OBD-II Code
P1633
Severity
Not yet reviewed
Nobody on our side has assessed this code yet, so we are not going to guess at how serious it is.
Can you drive it
We cannot say
Treat it as unknown until a shop reads it with live data. If the check-engine light is FLASHING, stop driving as soon as it is safe to — that means an active misfire, whatever the code.
Indicative cost (USD)
$5–$2,400
From this code's diagnostic notes. Varies by vehicle, diagnosis, parts, and local labor. This is not a repair quote.
$5 (fuse only) to $2,400 (ECM replacement + dealer programming).
P1633 is Lexus/Toyota's Battery Backup Circuit Open / ECM ROM Test Failure code — the ECM runs a power-on self-test at every key-on event that verifies the keep-alive memory (KAM) backup-battery circuit is intact and that the internal ROM checksum matches expected.
How a shop works this
Work from evidence to part, not the other way round. A code names a circuit or a symptom; it does not name the part that failed.
Note every code that is set, not just this one.
Codes that appear together usually narrow the cause faster than any one of them read alone.
Read the freeze frame — the conditions the fault was recorded under.
Cold or warm, idle or load, road speed and engine speed. A fault that only appears cold is a different fault from one that only appears under load, even with the same code on the scanner.
Confirm the fault is current rather than history.
Clear the code and see whether it returns under those same conditions. A code that does not come back may have been logged during a repair, a flat battery, or a bad connection that has since been disturbed.
Then follow the diagnostic notes further down this page, in the order they are written.
They are written in the order a shop works this code, cheapest checks first. Where they name a component, inspect its wiring and connector before replacing it.
If the code comes back after a repair, the underlying fault is still there. That is the point to bring in a scanner with live data and freeze-frame, or a shop that has one.
Common symptoms
- Check engine light
Read the full diagnostic procedureP1633 is Lexus/Toyota's Battery Backup Circuit Open / ECM ROM Test Failure code — the ECM runs a power-on self-test at every key-on event that verifi…
P1633 is Lexus/Toyota's Battery Backup Circuit Open / ECM ROM Test Failure code — the ECM runs a power-on self-test at every key-on event that verifies the keep-alive memory (KAM) backup-battery circuit is intact and that the internal ROM checksum matches expected. P1633 means one of those tests failed, and on Lexus platforms it's almost always the KAM backup-circuit fuse or wiring rather than a literal ROM-chip failure.
Shop ladder, cheapest first
- Check the ECM backup-battery fuse in the engine-bay fuse box (often labeled 'EFI' or 'ECU-B' on Lexus diagrams) — a blown backup fuse leaves the ECM without its 12V-always-on keep-alive memory feed and sets P1633 at the next key-on. Replace the fuse ($1 part), clear codes, see if it returns (if it blows again, you have a short in the KAM circuit).
- Verify the KAM circuit at the ECM connector — should be a constant 12V on the backup-power pin with the key OFF; if it's 0V at the ECM but 12V at the fuse, the wiring between them is broken (commonly from rodent damage or aftermarket alarm install).
- Battery-voltage check at the main battery — a weak battery that drops below 9V during cranking can momentarily lose the KAM feed and set P1633; load-test the battery before chasing wiring.
- ECM replacement is the last resort and requires dealer-level programming on most Lexus platforms ($800-2,200). The expensive misdiagnosis caveat: P1633 is NOT automatically an ECM failure — Lexus ECMs are extremely reliable, and the vast majority of P1633 codes resolve at the fuse or KAM-circuit wiring; don't quote a $1,500 ECM until you've verified 12V-always-on power is reaching the backup pin.
Vehicle-specific patterns1990-2000 Lexus LS400 (1UZ-FE V8) throws P1633 most often from a blown ECU-B fuse or aged wiring at the ECM connector — the original harness insulati…
- 1990-2000Lexus LS400 (1UZ-FE V8) throws P1633 most often from a blown ECU-B fuse or aged wiring at the ECM connector — the original harness insulation gets brittle past 25 years and rodents find the engine bay attractive.
- 2001-2006Lexus LS430 (3UZ-FE V8) shares the same architecture with slightly better harness insulation.
- 2007-2017Lexus LS460 (1UR-FSE V8) throws P1633 less frequently but the diagnostic flow is identical.
- 2006-2013Lexus GS300/GS350/GS430 (2GR-FE / 3UZ-FE / 1UR) and IS250/IS350 (4GR / 2GR) follow the same KAM-circuit architecture. RX300/RX330/RX350/RX450h (1MZ / 2GR / 2GR-FXE hybrid) — the hybrid RX450h has additional HV-system interactions but the P1633 KAM diagnostic is the same on the 12V side.
Estimated repair$5 (fuse only) to $2,400 (ECM replacement + dealer programming).
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