P1800 – Transmission Clutch Interlock Safety Switch Circuit Failure (Ford)


By Reinier (Contact Me)
Last Updated 2016-11-15
Automobile Repair Shop Owner

CodeFault LocationProbable Cause
P1800 P1800 – Transmission Clutch Interlock Safety Switch Circuit Failure (Ford)
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Manufacturer Specific Definitions for P1800

MakeFault Location
BuickECM to TCM Engine Coolant Signal
CadillacECM to TCM Engine Coolant Signal
ChevroletECM to TCM Engine Coolant Signal
CitroenTransmission Clutch Interlock Safety Switch Circuit Failure
GmECM to TCM Engine Coolant Signal (4L30-E Transmission)
InfinitiIntake manifold air control solenoid – circuit malfunction
KiaEngine torque signal – malfunction
NissanIntake manifold air control solenoid – malfunction
OpelSystem voltage,ignition - out of limits
PeugeotTransmission Clutch Interlock Safety Switch Circuit Failure

What Does Code P1800 Mean?

OBD II fault code P1800 is commonly defined by Ford as “Transmission Clutch Interlock Safety Switch Circuit Failure”, and is set when the switch that prevents a vehicle from being started while it is in gear, or when a vehicle can be started without depressing the clutch pedal. Note that depending on the application, warning lights may or may not accompany this code.

Most vehicles with manual transmissions have a built-in safety feature that prevents the vehicle from being started while it is in gear. In most cases, this feature consists of a simple normally-open switch that engages  with the clutch pedal. When the clutch pedal is depressed, the pedal acts on the switch, which closes a circuit that supplies power to the starter solenoid when the ignition key is turned to the “START” position.

Vehicles with automatic transmissions have a similar feature, called the “Neutral Safety Switch”, which is built into the gear selector. Its function and operation is however the same, in that it prevents the engine from being started while the transmission selector is in any position other than “Park” or “Neutral”.

Note that only the starter motor solenoid is affected by this circuit. All other systems such as the headlights, wipers, power window and seat adjustment, and everything else that do not depend on the engine to run to work will be operational.

The image below shows the typical layout and position of the clutch interlock safety switch on a vehicle with a manual transmission. Note that the actual design, location, and operation of the switch vary between applications. Note that on some applications; most notably many Ford products, the clutch interlock switch forms a part of the clutch master cylinder push rod, and as such, switches of this type are often more difficult to test or replace.


What are the common causes of code P1800?

The most common cause of code P1800 is the failure of the clutch safety switch itself. Other possible causes could include the following-

  • Incorrect adjustment of the switch
  • Damaged, burnt, disconnected, or shorted wiring, but note that with these causes, other codes will almost certainly be present.
  • Open circuits

What are the symptoms of code P1800?

Symptoms of code P1800 are much the same across all applications, and could include one or all of the following-

  • Vehicle may start without having to depress the clutch pedal
  • Vehicle may not start, even with the clutch pedal fully depressed
  • Vehicle may start with the clutch pedal fully depressed, but may not start at other times. Note that this problem usually occurs in a random and unpredictable manner.

How do you troubleshoot code P1800?

Step 1

Record all fault codes present, as well as all available freeze frame data. This information could be useful should an intermittent fault be diagnosed later on.

Step 2

Clutch safety switches can often not be accessed without removing at least some trim panels and /or parts of the dashboard and centre consoles. Consult the manual for the application being worked on for the correct procedure, and be sure to use extreme care not to damage trim panels when removing them to gain access to the switch. Place all removed trim panels and other components out of harm’s way (such as in the trunk) to avoid accidentally stepping on, and breaking something.

Step 3

WARNING: Be absolutely sure that the transmission is in neutral during this, or any other subsequent step that involves starting the engine. The safest method to ensure that the vehicle cannot lunge forward if the engine starts, is to lift the driving wheels off the ground, but be sure to support the vehicle with approved jack stands. 

Once you have gained access to the clutch safety switch, inspect it for signs of damage or brakeage. Clutch switches are in almost constant use, and as a result, it is not uncommon to see switches that have disintegrated.

If however, the switch seems to be undamaged, check to see if the switch is adjustable. If it is, check the adjustment since constant use can wear down the contact point where the clutch pedal acts on the switch. To test the adjustment, use a suitable object to depress the switch fully, and see if the engine starts by turning the key. If the engine starts, correct the switch adjustment until the engine starts if the clutch pedal is depressed fully.

NOTE: In cases where the switch is located on the clutch master cylinder, be sure to consult the manual on the correct procedure to remove the switch, since the process usually involves a degree of disassembly.

Step 4

If the switch is not adjustable, consult the manual to determine the function and colour coding of each wire in the connector. Disconnect the connector, and perform continuity and resistance checks on the wiring to verify that the correct current actually reaches the switch when the ignition key is turned to the “START” position. Make repairs as required to ensure that all electrical values fall within specifications if discrepancies are found. Note that if continuity and/or resistance issues are found, codes P1803 and/or P1803 may or may not to be present along with P1800. Both codes relate to short circuits.

Step 5

If obtained readings agree with stated values, and it is possible to fully depress the switch with a suitable implement, do so, and see if the engine starts. If the engine starts when the switch is depressed manually, it is likely that the contact point on the switch is worn down to the point where depressing the clutch pedal cannot close the circuit.

If this is suspected, replace the switch with an OEM replacement, and test the circuit again by starting the engine with the clutch pedal fully depressed. If the engine starts after a switch replacement, the repair can be seen as successful.

Step 6

If the problem occurs sporadically and there are no other ignition-related codes present, it is safe to assume that an intermittent fault in the switch is the cause. Clutch switches can typically not be repaired reliably, so if an intermittent fault is present or suspected, replace the switch with an OEM replacement part.

NOTE: If the switch is adjustable, be sure to adjust the replacement switch in small increments until the engine starts when the clutch pedal is depressed all the way to its stopper. Large adjustments can cause the moving parts in the switch to move too far, which can cause the switch to fail or break internally.

Step 7

Clear the code, and start the engine multiple times with the clutch pedal fully depressed to see if the code returns. At this point though, the engine should start every time, and it is highly unlikely that the code will return.

If the code does return, or if the engine fails to start intermittently, suspect a bad replacement switch, or a bad connection at the switch. Inspect all connections, and make repairs as required. If the intermittent fault persists, test the system current between the clutch switch and the starter solenoid. If a  multimeter displays the correct current (usually battery voltage) every time the ignition key is turned to the “START” position, it is likely that the starter solenoid is defective, but if this is the case,  it is almost certain that other codes relating to the starting or ignition circuit will be present.

Codes Related to P1800

  • P1801 – Relates to “Transmission Clutch Interlock Safety Switch Open Circuit”
  • P1802 – Relates to “Transmission Clutch Interlock Safety Switch Short Circuit To Battery”
  • P1803 – Relates to “Transmission Clutch Interlock Safety Switch Short Circuit To Ground”

NOTE: Some manufacturers have assigned different definitions to code P1800. Below are some examples-

  • General Motors – ECM (Engine Control Module) to TCM (Transmission Control Module) – “Engine Coolant Signal”. Note that code applies to applications where the 4L30-E transmission is fitted.
  • Infiniti – “Intake manifold air control solenoid – circuit malfunction”
  • Nissan – “VIAS (Variable Intake Air System) Control Solenoid Valve”
  • Toyota – “Automatic Transmission Operation Circuit Malfunction”
  • KIA – “Engine torque signal – malfunction”

Note that the above list is likely neither complete nor definitive. Other definitions of code P1800 that are not listed here may exist. Always consult the manual for the application being worked on for the correct definition of code P1800 as it applies to that particular application.