DEF & SCR REFERENCE
DEF (Diesel Exhaust Fluid) is 32.5% urea + 67.5% deionized water. Sprayed into the exhaust upstream of the SCR catalyst, urea decomposes to ammonia, which reacts with NOx → N₂ + H₂O. This reference covers spec sheets, sensor norms, system layout, and the failure-mode decoder. Print this for the bay floor.
DEF FLUID SPEC
| Property | Spec | Out-of-spec consequence |
|---|---|---|
| Urea concentration | 32.5% ± 0.7% | Below 30% → no ammonia generated, SCR efficiency tanks, P2BAA |
| Color (visual) | Clear to pale yellow | Cloudy / discolored → contamination, replace |
| Density @ 20°C | 1.087 – 1.093 g/mL | Off = wrong dilution |
| Freeze point | -11°C (12°F) | Tank heater required in cold climates |
| Boiling point | 103°C (217°F) | Crystallizes when dries on hot metal |
| Storage life (sealed) | 2 yr at < 25°C | Heat-aged DEF degrades urea, low test reading |
| Certification | ISO 22241 / API DEF | Off-brand may have impurities that poison SCR |
SYSTEM LAYOUT (TYPICAL CRDI)
- ①DEF Tank — typically 5–10 gal, heated, with level + temp + quality sensors.
- ②Suction line + DEF pump — pressurizes fluid to ~80–100 psi.
- ③DEF injector (dosing valve) — sprays into exhaust upstream of SCR. Heated to prevent crystallization.
- ④Mixer — turbulent section to vaporize urea into ammonia before reaching catalyst.
- ⑤SCR catalyst — vanadium or copper-zeolite substrate. NH₃ + NOx → N₂ + H₂O.
- ⑥NOx pre + post sensors — measure efficiency. NOx out / NOx in = SCR efficiency.
- ⑦Ammonia slip catalyst — final scrubber to neutralize any unreacted ammonia.
SENSOR & PID NORMS
| PID / Sensor | Healthy | Out of spec means |
|---|---|---|
| NOx upstream | 300–500 ppm cruise | 0 or pegged = sensor fault; very high = engine NOx output excessive |
| NOx downstream | < 50 ppm cruise | > 80 ppm = SCR inefficient (fluid, dose, sensor, or catalyst) |
| SCR efficiency | > 90% | < 80% sets P2BAA + derate countdown |
| DEF pressure (pump) | 80–100 psi (commanded) | Low = pump weak or line restricted |
| DEF tank level | live % | Below ~10% triggers warnings; 0% = countdown to limp |
| DEF tank temp | heater on < 0°C | If fluid frozen and heater dead → no dosing |
| DEF quality sensor | 32.5% reading | Diluted (water) or wrong fluid drops reading |
| Dosing pump amp draw | 4–6 A active | Below 1 A while commanded = pump motor failing |
| SCR temp | > 200°C for dosing | ECM blocks dose if SCR too cold (no reaction) |
FAILURE-MODE DECODER
| Symptom / Code | Most likely cause | Test to confirm |
|---|---|---|
| P20EE (SCR efficiency) | Bad catalyst, contaminated DEF, lazy NOx sensors | Refractometer DEF · scope NOx sensors · scan tool dose test |
| P2BAA (SCR NOx high) | DEF quality, dosing problem, SCR aged out | Refractometer DEF · dose actuation · SCR replacement if all above OK |
| P204F (DEF system performance) | Dosing pump weak, line clogged, injector clogged | Pump amp draw · injector flow test · visual for crystals |
| P207F (DEF quality) | Water / wrong fluid in tank | Refractometer reading — drain and refill if off-spec |
| P2453 (DPF / SCR sensor) | NOx sensor failure or drift | Scope traces, compare against known-good |
| White crystal at tailpipe / injector | DEF crystallization from drying / low-flow events | Visual + clean / replace injector tip |
| Countdown to limp mode active | Repeated failed dosing / efficiency cycles | Read freeze frame · fix root cause · reset countdown |
| "DEF tank empty" warning, tank full | Level sensor failure (often heater module) | Inspect level sensor float / replace integrated heater module |
REFRACTOMETER BEFORE ANYTHING
Every DEF/SCR complaint starts with a refractometer reading on the fluid in the tank. 30 seconds, $80 tool, definitive answer. Half the "bad SCR" diagnoses we see are diluted DEF — water topped off, wrong fluid added, or stale heat-aged stock from a half-empty jug in the bay. Don\'t skip this step.
After a confirmed fluid problem: drain tank completely (don\'t just top off — the contamination stays), refill with verified DEF, run scan tool system test, clear codes, drive cycle. If countdown was active, reset only after verifying repair held.