|Trouble Code||Fault Location||Probable Cause|
|P1701||Reverse Engagement Error (Ford)
Electronic Throttle Control System MIL Request (Chrysler/Dodge/Jeep)
Throttle Position Sensor Circuit (Daewoo)
Trans. MIL Request Circuit (GM)
Throttle Position Sensor (Hyundai)
Transfer Box Line Fault (Land Rover)
Cruise Control Set Signal Circuit Malfunction A/T (Subaru)
Transmission Control Module Power Supply (Infiniti / Infiniti)
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What Does Code P1701 Mean?
SPECIAL NOTES: Due to the large number of transmission designs in use today, this guide cannot provide comprehensive diagnostic and repair information for this code on all applications. However, the operating principles of all automatic transmissions are largely similar, meaning that the information provided here should enable average DIY mechanics to diagnose code P1701 on most applications with the aid of a repair manual for that particular application.
Nonetheless, the diagnostic and repair information presented here is intended for general informational purposes only, and as such, it should NOT be used to diagnose and/or repair code P1701 on any application without reference to the repair manual for the application being worked on. END OF SPECIAL NOTES.
OBD II fault code P1701 is a manufacturer specific code, which some manufacturers (Ford, Lincoln, Mazda, Mercedes, Mercury, and Oldsmobile) define as “Reverse Engagement Error”. On these applications, P1701 is set when the PCM (Powertrain Control Module) or TCM (Transmission Control Module) detects a failure of the automatic transmission to engage reverse gear. Note that in some cases, this code could also be set when sensors in the transmission and elsewhere fail to recognize that reverse gear had actually been engaged.
While design specifics of reverse gear engagement systems vary between applications, all transmissions use a system of electrically operated solenoid and other types of valves to control / direct the flow of pressurized transmission fluid to control and manage gearshifts. In addition, all automatic transmissions employ a system of planetary gears, clutches, and other means of power transmission to keep the transmission in any given selected gear until inputs from either a control module (PCM/TCM), or the driver, causes a change in the hydraulic circuits that control gearshifts.
In the case of reverse gear selection however, the actual input is made by the driver through activating a selector mechanism, which for the purposes of this guide, is assumed to be in perfect working order. From that point onward, reverse gear selection and proper engagement (or not) inside the transmission depends on several factors, such as;
- the transmissions’ various components and subsystems being fully functional,
- the presence of sound (valid) communication between the transmissions’ relevant sensors and all relevant control modules,
- system pressure,
- as well as on the level, quality, grade, and condition of the transmission fluid.
The image below shows a simplified schematic of the planetary gear sets in a typical automatic transmission that determine whether reverse, or a forward gear is selected. Note the clutch packs to the left of the gear sets; by locking one clutch pack and engaging the other, it becomes possible to alternate between reverse and forward gears. While all automatic transmissions, with the exception of CVT transmissions, work on this basic principle, the actual factors (as listed above) that ultimately determine whether reverse gear is engaged or not, are largely make and model specific.
What are the common causes of code P1701?
Some common causes of code P1701 could include the following-
- Defective, malfunctioning, or maladjusted gear selector mechanism
- Failure or malfunctions of mechanical components/systems inside the transmission
- Defective reverse gear sensor(s)
- Dirty, contaminated, or degraded transmission fluid
- Use of incorrect, or unsuitable grades/formulations of transmission fluid
- Damaged, burnt, shorted, or disconnected wiring/connectors both inside and outside of the transmission
- Loss of operating pressure when reverse gear is selected. A general reduction in pressure caused by a defective pressure pump will affect the overall functioning of the transmission, and not just reverse gear
- Failed, or failing PCM/TCM. Note that this is a rare event, and the fault must be sought elsewhere before any controller is replaced
What are the symptoms of code P1701?
Apart from a stored trouble code and an illuminated warning light, the most obvious symptom of this code is a failure to engage reverse gear. Other possible symptoms could include the following-
- In some cases, the engine may suffer from a severe loss of power when the transmission enters “limp mode” to prevent further damage to the transmission
- Depending on the exact nature of the problem, the transmission may slip out of reverse gear
How do you troubleshoot code P1701?
WARNING: Non-professional, or non-dealership technicians who do not have access to ALL relevant technical information on transmissions with “life-time” fills of fluid should NOT attempt a repair of this code if the repair entails draining the transmission fluid. On these transmissions, it is not always possible to determine the level of the transmission fluid during and after refilling, meaning that the transmission could be over -, or under filled. Both conditions are equally bad, and either state can, or likely will cause the destruction of the transmission.
NOTE #1: Some Ford transmissions, and most notably the third-generation A4ld, as well as the 44 -, and 55 series transmissions are fitted with a dedicated shift solenoid that has the function of applying or engaging reverse gear. This modification was intended to improve positive reverse gear engagement, and while the idea was sound, the execution left a lot to be desired. These dedicated reverse gear solenoids are known to be problematic in that they tend to stick for no discernable reason, which when it happens, causes a loss of operating pressure which in turn, can set a multitude of false codes that could include “Reverse Gear Engagement Failure”, and “Incorrect Gear Ratio.”
NOTE #2: When diagnosing transmission problems, it is always a good idea to test the transmission’s operating pressure to eliminate, or confirm, the loss of pressure as a cause of the problem, before starting a sometimes-difficult diagnostic procedure. By testing the operating pressure as a first step, a lot of time can be saved (and trouble avoided) if it is known whether the operating pressure falls within specification or not. Bear in mind though that to get reliable results; you need to have access to accurate reference data, as well as suitable and approved test equipment.
Record all fault codes present, as well as all available freeze frame data. This information can be of use should an intermittent fault be diagnosed later on.
Since low transmission fluid levels, as well as degraded transmission fluid are leading causes of code P1701, start the diagnostic procedure by inspecting both the level and the condition of the transmission fluid, but only on transmissions that are fitted with a means to check, and top off the level. Refer to the WARNING above.
The image below shows the various stages of transmission fluid degradation. If possible, compare the transmission fluid to the chart below, and replace the fluid if required. Note however that replacing the transmission fluid is NOT guaranteed to resolve code P1701.
If replacing the transmission fluid did not resolve the code, prepare to perform a thorough visual inspection of all associated wiring and accessible sensors/connectors/sensors/switches. Consult the manual to determine the location, function, color-coding, and routing of all associated wiring/connectors/ sensors/switches.
Look for damaged, burnt, shorted, disconnected, and/or corroded wiring and connectors. Make repairs as required if visible damage is found. Also, inspect the selector mechanism to look for loose, worn, broken, or damaged linkages and/or bushings/cables that can affect proper gear selection. Make repairs, or replace components as required if damage to the selector mechanism is found. Clear all codes after repairs are complete, and operate the vehicle to see if the code returns.
If no visible damage to wiring and the selector mechanism is found but the code persists, consult the manual on the correct procedure(s) to test the wiring for resistance, ground, reference voltage(s), and continuity. The easiest way to test all relevant circuits is to obtain a pin-out chart for the application that clearly shows which pin in the connector does what.
Compare all obtained readings with those stated in the manual, but bear in mind that if discrepancies between actual and expected values are found, some of those discrepancies could be the result of electrical issues within the transmission.
WARNING: Do NOT attempt to perform these tests if you do not have access to reference data, or if you do not have a complete understanding of the electrical circuit(s) that you are testing. One accidental short circuit can destroy a control module, so exercise extreme care when performing pinpoint testing.
If discrepancies between expected and actual electrical values are encountered, disconnect the transmission from the wiring harness. Consult the manual on the correct procedure to test the shift solenoids and their wiring, and compare all obtained readings with the manufacturer’s specifications.
Bear in mind that if electrical values for components inside the transmission do not comply with the manufacturer’s specifications, it may not be possible to find and repair the fault if the transmission has a “life-time” fill. Refer to the WARNING at the top of the “Trouble Shooting” section.
Consult the manual on the correct way to proceed from this point on. Either the manual will clearly spell out the possible repair options, or it will categorically state that the transmission should not be tampered with by non-professional mechanics.
In the former case, follow the directions to safely drain the transmission fluid, and remove the oil pan. Inspect the oil pan for the presence of large metal or friction material fragments. Metal fragments will collect on a collector magnet, and particles that are larger than the accumulated metal “sludge” is a sure sign that the transmission has suffered significant mechanical damage.
In the latter case, the vehicle must be referred to a competent dealer repair facility for professional diagnosis and repair.
Assuming then that the transmission can be serviced by non-professional mechanics, and that no large metal (or other types) of fragments are present in the oil pan, consult the manual to locate all relevant components, and the correct procedure to test all internal wiring, each shift solenoid, and all other components that are relevant to reverse gear selection.
Compare all obtained readings with the values stated in the manual, but bear in mind that even if all values fall within specifications, it does NOT mean that one or more hydraulic circuits may not be affected by sludge, or varnishes that form at high temperatures. Transmission fluid that is dark, or has a “burnt” smell is a good indicator of the presence of sludge or varnish in the valve body, which often requires replacement to fix the problem.
If however, the transmission fluid is in good condition, and there is no obvious electrical problem, suspect a sticky solenoid, sticky valve shuttle in the valve body, or a leak between the valve body and the transmission proper. Consult the manual on the correct torque values for all retaining bolts, and check all fasteners for tightness, but use extreme caution when checking the tightness of retaining bolts to prevent breaking bolts, or stripping threads in the transmission casing. Do NOT exceed recommended torque values.
If sticky valve shuttles are suspected, it is often more cost effective to replace the valve body as a complete assembly, as opposed to replacing individual components such as shuttles, springs, and check valves.
NOTE: In many, if not most cases, replacing the valve body will resolve code P1701, but be aware that in a significant percentage of instances, the problem might not involve the valve body, or even any component or components inside the transmission. It is entirely possible that the problem is caused by issues outside the transmission, such as software glitches/failures, failures in the CAN (Controller Area Network), or even intermittent wiring faults. For this reason, it is not recommended that non-professional mechanics proceed with this repair beyond replacing the valve body.
If the valve body replacement did not resolve the problem, it is strongly recommended that non-professional mechanics refer the vehicle to the authorized dealer or a specialist repair shop for professional diagnosis and repair.
Codes Related to P1701
There are no codes directly related to P1701, but several manufacturers have assigned definitions other than “Reverse Engagement Error” to this code. Below are some examples-
- Chrysler/Dodge/Jeep – Electronic Throttle Control System (MIL Request) (RESERVED for HAL Z.)
- Daewoo – Throttle Position Sensor Circuit
- GM – Trans. MIL Request Circuit
- Hyundai – Throttle Position Sensor
Land Rover – Transfer Box Line Fault
- Subaru – Cruise Control Set Signal Circuit Malfunction (A/T)
- Infiniti / Infiniti – Transmission Control Module Power Supply
Other Manufacturer Specific Definitions for P1701Transmission MIL Request Circuit (GM)
MIL Request Circuit (4L30-E Transmission) Conditions (GM)
Reverse Engagement Error Conditions (Ford)
Transmission MIL Request Circuit (Buick)
Transmission control module (TCM) -MIL request circuit (Cadillac)
Throttle Position Sensor (Hyundai)
Transmission Control Module Power Supply (Infiniti)
Transfer box - fault signal (Land Rover)
Transmission system -reverse engagement error (Lincoln)
Transmission solenoid malfunction – reverse gear engagement error (Mazda)
Transmission shift control valve – electrical fault (Mercedes-Benz)
Fuel trim (FT) -malfunction (Mercury)
Transmission Control Module Power Supply (Nissan)
Cruise control set signal -AT- circuit malfunction (Saab)
Cruise control set signal – AT- circuit malfunction (Subaru)
Transmission control module (TCM), CAN data bus – communication malfunction (Suzuki)
Transmission MIL Request Circuit (Chevrolet)