Fix P0730: Mechanic’s solutions that WORKED

The code P0730: Incorrect Gear Ratio points out a discrepancy between the expected and actual gear ratios in the transmission. It most likely indicates issues with the transmission system or related components.

P0730 quick overview

Meaning	P0730: Incorrect Gear Ratio
Is it serious?	Yes, this code is serious. Ignoring it can lead to poor vehicle performance, reduced fuel efficiency, or potential transmission damage.
Possible causes	– Worn or damaged internal transmission components – Faulty transmission control module (TCM) – Incorrect transmission fluid type or level – Malfunctioning solenoids or solenoid valve – Electrical issues, such as faulty sensors or wiring harnesses – Issues with the torque converter – External factors affecting the transmission, like a slipping tone ring
How to diagnose?	1. Scan the vehicle using an OBD2 scanner to confirm the presence of the P0730 code and check for any additional related codes. 2. Check the transmission fluid level and quality. If the fluid is dirty, burnt, or low, it needs to be changed or topped up. 3. Examine the transmission’s electrical connections and harness for signs of damage, corrosion, or disconnection. 4. Test the solenoids and the solenoid valve in the transmission to ensure they are functioning correctly. 5. Check for any service information bulletins (SIBs) related to the vehicle’s make and model that address the P0730 code. 6. If the above steps don’t identify the problem, consider consulting a specialist or conducting a deeper inspection of the transmission’s internal components.
What does this mean?	This code indicates that the actual gear ratio detected by the transmission control module (TCM) does not match the expected gear ratio based on the current conditions and input.

How to fix P0730 – learn from mechanics

I looked into P0730 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: Buick

Vehicle Information

• 2000 Buick LeSabre Custom 3.8L
  

Issue + Repair History

• MIL Code P730 frequently sets when driving from stop to highway speeds using drive or OD.
• Previous codes included P730, P1811, PO742.
• Pressure control solenoid and valve body were replaced.
• Harsh shifts observed.
  

Other OBD2 Codes + Test Results

• Code P0730 sets frequently.
• Line pressure tests revealed it to be excessively high.
  

Suggestions from Community

• Check the PCM for low PCS amperage and possibly suspect the Input Speed Sensor.
• Reset the TAPS after repairs (trans adaptive strategy).
• Ensure adaptive memory has been reset and verify if commanded gear matches specified gear ratios.
• Recommendation to pull and rebuild the unit.
  

Final Fix

• Code P730 is related to a gear ratio fault.
• The gear ratio exceeded the specified limits in 3rd gear.
• Even after transmission removal and clutch inspection, no obvious causes were found.
• The issue was with the output speed sensor ring on the differential which spun freely.
• The problem was resolved by replacing the output speed sensor ring.

Case #2: Dodge

Vehicle Information

• 2007 Dodge Sprinter 2500 3.0L
  

Issue + Repair History

• CEL (Check Engine Light) illuminated with code P0730.
• Transmission pan was removed and filter and fluid inspected, which appeared normal.
• The vehicle tends to go into neutral until it slows down to about 18 mph.
• This is a NAG1 transmission, possibly a W5A 300,380, or 580, similar to the M B 722.6 transmission.
  

Suggestions from Community

• The code pertains to an incorrect gear ratio; checking speed sensors and monitoring them with a scan tool is advised. A bad sensor is the usual cause, but a stuck valve in the valve body can also be the culprit.
• The NAG1 transmission from Chrysler might set this code due to a valve sticking in the 3-4 shift group.
• The same or a similar transmission used in the 300C sedans sticks in the gear the problem occurs in. A specialized scan tool is required to obtain all freeze-frame data. The speed sensors are part of the valve body’s plastic circuit board.
• P0730, according to a German scan tool, indicates an “Impermissible transmission ratio.” The circuit board, known as the lead frame, is connected directly to the valve body. When this lead frame is faulty, it typically associates with N2 and N3 speed sensor errors. Other CAN bussed modules should be scanned for faults, and verifying rear tire size and any changes to the rear axle is suggested.
  

Final Fix

• After consulting with the customer, the transmission was pulled and inspected. No internal problems were found. All frictions and the electronic component above the valve body were replaced. Everything is now functioning correctly after a week since the vehicle left the shop.

Case #3: Honda

Vehicle Information

• 1999 Honda Accord LX 2.3L Auto Trans
  

Issue + Repair History

• Car was towed in from a body shop.
• When the car is put into drive, there’s a delay. It then alternates between going into drive and falling out of drive multiple times. Once it’s moving, it shifts fine until it comes to a stop, where it repeats the same behavior. This issue doesn’t occur in reverse.
• Alldata doesn’t provide a diagnostic procedure for the PO730D code.
• P1259 might be related, but it’s unclear.
• The car had sustained front-end damage, and a new radiator and support were installed at the body shop.
• No apparent damage to the transmission’s external casing.
  

Diagnostic Codes

• PO730D: Gear ratio issue.
• P1259: VTEC system malfunction.
  

Suggestions from Community

• P1259 can occur if the vehicle is low on oil, and it’s usually unrelated to the transmission issue. The recommendation is often to replace the transmission when the P0730 code appears.
• Possible electronic or electrical issues due to the collision. Recommendations include checking all grounds and connections, even those not in the immediate area of damage.
• The VTEC code might be due to switched underhood plugs or damaged wiring from the accident or repair process. If the VTEC code returns after being cleared, consider replacing the VTEC valve on the cylinder head’s back.
• Honda had issues with clutch design changes starting around 1998, potentially leading to transmission failures. An extended warranty or service bulletin might exist for this problem.
• The code could relate to shift solenoids. Depending on the accident’s severity, a valve body problem might exist. Pressure tests are suggested. P1259 VTEC issues might be due to open wires to the solenoid, loose head bolts, manifold issues, or other VTEC components.
  

Final Fix

• Many technicians suggest the transmission might be faulty. The course of action is to check all pressures and potentially replace the transmission. For the VTEC code, the advice is to clear the code and replace the valve if it returns. It’s also recommended to inspect all wiring for possible damage due to the accident. The actual work on the vehicle will occur after addressing the transmission issue.

Case #4: Chevrolet

Vehicle Information 2001 Chevrolet Impala LS 3.8L Gear Ratio Error

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Issue + Repair History After reinstalling the transmission, gear ratios were not steady and began going out of spec after some driving. Code P0730 set again. Gear ratio discrepancies noted across all gears when the transmission warms up. Other symptoms include surging cruise control and speedometer. Previous issues: Harsh shift after 5-10 miles, with codes P0730 and P1811. Transmission had been removed and overhauled by another shop, and a new torque converter and recalibrated PCM were installed. The problems persisted.

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Other OBD2 Codes + Test Results P0730 – Gear ratio error

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Suggestions from Community

1. Replace input and output speed sensors, as they can test good but still have a fault.
2. Replace the pressure control solenoid inside the side cover, reset trans adaptives afterwards.
3. Consider the possibility of issues with new converter materials GM is using that are designed to slip constantly.
4. Check the tone ring on the differential, as it might slip on the carrier when it warms up.

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Final Fix Issue was traced back to the tone ring on the differential slipping on the carrier when it warmed up. This was confirmed after rechecking upon heating the car.

Case #5: BMW

Vehicle Information 1997 BMW 528i 2.8L

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Issue + Repair History The check engine light is activated with codes P0740 (TCC circuit malfunctions) and P0730 (Gear ratio incorrect). Vehicle enters limp mode, and the issue recurs within 5 miles after clearing the codes. Mechanic wonders if it’s a transmission issue or perhaps an internal harness fault.

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Suggestions from Community

1. Inquire if there’s been any transmission service and if original BMW fluid was used.
2. Disconnect the transmission program switch next to the shifter and try again. Also, disconnect the battery to clear codes.
3. There’s a known Service Information Bulletin about the Torque Converter Lock-up Solenoid that might relate to the TCC code.
4. The first fault could be a pressure regulator, which is a good starting point for investigation.

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Final Fix Initially, the solenoid in the valve body was replaced but didn’t resolve the issue. The final solution was replacing the box with a remanufactured unit. The unit in question was a GM box, not the typical ZF.