FC 1691 — Aftertreatment Diesel Oxidation Catalyst Conversion Efficiency – Below Normal — Cummins ISX15
Diagnostic reference for Cummins ISX15 (2010–2017) FC 1691.
What it means
The temperature rise across the diesel oxidation catalyst is lower than expected. On Cummins ISX15 (2010–2017) this affects DOC, DPF regeneration.1
Most likely causes
- Face-plugged or contaminated DOC — Exhaust flow restriction or catalyst contamination reduces conversion and temperature rise across the DOC. Typical on this platform.1
- Exhaust leak upstream or at DOC inlet/outlet — Leak reduces measured temperature rise and conversion efficiency. Typical on this platform.1
- Temperature sensor problem for DOC inlet/outlet — Erroneous temperature readings cause calculated conversion efficiency to appear low. Typical on this platform.1
How to diagnose
- Confirm FC 1691 is active and note accompanying codes; DPF regeneration-related codes can co-occur.
- Review aftertreatment temperature data: compare DOC inlet vs outlet temperatures under steady load; low delta suggests conversion issue.
- Inspect exhaust system for leaks at DOC connections and upstream joints; leaks affect temperature rise.
- Check DOC differential pressure / face plugging indicators and DPF soot load; a heavily loaded DPF can influence regeneration and DOC performance.
- Verify temperature sensor readings and plausibility against other exhaust temperature sensors; sensor faults can mimic low conversion.
- If regeneration performance is degraded, evaluate DPF regeneration history and related DPF restriction codes.
Parts and tools you may need
- Diagnostic scan tool with Cummins Insite/J1939 live data
- Exhaust back-pressure gauge
- Thermal imaging camera or contact thermometers
- Exhaust inspection mirror and light
Tools you’ll need
- diagnostic scan tool — Read active FC 1691 and related aftertreatment codes, then log DOC inlet/outlet temperatures under load to quantify delta-T.
- infrared thermometer — Non-contact spot check of DOC inlet/outlet surface temps to quickly confirm low temperature rise and rule out sensor error.
- exhaust backpressure gauge — Measure DOC/DPF differential pressure to detect face plugging or restriction that reduces conversion and regeneration efficiency.
- digital multimeter — Verify DOC temperature sensor supply, ground, and signal voltages/resistance to confirm sensor plausibility versus actual temps.
Rough cost and difficulty
| Likely repair | Parts | Shop labor | Who usually does it |
|---|---|---|---|
| Exhaust gas temperature sensor | $50–$250 | 0.5–1.5 hrs | DIY with tools and a manual |
| Exhaust manifold, gasket or up-pipe leak repair | $50–$400 | 2–6 hrs | DIY with tools and a manual |
| Diesel oxidation catalyst (DOC) | $1,500–$4,000 | 1–4 hrs | Shop |
Typical US independent-shop figures; labor runs about $140–$200 per hour (dealers charge more). Confirm the diagnosis before buying parts.
When to see a pro
If the DOC is suspected plugged or contaminated, or exhaust repairs are needed, have a qualified diesel technician inspect and service the aftertreatment system. Do not attempt high-pressure fuel system service. If SCR inducement codes appear, diagnose the underlying fault before clearing derates.
Check for recalls
Recalls are tied to your truck’s VIN, so check yours directly: enter the VIN at NHTSA’s recall lookup. Open safety recalls are repaired free at a dealer.
Sources
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