Intermittent Jerking and Idle Stall on 2007 Yaris 1.3 (2SZ-FE) with –20 % STFT

[Or05230]

Hello everyone,

I’ve been battling an intermittent drivability issue on my 2007 Toyota Yaris 1.3 L (2SZ-FE) and could really use your insights. Below is a summary of what’s happening, what I’ve tested so far, and some live-data readings. Any ideas or suggestions are greatly appreciated!

1.Primary Symptoms:

- Jerking under light throttle or cruising – It feels like the ECU is abruptly cutting fuel when the STFT drops into large negative trims.

Occasional idle stalls – Usually when stopping or coming to a slow roll (e.g., at a stoplight), the engine will dip toward 600 RPM and sometimes stall altogether.

Variable occurrence – It does not happen every time; sometimes I drive for 30 minutes without incident, other times it happens multiple times in a single trip.

in any case, once warm, the idle is pretty low w/ the AC off.

2. Key Scan-Tool Data (Warm Idle)
Short-Term Fuel Trim (STFT): Dips as far as –20 % right before the jerking/stall.


Upstream O₂ (Bank 1 Sensor 1): Cycles smoothly between 0.07 V ↔ 0.70 V at idle (≈1 Hz oscillation)—indicating a healthy heated zirconia sensor.

Downstream O₂ (Bank 1 Sensor 2): Steady at 0.60 V – 0.65 V, as expected with a good catalytic converter.

3. What I’ve Already Checked
– Pretty new NGK OEM Plugs.

MAF Sensor & Intake:
– MAF reading at idle = 1.2–1.4 g/s
– Throttle body cleaned (top and bottom).
– Intake manifold and vacuum hoses looks ok.

-Coolant temp looks accurate.

No Stored DTCs:
– ECM logs no check-engine light, no pending codes, no misfire codes, no O₂ or sensor faults. also tested by 2 mechanics with Toyota special OBD.

Thank you!!!