How to identify which GM 6T transmission your car has: RPO, VIN and GM records

Why the model name is not enough

The cleanest example in our controlled evidence set is the Chevrolet Sonic.

A GM recall document identifies the 2012 Sonic 1.8L LUW automatic as 6T30, RPO MH9. [E101]

A different GM bulletin identifies 2012–2020 Sonic 1.4L LUJ/LUV automatic applications as 6T40, RPO MH8. [E104]

So even within the same model name — and even overlapping the same model year — the engine/application can change the transmission conclusion.

The full Sonic case — including which engine codes point to which transmission — is covered in does the Chevrolet Sonic use a 6T30 or a 6T40?.

That is why the workflow should be:

vehicle model → exact model year → exact engine/application → source-specific RPO/transmission mapping

not:

vehicle model name → assumed transmission

For the engineering differences between the 6T30, 6T40 and 6T45 themselves, see our GM 6T family comparison. This article is about identifying the installed unit, not comparing the family mechanically.

Step 1: lock the exact vehicle configuration first

Before looking at any transmission code, record the information that defines the vehicle application:

• model;
• model year;
• engine / engine code where available;
• automatic vs manual configuration;
• market or application boundary when the source specifies one.

This is not paperwork for its own sake. The source scope can change the answer.

For example, PIP5353F supports 2011–2015 Cruze 1.4L LUJ/LUV automatic = 6T40 / MH8. [E103]

That evidence does not justify the broader sentence “every Cruze uses a 6T40,” and it does not justify extending the 1.4L result to a 1.8L application.

Likewise, PI0724B supports 2011 Captiva / Equinox / Terrain 2.4L LAF = 6T45, RPO MHC/MH7 within that document’s stated application. [E106]

It does not support “every Captiva in every market uses 6T45.”

The first job is therefore to make the vehicle description as exact as the source you intend to use.

Step 2: use RPO only when the source gives an explicit mapping

RPO information is useful because GM documents can connect a specific vehicle/application to a transmission designation.

Examples in the controlled evidence set include:

The important phrase is “supported by that source.”

This table is not a universal decoder. It is a set of document-scoped examples.

A code that appears beside one transmission in one bulletin should not automatically be promoted into an all-years, all-models rule.

Why we do not publish a universal RPO → 6T decoder

A broad GM bulletin can list many vehicles and several 6T-family transmissions in the same applicability section without supplying a one-to-one row mapping for every vehicle. [E112]

If the document does not explicitly say which row belongs to which transmission, splitting the list into a precise model-by-model decoder would add information that the source itself did not provide.

That is exactly the kind of inference OEProof avoids.

So this article uses a stricter rule:

Only publish an RPO/transmission pairing when the controlled source explicitly supports that pairing in a defined application.

This is also why “MH8 = 6T40” should not be read as a context-free law. The safe statement is narrower: the cited GM document supports MH8 with the specified 6T40 application.

Step 3: use VIN-aware parts verification when configuration still matters

GM service documentation in our evidence set instructs technicians to use VIN/EPC information when selecting parts and checking supersession rather than inferring the answer from number appearance. [E206]

That evidence is about parts-selection workflow, not a public VIN decoder for 6T transmissions.

The safe practical lesson is:

• if the exact application is still ambiguous, keep the VIN attached to the parts inquiry;
• use a VIN-aware EPC/service-parts lookup when available to the technician, dealer or parts system;
• do not invent a transmission from VIN digits unless a controlled source actually provides that decoding rule.

OEProof does not claim to provide a GM VIN-database lookup service on this page.

VIN is a verification input, not a magic shortcut.

Step 4: check the exact vehicle documentation, not a family-wide maintenance page

Chevrolet’s official manuals/support system directs vehicle information to the exact year / make / model documentation. [E406]

That same discipline applies to transmission identification: the closer the source is to the exact vehicle configuration, the safer the conclusion.

A generic “GM 6T” article can explain the family.

It cannot replace a document that identifies the actual application installed in one specific vehicle configuration.

What if two sources appear to disagree?

Do not average them and do not choose the one with the more specific-looking title.

First compare the scope:

• same model year?
• same engine?
• same automatic/manual configuration?
• same model naming and market context?
• same transmission generation or application boundary?

Many apparent contradictions disappear once the compared applications are separated correctly.

If the records really do make mutually exclusive claims about the same application, stop the fitment decision and move to a stronger independent source.

This is the same evidence principle behind our supplier-catalogue self-verification audit: one catalogue record is useful for data collection, but it is not the end of verification.

Transmission model and transmission generation are different questions

Identifying 6T30 vs 6T40 vs 6T45 is not the same as identifying Gen 1 vs Gen 2.

A buyer can get the family/model right and still order the wrong generation-sensitive part.

GM PI0604C documents substantive Gen 2 changes in the 6T family, including valve-body/filter-plate changes. That generation question is handled separately in how to identify GM 6T Gen 1 vs Gen 2.

The safe sequence is therefore:

identify the installed 6T model → identify the relevant generation/design boundary → verify the exact part

not all three steps at once from a product title.

A practical pre-order checklist

Before ordering a 6T transmission part, write down:

1. Vehicle: exact model and model year. 2. Engine: exact engine/application where available. 3. Transmission evidence: the specific GM/manual record or VIN-aware parts record that supports the installed transmission. 4. RPO: only when its mapping is supported in the application you are using. 5. Generation/design boundary: if the part is generation-sensitive. 6. Exact OE / part-number reference: only after the transmission/application has been established.

If one of those fields remains unresolved, leaving it unresolved is safer than filling it from another seller’s title.

What we do not claim here

• We do not publish a universal GM RPO decoder.
• We do not claim that MH8, MH9, MHC or MH7 has one unconditional meaning across every year and vehicle.
• We do not provide a VIN digit-by-digit transmission decoder.
• We do not promise access to GM’s internal VIN/EPC systems.
• We do not use a broad bulletin to assign one transmission to each model unless the bulletin itself provides that mapping.
• We do not infer a specific seal, gasket or filter fitment merely from the transmission family name.
• We do not claim a universal location or format for a Service Parts Label in this evidence set.

FAQ

Can I identify a 6T30, 6T40 or 6T45 from the car model name alone?

No. The controlled GM evidence contains direct counterexamples: 2012 Sonic 1.8L LUW automatic is supported as 6T30/MH9, while Sonic 1.4L LUJ/LUV automatic applications in PIP5353F are supported as 6T40/MH8. [E101][E104]

Does MH8 always mean 6T40?

This article does not make that universal claim. PIP5353F supports MH8 in the specific 6T40 applications cited here. The mapping must stay inside the source scope. [E102][E103][E104]

Can I decode the transmission directly from the VIN characters?

Not from the evidence used here. GM documentation supports VIN/EPC-aware parts verification, but this evidence set does not contain a public VIN digit-by-digit transmission decoder. [E206]

Why do you ask for the engine if I already know the vehicle model?

Because the engine/application can change the transmission conclusion. Within one model name, GM documents can tie different engine applications to different 6T models — the worked example is in our dedicated Sonic article.

Is identifying 6T40 enough to order any 6T40 part?

No. Generation/design changes and exact OE fitment remain separate questions. Identify the transmission first, then the generation when relevant, then the exact part.

What should I send with an RFQ if I am not sure which 6T transmission is installed?

Send the exact vehicle, model year, engine information, VIN when appropriate for application lookup, and any known RPO/transmission information. Do not guess a transmission code just to complete the request.