U1135 — SCP (J1850) Invalid or Missing Data for Ignition Switch / Starter #

Please note: The code description may differ depending on the vehicle manufacturer.
Known label variations:

• Ford: SCP (J1850) Invalid or Missing Data for Ignition Switch/Starter
• General Motors: Loss of Communication with Ignition Switch Module
• Chrysler: Ignition Switch Data Invalid or Absent (SCP)
• Jaguar/Land Rover: Ignition Switch Signal Missing via SCP Network

Summary for code U1135 #

The U1135 diagnostic trouble code (DTC) points to a communication issue within the vehicle’s Serial Communication Protocol (SCP, also known as J1850) network, specifically regarding the ignition switch or starter system. In simple terms, the vehicle’s modules are not receiving valid or expected data from the ignition switch or starter circuit. This can be caused by wiring faults, module failures, or network issues. While not always causing immediate breakdown, this code can create unpredictable starting issues or even prevent the engine from cranking. Addressing it swiftly helps avoid getting stranded and ensures proper system performance.

Common symptoms for code U1135 #

When U1135 is logged, your vehicle may display a range of subtle to obvious symptoms. Some may be intermittent, making diagnosis tricky. Typical signs include:

• Intermittent or complete no-start conditions
• Starter does not engage when turning the key or pressing the start button
• Warning lights (such as the check engine or security indicator) on the dashboard
• Failure of accessories powered by the ignition switch (radio, power windows, etc.)
• Occasional loss of communication-related DTCs stored in other modules

Probable causes for code U1135 (by likely frequency) #

The U1135 code is most often triggered by network, wiring, or module-related problems. Here’s what typically lies behind this fault, ranked from most to least common:

1. Corroded, loose, or damaged SCP (J1850) wiring or connectors near the ignition switch or starter circuit
2. Faulty ignition switch not broadcasting proper data to the network
3. Defective starter relay or associated module unable to relay data
4. Failed or unresponsive body control module (BCM) or other related control module
5. Low battery voltage or intermittent power supply issues
6. Poor ground connections affecting communication lines
7. Aftermarket alarm or remote start installations interfering with SCP/J1850 data lines

Step-by-step diagnosis for code U1135 #

Diagnosing U1135 requires a methodical approach. Because the fault can be intermittent, patience and attention to detail are your best allies. Here’s a proven sequence:

• 1. Check for related codes: Scan all modules for additional communication or ignition-related DTCs.
• 2. Visually inspect SCP (J1850) wiring and connectors: Focus on areas near the ignition switch, starter, and BCM.
• 3. Check ignition switch operation: Using a scan tool, monitor live data to see if the ignition switch status updates as you turn the key/button.
• 4. Test for power and ground at the ignition switch: Ensure correct voltage and ground continuity.
• 5. Check for aftermarket device interference: Disconnect or bypass any non-OEM alarm or remote starter systems.
• 6. Test the starter relay/module: Confirm it communicates and functions properly.
• 7. If all else checks out, suspect a control module issue: Consult OEM testing procedures for the suspected module (often BCM).

Possible repairs for code U1135 #

Once the root cause is identified, repairs may range from simple wiring fixes to module replacement. Here are typical solutions matched to the causes above:

• Repair or replace damaged SCP (J1850) wiring and clean/secure connectors
• Replace a faulty ignition switch (programming may be required on newer vehicles)
• Replace a defective starter relay or related module
• Repair or replace the body control module (BCM), and reprogram if necessary
• Charge or replace the battery, and fix any power supply issues
• Clean and secure all ground connections associated with the network
• Remove or properly integrate aftermarket alarm or remote starter systems

Is it safe to drive with code U1135? #

Limited — While some vehicles may continue to operate with this code present, you risk sudden no-start situations or loss of critical functions. If the code is intermittent, you might get stranded unexpectedly. For safety and reliability, address this issue as soon as possible. If the vehicle won’t start, arrange for professional diagnosis and repair.

Vehicles most commonly affected by code U1135 #

While U1135 can appear on any make using the SCP (J1850) network, it’s most often reported on certain models from the late 1990s through mid-2000s that use this protocol, especially:

• Ford (F-Series, Explorer, Taurus, Focus, Expedition)
• Lincoln (Town Car, Navigator)
• Mercury (Grand Marquis, Mountaineer)
• Chrysler and Dodge (Grand Caravan, PT Cruiser, Ram trucks)
• Some General Motors vehicles (Chevrolet Silverado, GMC Sierra with early multiplexing)
• Jaguar and Land Rover models from the same era

Common mistakes & best practices for code U1135 #

Misdiagnosis is common with network codes like U1135. Avoid these pitfalls for a smoother repair:

• Replacing the ignition switch without confirming network integrity
• Overlooking damaged wiring or loose connectors, especially after accessory installations
• Ignoring battery or ground problems that can cause communication faults
• Forgetting to clear all codes and retest after repairs
• Skipping a full module scan — other codes may point to the real culprit

Quick FAQ for code U1135 #

Q1: Can a weak battery cause U1135?
A: Yes, low voltage can disrupt communication and cause network-related codes like U1135. Always check battery health during diagnosis.

Q2: Does replacing the ignition switch always fix U1135?
A: Not necessarily. While a faulty ignition switch is possible, wiring faults and module issues are just as likely. Proper diagnosis is crucial before replacing parts.