P0734 Code: Gear 4 Incorrect Ratio

A slipping transmission is never a good sign. Generally, any error in shifting gears warrants a trip to your mechanic. The code P0734 is one of the many OBD-II codes related to this issue. If your scan tool reads this code, it pays to know what it means and how to address it.

Below is an informative guide to help you understand code P0734 better.

What Does the P0734 Code Mean?

Diagnostic trouble code (DTC) P0734 stands for “Gear 4 Incorrect Ratio.” It is triggered once your PCM detects that the RPM difference between the rotational speed of your input speed sensor and the transmission output speed sensor doesn’t match what the ECM/PCM expects to see in 4th gear.

This error code sometimes indicates that the transmission is slipping. It can happen when your vehicle is shifting to 4th gear or even when driving at steady speeds. It can also mean that the shift to 4th gear never happened for some reason.

If you’re planning a DIY solution to code P0734, you should read our technical discussion about gear ratios for more information. Otherwise, you can read the following section for the most likely causes of P0734.

Note: The definition of code P0734 may be different depending on the vehicle manufacturer. Consult the appropriate repair manual or repair database for the exact code definition.

What are the Possible Causes of the P0734 Code?

Error code P0734 can be triggered due to several reasons. Here are some of the possible causes of this code:

• Low transmission fluid
• Defective 4th gear-related shift solenoid
• Mechanical failure in the transmission
• Malfunctioning transmission control module (rare)
• Faulty valve body
• Blocked hydraulic passages due to dirty transmission fluid

What are the Common Symptoms of the P0734 Code?

Here are some of the common symptoms you should watch for in relation to the code:

• Check engine light
• Inability to shift to 4th gear/delayed shift
• Transmission slipping
• Decreased fuel economy

How to Diagnose the P0734 Code

Diagnosing code P0734 isn’t easy. Since this code can be triggered due to several causes, it may be difficult to pinpoint the underlying fault that is causing it. If you’re not well-versed with auto repair, it is best to leave the job to a mechanic.

However, if you have great DIY repair skills, you may handle the task yourself. Consult your manufacturer’s repair manual for the diagnostic steps that are fit for your vehicle.

How to Fix the P0734 Code

Just like any other diagnostic trouble code, there is no one fix for P0734 that can work for all makes and models. For example, repair steps for a P0734 in Dodge may not be the same for a P0734 in Subaru. Again, if you’re not well-versed with auto repair, it is best to take your vehicle to your auto repair shop for proper diagnosis and repair.

If you’re determined to fix this code yourself, you can use online auto repair sources and guides to help you identify the repair steps compatible with your make and model. You may get an ALLDATA single-vehicle subscription, which should be useful in fixing this code and other issues with your car you may potentially face in the future.

OBD Codes and Gear Ratios

The ECM/PCM knows how fast the output shaft should be spinning in each gear because the gear ratios are written into the algorithms. It watches crankshaft rpm, turbine shaft (transmission input shaft) rpm, and output shaft rpm. It knows what gear it is commanding and as it watches the speed of the engine, the input shaft, and the output shaft, it knows if the gear it has commanded is in play.

You might notice the same thing watching the tach on the dash. You can tell when the transmission shifts by watching the tach and the speedometer. If, for example, you’re accustomed to seeing 1500 rpm at 50 mph on your particular vehicle with a steady throttle but suddenly you notice that the tach is showing 2400 rpm at 50 mph under the same conditions, you know 4th gear never engaged or that the clutches for that gear are slipping.

Automatic transmissions use multiple gear ratios to manipulate engine output. Your vehicle’s powertrain control module (PCM) or transmission control module (TCM) uses the data, such as information regarding throttle position vs. vehicle speed, to determine when to shift up and down between gears.

The data collected from different sensors are also used to check if all transmission components are working as they should. For example, the engine speed is typically calculated against the transmission speed sensor. This data is used to analyze gear ratio and detect torque converter slip.

Once your PCM detects that the data does not match the expected value, it will most likely trigger DTC P0734 and illuminate the check engine light.

Products Mentioned in this Guide

Automatic Transmission Fluid Engine Control Module Transmission Control Module Valve Body