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MORE_GAS (0x167) byte 7 carries the cylinder mode: 41 = all cylinders, 10-17 = the PCM confirmation ramp toward deactivation, 20 = two cylinders latched off. The byte reports all-cylinders during fuel cut, so engineBraking is inferred from pedal-up and converter coupling (a flat rpm/speed ratio) with a 0.5 s confirm; there is no decel gate, because fuel cut survives grades. MORE_GAS is registered with the pt parser up front, unmapped modes hold the resolved state, and the DBC names the CYL_DEACT_STATE values.
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Idle creep in Drive holds the rpm/speed ratio flat at idle rpm, so the coupling gate passes without fuel cut and the ring shows engine braking in parking lots: 63 windows under 20 kph across the 12-route rlog corpus, 60 of them fully below 900 rpm (the idle band). Fuel cut never runs at idle rpm. Gate the overrun on EB_MIN_RPM = 1000. Corpus replay through this code: sub-20 kph engine braking 92.0 s -> 7.9 s; 40+ kph unchanged (863.1 s -> 861.1 s). Mazda tests: 571 pass.
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Why
Mazda reports cylinder deactivation on the bus but nothing decodes it. The PCM encodes it as a state machine in MORE_GAS (0x167) byte 7. There is no discrete fuel-cut flag anywhere: all 128 addresses in a driving capture were checked.
What
mazda_2017.dbc: newCYL_DEACT_STATEon MORE_GAS, value table in the signal comment: 41 all cylinders (also during fuel cut), 10-17 entry ramp, 20 two cylinders active, 30/40 exit transients.carstate_ext.py:CylinderState(normal / entry / deactivated / engineBraking) pluscyl_entry_progress, the ramp mapped to 0-1.engineBrakingis inferred: pedal up, rolling, converter coupled, confirmed 0.5 s. Coupling means the trailing 0.5 s of rpm/kph ratio stays flat (p95-p5 under 2.5 rpm/kph, above 8 kph). The inference is needed because the byte reads 41 during fuel cut, and fuel cut survives grades, so a decel gate cannot detect it.Validation
test_mazda_cyl_deact.py: 17 cases, all pass. Covers the state boundaries, ramp progress, coupling lock/unlock, and confirm timing.Known limitation
Merge note
Prerequisite for zoompilot/zoompilot#18
AI Usage
Disclaimer: GLM-5.3 by Z.ai was used to help develop, debug, and document this submission. All changes were reviewed and validated by a human.