- P0758 indicates a fault in the B shift solenoid’s electrical circuit
- Solenoid controls hydraulic pressure; PCM detects mismatch and triggers the code
- Symptoms range from abrupt shifts to limp mode and overheating
- Common causes include faulty solenoid, hydraulic issues, low or contaminated fluid, or electrical faults
Introduction OBD2 error code P0758 points to an issue with the B shift solenoid, specifically an electrical circuit fault. This error can affect the operation of the automatic transmission and requires careful diagnosis to identify the root cause.
Modern automatic transmissions rely on a complex network of electronically controlled solenoids to manage gear changes efficiently and precisely.
What P0758 Code Means
PCM (Powertrain Control Module) has detected an anomaly at the B shift solenoid. An electronically controlled automatic transmission uses multiple shift solenoids to manage hydraulic fluid flow between circuits and to initiate gear changes at the optimal moment.
These solenoids function as electronic valves that control hydraulic pressure in different parts of the transmission. When the PCM detects that the B solenoid is not operating within preset parameters, the P0758 code is generated.
Code parameter settings
Parameter activation for this fault varies considerably between manufacturers. Most systems use complex calculations based on:
- Throttle position
- Engine load
- Engine speed
- Vehicle speed
- Transmission fluid temperature
The PCM continuously compares the desired solenoid operating value with the actual value. If there is a significant discrepancy between these values, the P0758 code is stored and the check engine light on the dashboard is illuminated.
Common symptoms of P0758
Mild symptoms
- Shifts are more abrupt or hesitant
- Reduced fuel efficiency
- Check engine light illuminated
Severe symptoms
- Inability to shift into certain gears
- Transmission enters limp mode (safety mode)
- Transmission overheating
- Abnormal vibrations during gear changes
The transmission may enter safety mode to prevent internal damage, limiting the vehicle to a single gear or a few specific gears.
Common causes of P0758
Solenoid problems
- Shift solenoid defective or stuck
- Solenoid coil burned
- Corroded contacts at the solenoid
Hydraulic problems
- Blockages in hydraulic circuits of the transmission fluid
- Insufficient pressure in the hydraulic system
- Defective torque converter
Fluid problems
- Low transmission fluid level
- Degraded or contaminated transmission fluid
- Leaks in the transmission system
Electrical problems
- PCM faulty
- Transmission control module faulty
- Damaged wiring or corroded connections
Common misdiagnoses
Many technicians tend to replace the shift solenoid immediately without a full diagnosis. This can be an erroneous approach, as the problem may be caused by:
- Electrical circuit issues
- Inadequate transmission fluid
- Hydraulic problems in the transmission system
- PCM or transmission control module failure
P0758 Diagnostic Guide
Required equipment
- Professional OBD2 scanner
- Digital multimeter
- Transmission pressure gauge
- Basic automotive tools
Step 1: Check the transmission fluid
Always start by checking the transmission fluid level and condition:
- Check the fluid level according to the manufacturer’s specifications
- Inspect the color and smell of the fluid
- Burnt fluid (dark brown/black) or with an unusual odor indicates serious problems
Step 2: Visual inspection
- Check all wiring and connectors in the transmission area
- Look for damaged or corroded wires/connectors
- Inspect the transmission pan for metal debris
Step 3: Code scanning
- Connect the scanner to the diagnostic port
- Note all stored trouble codes
- Clear the codes and perform a test drive to check for recurrence
Step 4: Hydraulic pressure testing
If the fluid appears normal:
- Use a pressure gauge to measure the hydraulic pump pressure
- Compare results with the manufacturer’s specifications
- Insufficient pressure may indicate pump problems or blockages
Step 5: Electrical testing of the solenoid
- Check voltage and ground signals of the solenoid
- Measure the solenoid coil resistance
- Compare values with the technical specifications
Step 6: Functional test
- Use the scanner to manually activate the solenoid
- Check if the solenoid responds to PCM commands
- Monitor real-time parameters during the test
Repair procedure
If the issue is the fluid
- Replace the transmission fluid if degraded
- Top up the level if it is low
- Repair any leaks identified
If the issue is electrical
- Repair or replace damaged wiring
- Clean corroded contacts
- Check and repair ground connections
If the solenoid is defective
- Remove the transmission pan for access
- Replace the defective solenoid with an OEM or equivalent
- Check the condition of the other solenoids at the same time
If the problem persists
- Test the PCM
- Check the transmission control module
- Consider internal transmission issues
Prevention of future problems
To prevent recurrence of P0758:
- Perform transmission fluid changes according to the maintenance schedule
- Use only the fluid recommended by the manufacturer
- Avoid aggressive driving or overloading the transmission
- Perform periodic transmission fluid level checks
Estimated repair costs
- Transmission fluid replacement: 300-600 RON
- Solenoid replacement: 400-800 RON (plus labor)
- Wiring repairs: 200-500 RON
- PCM module replacement: 1500-3000 RON
Costs can vary significantly depending on the vehicle make, system complexity, and shop rates.
Accurate diagnosis of P0758 requires a methodical approach and proper equipment. Do not ignore this error, as it can lead to serious and costly transmission damage if not addressed promptly.
