Fix P0700: Mechanic’s solutions that WORKED

The code P0700: Transmission Control System Malfunction points out the problem in your transmission control module, which could be caused by TCM (transmission control module) failure, wiring damage, or transmission itself. It is not that rare for this code to be set by other issues as well.

P0700 quick overview

Meaning	P0700: Transmission Control System Malfunction
Is it serious?	While P0700 itself doesn’t pinpoint a specific issue, it signals a potential problem within the transmission control system, which can affect the vehicle’s performance and reliability.
Possible causes	– Faulty transmission control module (TCM). – Wiring or connection issues in the transmission control circuit. – Problems with the transmission itself. – Sensor or solenoid malfunctions within the transmission. – Insufficient transmission fluid or fluid contamination. – Other related codes stored in the TCM.
How to diagnose?	1. Use a scan tool to retrieve the specific transmission codes associated with P0700. 2. Check for any visible damage or disconnection in the wiring and connectors related to the transmission control system. 3. Inspect the transmission fluid level and condition, ensuring it’s at the correct level and not contaminated. 4. Address any additional fault codes stored in the TCM, which can provide more detailed information about the issue. 5. Test the TCM and associated components, such as sensors and solenoids, for proper functioning. 6. If necessary, consult a professional mechanic or technician with experience in transmission diagnostics for further evaluation and repair.
What is TCM?	TCM stands for Transmission Control Module. It’s a computer in your car that manages and controls the automatic transmission to ensure smooth shifting and optimal performance.

How to fix P0700 – learn from mechanics

I looked into P0700 service cases at iatn.com website where in the private forums the mechanics seek advice from other mechanics. I looked up a bunch of cases to help you decide how to fix your issue and see what would a real mechanic do to solve it.

Case #1: Mitsubishi

• Make: 1999 Mitsubishi Eclipse RS
• Model: Eclipse RS
• Year: 1999
• Engine: 2.0L

Issue + Repair History:

• Car in limp-in mode with the check engine light on.
• Initial code: P0700 (Internal controller fault).
• Replaced TCM with a remanufactured unit from Mitsubishi, but problem persisted.
• Replaced TCM with a newer remanufactured unit, but the same issue occurred.
• Limp-in mode and P0700 code set as soon as the transmission shifts to second gear.
• Input speed sensor code discovered using Global OBD11.
• Replaced input speed sensor and reinstalled the original TCM, but no speedometer and no shift from second gear.
• Installed the remanufactured TCM with the new input sensor, and the car ran properly.

Other OBD2 Codes + Test Results:

• Additional Codes: None mentioned.
• Fluid level: Full and not burnt.
• Pinout to TCM: Good.

Suggestions to Fix Issue (from Community):

1. Access the transmission controller to determine exact transmission codes stored.
2. Use a scanner, such as Snap-on, to switch to TCM mode and retrieve codes.
3. Confirm if the P0700 codes are through the PCM or TCM.
4. Jump out pin 12 and 6 at the connector to flash out the transmission code on the O/D light.
5. Check for input or output shaft speed sensor codes, as these transmissions commonly have issues with those.

Final Fix:

• Replaced the input speed sensor and installed the remanufactured TCM with the new input sensor. The car ran properly after these actions.

Case #2: GMC

• Make: 2007 GMC Yukon Denali
• Model: Yukon Denali
• Year: 2007
• Engine: 6.2L

Issue + Repair History:

• Initially had a U0101 communication fault with the TCM.
• No power from the fuse box to the TCM.
• Repaired the circuit from the fuse box to the TCM.
• Restored communication with the TCM.
• P0700 code persisted in the Engine DTC (no codes in Transmission DTC).
• Good power and ground to the TCM.
• Seeking help for diagnosing the P0700 code.

Other OBD2 Codes + Test Results:

• Additional Codes: None mentioned.

Suggestions to Fix Issue (from Community):

1. Check if the truck drives and shifts or is stuck in gear.
2. Verify the scanner being used to communicate with the TCM.
3. Understand that P0700 indicates a transmission issue.
4. Clear the ECU memory after handling the transmission communication code.
5. Clear codes on both the PCM and TCM, then retest.
6. Suggest disconnecting both battery cables and rubbing them together for a minute.
7. Consider just clearing the code if there are no codes in the TCM, as P0700 may indicate a resolved TCM issue.

Final Fix:

• Discovered that the TCM in the transmission was not receiving battery voltage from the underhood fuse panel.
• The fuse checked out okay, but no power was coming out from the fuse circuit board.
• Key-on voltage from the fuse box was also insufficient.
• Installed a new fuse box, which fixed the vehicle. The defective circuit board in the underhood fuse box was the root cause of the problem.

Case #3: Chrysler

• Make: 2005 Chrysler 300 C
• Model: 300 C
• Year: 2005
• Engine: 5.7L

Issue + Repair History:

• Check engine lamp came on but went off a day or two later.
• Stored Diagnostic code P2638 in the TCM.
• Previous incorrect code posted (P2368, incorrect).
• No reset of codes in over a week.
• Vehicle was performing normally with no symptoms.
• Car equipped with NAG-1 auto trans.
• Inspection revealed damaged wiring harness near the firewall caused by mice.
• Insulation on a couple of wires was penetrated, and wires may have been touching each other.

Other OBD2 Codes + Test Results:

• Additional Codes: U0002 (Bus communication fault), P0700 (Trans request MIL).
• P2368 was initially posted but later corrected to P2638.
• No freeze frame data retrieved.

Suggestions to Fix Issue (from Community):

1. Investigate U0002 code, which indicates a Can C bus performance issue.
2. Scan the TCM to retrieve the fault code stored there.
3. Check for additional codes in the TCM and other modules.
4. Consider the possibility of a sticking throttle body causing the torque management code.

Final Fix:

• Diagnostic codes (P2638, U0002, P0700) never returned after extended driving.
• Inspection revealed damaged wiring harness near the firewall caused by mice.
• Some wire insulation was penetrated, and wires may have been touching.
• Uncertain if the damaged wiring was the root cause of the malfunction, but it was the only issue found.
• Codes did not return, and the vehicle continued to perform normally after the wiring issue was addressed.

Case #4: Dodge

• Make: 2004 Dodge Ram 1500 ST
• Model: Ram 1500 ST
• Year: 2004
• Engine: 5.7L

Issue + Repair History:

• Vehicle diagnosed as needing a fuel pump.
• RPM on scan tool observed when cranking the motor, but no fuel pump activation.
• Checked theft status with no codes found.
• Persistent P0700 code that won’t clear.
• Vehicle was down four quarts of oil.

Suggestions to Fix Issue (from Community):

1. Check for voltage and ground at the fuel pump.
2. Ensure the ASD relay is supplying voltage and energizing.
3. Inspect fuses related to the fuel pump.
4. Verify the ground for the fuel pump is in good condition.
5. Scan the TCM to check for any codes there.
6. Test if the vehicle will run on alternate fuel sources like propane or carb spray.

Final Fix:

• The problem was resolved by checking for voltage to the fuel pump.
• Voltage and ground were present at the pump.
• A chafed power feed wire for the pump was found about 8 inches from the pump connector.
• The wire had green corrosion and had broken the last two strands of wire when wiggled.
• The wire was repaired, fixing the issue.
• This was identified as a potential problem on many of these vehicles in the future.

Case #5: Ford

• Make: 2004 Ford F-350 Super Duty XL
• Model: F-350 Super Duty XL
• Year: 2004
• Engine: 6.0L

Issue + Repair History:

• Initial problem: Hard start when hot.
• Replaced FICM (Fuel Injector Control Module) and had it reprogrammed at the dealer.
• Replaced EGR (Exhaust Gas Recirculation) and cleaned it.
• A P0700 code (Transmission Control System) appeared, and the truck experienced intermittent hard starts when hot.
• On hard starts, the vehicle lacked power and seemed to run on fewer cylinders before eventually running okay.
• Checked harness for issues but couldn’t find any.
• Reverted to the old FICM, and a code for injector 1 fault appeared.
• Repeatedly checked connections.
• Noticed that the transmission temperature gauge went to cold, and the tow/haul light flashed (potential related issue).

Other OBD2 Codes + Test Results:

• Additional Codes: P0611 (Fuel Injector Control Module), P1335 (EGR Position Sensor), P2614 (Cam Position Output Circuit/Open), P2617 (Crank Position Output Circuit/Open).
• Trans temp gauge goes to cold, and the tow/haul light flashes.

Suggestions to Fix Issue (from Community):

1. Check data on the scan tool for ICP (Injector Control Pressure), RPM, ICP voltage, fuel pulse width, FICM sync, and crank signal.
2. Verify ICP pressure (needs 500 psi to start) and check for a good crank signal when not starting.
3. Check the TFT sensor (Transmission Fluid Temperature) for potential issues.
4. Use Ford-specific scan software to retrieve the transmission module’s specific code related to P0700.
5. Perform an air pressure test on the high-pressure oil system to identify potential leaks.
6. Ensure PMI (Parameter Initialization) was correctly performed on the FICM.
7. Investigate high-pressure oil system issues, and consider replacing the high-pressure oil pump.
8. Look for harness rub-through and potential harness issues related to the transmission.
9. Test high-pressure oil psi with a scan tool to assess if there’s an issue with high-pressure oil.
10. Consult a Ford dealer with a VCM (Vehicle Communication Module) to monitor high-pressure oil pump readings during startup.

Final Fix:

• The issue was resolved by replacing the FICM again and having it reprogrammed at the dealer.
• The FICM sync was also checked during this process.
• This combination of replacing the FICM and reprogramming resolved the intermittent hard start and power loss issues.