P0734 Code: Gear 4 Incorrect Ratio (GM 6L80/8L90 Guide)

P0734 is "Gear 4 Incorrect Ratio." The transmission module commanded fourth gear, but the input and output shaft speeds showed the transmission was not actually holding fourth. On a 6L80 or 6L90, fourth is a handoff gear. It is the last gear that uses the 1-2-3-4 clutch and the first that uses the 4-5-6 clutch, so a fourth-gear fault usually points to one of those two. The pattern "first to third are fine, fourth, fifth and sixth slip" is a well-known 6L80 failure. On the 8L90 and 8L45 there is also a specific cause, a neutral on the 3-4 shift after valve body work, that no generic page mentions.

What Does P0734 Mean?

Each gear is a fixed set of applied clutches with a fixed ratio. On a 6L80, fourth is 1.152:1, so the input shaft should turn only a little faster than the output shaft. The transmission control module (TCM) divides input (turbine) speed by output speed. If the result stays away from the fourth-gear ratio while fourth is commanded, it records a fourth-gear ratio fault. In the generic SAE list that is P0734.

P0734 tells you what happened, not what failed. The input shaft is usually running faster than fourth gear allows, so a clutch is slipping. The general method for any ratio code is in our P0730 incorrect gear ratio guide: calculating ratio, ruling out speed sensors, and fluid and valve body basics. This page covers the clutches that make fourth gear.

Which Clutches Are Applied in 4th Gear?

On the 6L80, the 3-4 shift releases the 3-5-R clutch and applies the 4-5-6 clutch, while the 1-2-3-4 clutch stays on. Fourth is the first time the 4-5-6 clutch has to carry torque on the way up through the gears. A weak 4-5-6 clutch shows up here first.

Use the shared clutches to narrow it down:

• 4th, 5th and 6th all slip or are missing, while 1st to 3rd are fine: the 4-5-6 clutch or its hydraulic feed. This is the most recognisable 6L80 fourth-gear pattern.
• 4th slips along with 1st, 2nd and 3rd, while 5th and 6th are fine: the 1-2-3-4 clutch, the one element 4th shares with the lower gears.
• Only the 3-4 shift misbehaves: shift timing, adapts or valve body control, before a failed pack.

How Does GM's TCM Decide Fourth Gear Is Wrong?

The principle is arithmetic. A 6L80 in fourth with the output shaft at 2,000 RPM should have input speed near 2,000 × 1.152, which is about 2,300 RPM. A reading well above that is slip. (These are example numbers to show the method, not a GM specification.)

GM's 2011 OBD documentation for the 6L80/6L90, which we read directly, describes a steady-state fourth-gear test. With fourth commanded and gearbox slip at 400 RPM or more, the TCM runs an intrusive test and commands fifth gear:

• If fifth is not achieved, the failure is counted against the 4-5-6 clutch, the element fourth and fifth share.
• If fifth is achieved, the failure is counted against the 1-2-3-4 clutch. Fifth does not use the 1-2-3-4 clutch, so a good fifth clears the 4-5-6 clutch and leaves the 1-2-3-4.

The same document runs matching checks in fifth and sixth gear for the 4-5-6 clutch, and a code needs repeated fail counts, so a single flare will not set it. ATRA's summary of GM's criteria for the 4-5-6 circuit code describes it setting on a flare, with input speed more than 75 RPM away from the anticipated speed. Thresholds vary by year and calibration, so treat them as the logic, not a spec for your VIN.

Rule out the speed sensors first. A bad input or output speed signal produces a false ratio. GM disables these clutch tests while speed sensor codes are active, and you should fix those codes before trusting any ratio code.

Why Might a GM Truck Show P0796 Instead of P0734?

ATSG's 2007 6L80 manual lists P0734 "Incorrect 4th Gear Ratio" as a 6L80 DTC. But GM's own later OBD documents (2011 for the 6L80/6L90, 2017 for the 8L90/8L45 family) report these failures by the clutch control circuit, not by gear. For fourth gear, the relevant codes are:

Note that the same code number means a different clutch on the 6-speed and the 8-speed. P0796 is the fourth-gear clutch on a 6L80 but not on an 8L90. Always read the code against the correct transmission.

On a 6L80, the 4-5-6 clutch and the low/reverse clutch share one pressure control solenoid through the clutch select valves, which is why ATRA calls the P0796 circuit "low and reverse/4-5-6" and the valve the "CBR1/4-5-6 regulator valve." Despite the "solenoid" wording, the code only proves slip happened when that clutch was commanded on.

How Do the 6L80 and 8L90 Differ for a Fourth-Gear Ratio Fault?

What Is Commonly Behind a Fourth-Gear Ratio Fault on These Units?

6L80/6L90: the 4-5-6 clutch. A typical owner report on a 2012 Yukon XL described losing 4th, 5th and 6th while 1st to 3rd worked, with the engine flaring on the shift to fourth. Experienced members described it as a common 4-5-6 clutch failure, with a cracked 4-5-6 piston that can no longer clamp the pack. That is a forum report, not a GM document, but it matches the clutch chart exactly. Trade sources add other 4-5-6 causes:

• Regulator valve wear. ATRA says to inspect the CBR1/4-5-6 clutch regulator valve in the upper valve body for wear and sticking, and the seals and circuit for leaks.
• Parts that fail upstream. Gears Magazine notes that a fracture in the 4-5-6 dampener shaft can make it seem the 4-5-6 clutch is no longer applying, and that stator support tube cracks and cooler bypass valve failures can lead to failure.
• Check balls, spacer plate, bore plugs and debris. ATRA lists these as common to every 6L80 clutch stuck-off code and warns not to replace the TEHCM for them.

6L80/6L90: the 1-2-3-4 clutch. ATRA reports that units built before June 2010 could crack the 1-2-3-4 piston, causing slip in more than one forward gear. If fourth slips together with the lower gears, look here.

8L90/8L45. Gears Magazine lists incorrect shift adapts, wrong fluid, incorrect programming, incorrect clutch piston travel and outdated software as root causes of 8-speed flares and slips. Sonnax lists ratio codes among the symptoms of worn clutch control valves in the 8L valve body. The 3-4 neutral below is the most gear-specific one.

More on 6L80 patterns: 6L80/6L90 problems explained.

The 8L90/8L45 3-4 Neutral After Valve Body Work

If an 8-speed truck goes to neutral on the 3-4 shift and it started after transmission work, check this first.

Sonnax and Transmission Digest describe the case. After the valve body is removed and refitted, commonly to replace the fluid temperature sensor and wiring harness, the S8 default control solenoid and S9 1-2-3-4-5-R boost solenoid connectors get swapped. They are identical solenoids with matching connectors, so this is easy to do. With them swapped, one of the elements fourth gear needs is exhausted when fourth is commanded, and the 3-4 shift ends in neutral. The two sources describe the hydraulics differently. Sonnax says the default override valve fails to latch and 4-5-6-7-8-R clutch apply oil is exhausted. Transmission Digest says S9 ends up stroking the boost valve and exhausting the 1-2-3-4-5-R clutch. Both of those elements are applied in fourth, and either way the result and the fix are the same.

Sonnax gives a way to tell the connectors apart: S8 has about 3.75 inches of conduit and a green wire with a blue tracer to terminal 1, and S9 has about 1.75 inches of conduit and a yellow wire with a red tracer to terminal 7. If the truck was just in for valve body or harness work, ask for the connectors to be checked before anything is condemned.

Related Codes You May See With P0734

Symptoms of P0734

• Check engine light, with P0700 in the engine module
• Engine RPM flares on the 3-4 shift, or no shift into fourth
• Loss of 4th, 5th and 6th with normal 1st to 3rd (the 6L80 4-5-6 pattern)
• Neutral on the 3-4 shift (the 8-speed connector swap case)
• Slipping at highway cruise, with RPM rising and road speed staying the same
• Firmer shifts and no converter lockup after the code sets. ATRA reports the TCM commands maximum line pressure, inhibits the TCC and freezes adapts for these clutch codes

On urgency: fourth and above are the cruising gears, so a slipping fourth builds heat quickly at highway speed. Keep speeds down, do not tow, and get it diagnosed before the debris spreads through the valve body.

Causes and How to Confirm Each

How Is P0734 Diagnosed?

1. Read every code in the TCM and save the freeze frame. A P0796 or P2723 alongside tells you which clutch circuit the TCM suspects. Read it against the correct transmission, because the codes map differently on the 6- and 8-speed.
2. Rule out speed sensor codes before trusting the ratio.
3. Check fluid level and condition with the correct procedure at operating temperature.
4. Ask about recent work. On an 8-speed, a new fluid temperature sensor or harness followed by a 3-4 neutral means checking the S8/S9 connectors first.
5. Check for calibration updates and bulletins for the VIN.
6. Road test with a scan tool showing commanded gear, input speed and output speed. Find out whether 5th and 6th also slip, or whether the lower gears do too.
7. Check the pan and filter. Heavy friction material means disassembly. A clean pan makes a valve body cause more likely.
8. Repair the cause as well as the clutch: piston, regulator valve, seals or dampener shaft, depending on what is found.
9. Clear the codes, relearn adapts where required, and road test through the freeze frame conditions.

Parts and Cost

General estimates only. Actual prices depend on region, shop and damage.

What Other P0734 Pages Get Wrong

• Not naming the 4-5-6 clutch. On a 6L80, lost or slipping 4th, 5th and 6th is a clutch pattern you can read straight off the application chart.
• Assuming a code means the same thing on every GM transmission. P0796 and P2723 point to different clutches on the 6L80 and the 8L90.
• Missing the 8-speed 3-4 neutral. Swapped S8/S9 connectors after valve body work cause it, and the fix costs no parts.
• Replacing the TEHCM. ATRA warns against it for these stuck-off codes. The causes are in the valve body and clutches.
• Ignoring the TCM's own test. GM's intrusive command to fifth separates a 4-5-6 fault from a 1-2-3-4 fault, and a good technician can do the same with a scan tool.

Frequently Asked Questions

What clutches are applied in 4th gear on a 6L80?

The 1-2-3-4 clutch and the 4-5-6 clutch. Fourth is the last gear that uses the 1-2-3-4 clutch and the first that uses the 4-5-6 clutch, so a fourth-gear fault usually involves one of those two.

My 6L80 lost 4th, 5th and 6th but 1st to 3rd work. What is it?

That pattern points to the 4-5-6 clutch, the one element 4th, 5th and 6th share. Owners and rebuilders commonly report a cracked 4-5-6 piston. A worn regulator valve or a broken dampener shaft can cause the same symptom, so confirm it before buying parts.

My 8L90 goes to neutral on the 3-4 shift after a repair. Why?

If the valve body was removed, for example to replace the fluid temperature sensor or harness, the S8 and S9 solenoid connectors may have been swapped. Sonnax and Transmission Digest both describe the swap exhausting an element that fourth gear needs, which produces the neutral. Correcting the connectors costs no parts.

Why does my scan tool show P0796 and not P0734?

GM's OBD documents report these failures by the clutch circuit involved. On a 6L80, P0796 is the 4-5-6 (and low/reverse) circuit. On an 8L90 or 8L45, the same number points to the 1-3-5-6-7 clutch.

Can I drive with P0734?

Gently and briefly. Fourth and above are cruising gears, and slip at highway speed makes heat fast. Do not tow.

Will a software update fix it?

Sometimes, especially on the 8-speeds, where calibration and adapts are common causes of flares. It will not fix a cracked piston or burned clutches. If the pan shows friction material, plan on a mechanical repair.

A Note on Sourcing for This Code

Verified from documents we read: the 6L80 clutch application chart, ratios and DTC list come from ATSG's 6L80 manual. The steady-state fourth-gear test with its intrusive command to fifth, and the 6L80 clutch-to-code mapping, come from GM's 2011 OBD documentation. The 8L90/8L45 mapping comes from GM's 2017 OBD documentation, and the 8L90 ratios from GM Powertrain's specification sheet.

Reported, not verified by us: the cracked 4-5-6 piston pattern and cost figure come from an owner forum thread. The dampener shaft and stator support causes come from Gears Magazine. The regulator valve and valve body causes come from ATRA seminar material. The S8/S9 3-4 neutral comes from Sonnax and Transmission Digest. We found no GM bulletin specific to P0734, so we cite none.

Sources We Checked

GM documents and industry references used for this guide. Calibrations and thresholds vary by model year — confirm against current GM service information for your VIN.

• GM OBD system operation summary (transmission diagnostics)
• GM Powertrain 8L90 specifications
• GEARS Magazine (ATRA)