Cat C15 Fault Codes Reference (Top 18) | HDT
Caterpillar C15 (and the smaller C13, larger C16) was Cat’s last on-highway HD lineup before Cat exited US Class 8 in 2010. Plenty of legacy C15 / C13 trucks are still in service — particularly in fleet, agricultural, and vocational use — and the same engine architecture lives on in Cat off-highway, marine, industrial, and genset applications. The 18 codes below are the ones we see most often. Cat ET (Electronic Technician) is the OEM platform; aftermarket platforms (JPRO, Jaltest CV, TEXA) cover diagnostic and bidirectional work.
Note: Cat fault codes don’t follow standard SAE J1939 SPN/FMI mapping consistently. Cat uses a proprietary diagnostic structure with PID/CID/MID identifiers in many cases. The codes below are presented in the format Cat ET displays them, with SPN/FMI equivalents where applicable.
Fuel system codes
| Cat code | Description | Common cause | Next step |
|---|---|---|---|
| 164-3 / 164-4 | Fuel rail pressure sensor circuit fault | Failed rail pressure sensor, harness chafe | Backprobe sensor; replace if voltage out of spec. |
| 164-1 | Fuel rail pressure low | Plugged fuel filter, weak transfer pump, dropping injector | Check primary fuel filter restriction; verify transfer pump pressure. Run cylinder cutout in Cat ET if both OK. |
| 168-0 | System voltage above normal | Alternator over-charging, failed voltage regulator | Test alternator output under load. Replace alternator if over-charging. |
| 1-1 through 6-1 (cylinder injector codes) | Cylinder N injector circuit fault | Failed Cat HEUI / common-rail injector, harness fault | Cat ET injector solenoid test. C15 ACERT-era engines (2007-2010) use common-rail; older C15 / C13 use HEUI. Test method differs. |
Air handling and turbo codes
| Cat code | Description | Common cause | Next step |
|---|---|---|---|
| 411-3 / 411-4 | EGR valve position circuit fault | Carbon-coked EGR valve (ACERT engines), failed sensor | EGR valve sweep test in Cat ET. ACERT-era EGR valves carbon up faster than highway fleets expect. |
| 27-3 / 27-4 | Boost pressure sensor circuit fault | Failed boost sensor, oil contamination of sensor port | Replace sensor; check intake manifold for excessive oil indicating turbo seal wear. |
| 105-15 | Intake manifold air temperature high | Plugged charge air cooler, failed temp sensor | Compare CAC inlet/outlet temp. Excessive temp rise = airflow restriction. |
| 2789-7 | VGT actuator commanded position not reached (ACERT) | Carbon on VGT vanes, failed VGT actuator | VGT sweep test. Decarbon or replace turbo for stuck vanes. |
Aftertreatment codes (ACERT-era 2007-2010)
| Cat code | Description | Common cause | Next step |
|---|---|---|---|
| 3251-0 | DPF differential pressure too high | DPF heavily soot-loaded | Force a stationary regen via Cat ET. If dP doesn’t drop after a complete regen, DPF needs cleaning or replacement. |
| 3563-0 | Intake throttle position fault | Carbon on intake throttle (used during regen), failed actuator | Cat ET intake throttle bidirectional test. Decarbon or replace. |
| 3936-0 | SCR efficiency fault (later ACERT) | Failed NOx sensor, expired DEF, mechanical (intake leak) | Compare upstream/downstream NOx sensor data; check DEF concentration. |
Sensors and electrical codes
| Cat code | Description | Common cause | Next step |
|---|---|---|---|
| 110-3 / 110-4 | Coolant temperature sensor circuit fault | Failed ECT, harness chafe | Backprobe sensor; replace if out-of-spec resistance. |
| 100-1 / 100-3 | Engine oil pressure low / sensor fault | Mechanical low pressure or failed sensor | Confirm with mechanical gauge before assuming sensor. |
| 91-3 / 91-4 | Throttle position sensor circuit fault | Failed throttle position sensor, harness fault | Test sensor output through full pedal range; replace if signal abnormal. |
| 97-3 / 97-4 | Water in fuel sensor circuit fault | Failed WIF sensor, contaminated fuel | Drain water separator; inspect WIF sensor. |
Cat-specific notes
- Pre-ACERT vs ACERT: C15 / C13 produced before 2007 are pre-ACERT (no DPF, no EGR, no DEF). 2007-2010 are ACERT (DPF + EGR; no DEF on Cat — they used aggressive EGR instead of SCR). Diagnostic logic differs significantly between the two.
- HEUI vs common rail: Older C15 / C13 use HEUI (Hydraulically actuated Electronically controlled Unit Injector). 2007+ ACERT engines use common-rail. Injector test procedures differ — Cat ET handles both.
- Cat exited US Class 8 in 2010: Cat stopped producing on-highway Class 8 engines in 2010 due to EPA emissions challenges (no DEF strategy didn’t scale). Cat ET continues to support legacy Class 8 plus current off-highway, marine, and industrial Cat engines.
- Off-highway / industrial: Cat 3406, 3408, 3412, 3500-series, 3600-series engines remain in production for off-highway, marine, and stationary power. Cat ET coverage on these remains current.
- Cat Communication Adapter III: Cat’s preferred adapter. NEXIQ USB-Link 3 supported via RP1210 for some Cat ET versions.
Tools we recommend
- Cat ET (Electronic Technician) — OEM platform for full Cat coverage including reflash and parameter programming. Cat ET packages
- Cat Communication Adapter III — Cat’s preferred datalink adapter
- JPRO Professional — covers legacy Cat C13/C15 on-highway at aftermarket depth. JPRO packages
- Jaltest CV — multi-brand including legacy Cat. Jaltest packages
- JDC565.9 (Cojali Jaltest cable) — Mitsubishi/Caterpillar diagnostic cable for some legacy Cat applications
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