What Does P1594 Mean?
This code indicates a general circuit malfunction in the automatic transmission diagnosis input signal. The PCM is receiving a signal from the transmission control circuit that does not conform to expected parameters—it may be erratic, intermittent, or outside the valid voltage range. This affects the PCM's ability to coordinate engine and transmission operations.
Common Causes
30%
Faulty transmission range sensor or internal transmission input sensor
25%
Damaged wiring harness between the transmission and PCM/TCM
20%
Corroded transmission connector (external case connector)
15%
Low or contaminated automatic transmission fluid affecting internal sensors
10%
TCM or PCM internal communication circuit fault
Diagnostic Steps
1
Check the ATF level and condition; low or burnt fluid can affect internal sensor readings and solenoid operation.
2
Inspect the main transmission electrical connector on the case for corrosion, bent pins, and fluid contamination.
3
Monitor the transmission input signal voltage on live data; compare readings to manufacturer specifications across all operating conditions.
4
Check the wiring harness from the transmission to the PCM/TCM for damage, especially where it routes near exhaust or suspension components.
5
If an internal sensor is suspect, verify by comparing its readings to known-good values and replace the internal harness or sensor as needed.
Estimated Repair Cost
$150 - $800
Parts + labor, varies by vehicle and location
This code indicates a general circuit malfunction in the automatic transmission diagnosis input signal. The PCM is receiving a signal from the transmission control circuit that does not conform to expected parameters—it may be erratic, intermittent, or outside the valid voltage range. This affects t...
The most common cause of P1594 (Automatic transmission diagnosis input signal circuit malfunction) is: Faulty transmission range sensor or internal transmission input sensor
Typical repair costs for P1594 range from $150 to $800, depending on the vehicle, location, and whether you do it yourself or go to a shop.
Erratic transmission input signals can lead to harsh or delayed shifts, torque converter lockup issues, or failure to shift. In severe cases, the transmission may default to limp mode (single gear).
Start by connecting an OBD2 scanner to read the code and any freeze frame data. Then follow the diagnostic steps specific to P1594 to identify the root cause.
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Quick Info
Category
Powertrain
System
Automatic Transmission
Difficulty
Type
Manufacturer
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