Safety Warning
DIY auto repair can cause serious injury, fire, or vehicle damage. These guides are for informational purposes only. Always follow OEM torque specs, wear PPE, and consult a certified mechanic if you are unsure. You are solely responsible for your safety.
DTC Advanced Diagnostics: P1A4B
DTC P1A4B : TCM Incorrect Combination of Clutch Engage 1
General Description
When the transmission control module (TCM) shifting control malfunctions, it may cause unintended rapid deceleration, caused by the lock-up of the countershaft. The lock-up of the countershaft may occur by a cause of abnormal engagement combinations of the clutches and the brakes or abnormal switching of the two-way clutch. The TCM monitors the conditions of the clutch, the brake, and the two-way clutch to secure safety before the vehicle leads to unintended rapid deceleration. The TCM presumes that there is abnormal engagement of the clutches and the brakes or an abnormal switching of the two-way clutch from the values of the transmission fluid pressure sensor and the two-way clutch actuator valve stroke sensor. If the TCM determines a shifting control malfunction, a DTC is stored.
Monitor Execution, Sequence, Duration, DTC Type
| Execution | Continuous |
| Sequence | None |
| Duration | 0.4 second or less |
| DTC Type | One drive cycle, MIL on, shift position indicator blinks |
Enable Conditions
| Condition | Minimum | Maximum | |
|---|---|---|---|
| Vehicle speed [Vehicle Speed] | 7 mph (10 km/h) | - | |
[ ]: HDS Parameter
Malfunction Threshold
From the values of the transmission fluid pressure sensor and the two-way clutch actuator valve stroke sensor, the TCM determines as abnormal engagement of the clutch and the brake or an abnormal switching of the two-way clutch at least 2 times.
Possible Cause
The causes shown may not be a complete list of all potential problems, and it is possible that there may be other causes.
- TCM internal circuit failure
Diagnosis Details
Conditions for setting the DTC
When a malfunction is detected, the MIL comes on, the shift position indicator blinks, and a Pending DTC, a Confirmed DTC, and the freeze data are stored in the TCM memory.
Conditions for clearing the DTC
The shift position indicator comes off by turning the vehicle to the OFF (LOCK) mode. The MIL is cleared if the malfunction does not return in three consecutive trips in which the diagnostic runs. The MIL, the Pending DTC, the Confirmed DTC, and the freeze data can be cleared with the scan tool clear command.
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When to See a Mechanic
Stop DIY work and contact a certified mechanic immediately if any of the following apply:
- • You smell fuel, burning insulation, or see smoke.
- • Brakes feel soft, pull hard to one side, or make grinding noises.
- • The engine overheats, stalls repeatedly, or misfires under load.
- • You are missing required tools, torque specs, or safe lifting equipment.
- • You are not confident in the next step or safety outcome.
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