame data records engine conditions when a malfunction occurs. This information can be useful when troubleshooting.
* When confirming the freeze frame data, be sure to check all 5 sets of freeze frame data See: Computers and Control Systems > Scan Tool Testing and Procedures > Freeze Frame Data.
* When confirming freeze frame data, if there are multiple items related to the cause of the malfunction, perform troubleshooting for all related items.
* Try to start the vehicle under the conditions recorded in the freeze frame data which were present when the malfunction occurred. Confirm the data at this time and compare it with the freeze frame data.
* If the malfunction does not reoccur, carefully check the vehicle conditions from when the malfunction occurred using freeze frame data.
* When performing inspections, jiggle the relevant wire harnesses and connectors in an attempt to reproduce malfunctions that do not always occur.
* If the same inspection or replacement procedure appears 2 times when performing an inspection procedure, it is not necessary to repeat the procedure the second time.
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