Cummins and Caterpillar Engine Serial Numbers Explained
How Cummins 8-digit ESNs and Caterpillar 3-character prefix serial numbers work, with a table of common CAT prefixes (6NZ, BXS, S3C, 92U) and what they identify.
Cummins: 8 sequential digits
Modern Cummins ESNs are exactly 8 digits with no letters and no model information embedded — they are purely sequential. To get the engine model, build date, and parts list you look the ESN up in Cummins QuickServe. Pre-1990 Cummins engines can have shorter all-numeric serials of 5-7 digits.
The Cummins dataplate is on the rocker cover or the side of the gear housing; the ESN is also stamped on the block, commonly on the fuel pump side above the oil cooler.
Caterpillar: 3-character prefix + serial
CAT ESNs start with a 3-character prefix followed by a 5-7 digit sequential serial (e.g., 6NZ58131). The second and third characters of the prefix are always letters. The prefix identifies the engine model and often the emissions generation:
| Prefix examples | Engine model |
|---|---|
| KAL, C7S | C7 |
| 1YN, 2KS, MBL, TME | C12 |
| S3C, KCB, LGK, KWJ | C13 |
| 6NZ, 9NZ, BXS, MXS, NXS | C15 / C15 ACERT |
| 1BW, 4ES, 7JZ | 3126 |
| 92U | 3406 |
| 2EK, 4MG, 7FB | 3406B |
| 1LW, 2WS, 5EK, 6TS | 3406E |
Why the CAT prefix matters
Two engines with the same displacement can take very different parts depending on prefix — a 6NZ C15 (pre-ACERT single turbo) and a BXS C15 ACERT (twin turbo) share almost no top-end components. Always quote the full prefix + serial when ordering CAT parts.
Frequently asked questions
- Can I tell the Cummins engine model from the serial number alone?
- No — Cummins ESNs are purely sequential 8-digit numbers. Look the ESN up in Cummins QuickServe (or a decoder that references it) to get the model, CPL, and build date.
- What engine is a 6NZ?
- 6NZ is one of the best-known Caterpillar C15 prefixes — a pre-ACERT, single-turbo C15 widely regarded for simplicity and rebuildability.
- Where is the ESN on a Caterpillar truck engine?
- On the information plate on the left side of the block near the front, and stamped on the block itself. On many 3406/C15 engines it is behind the air compressor.