Cause
Soot-fouled EGR cooler
The cooler's internal passages blocked with soot, so recirculated gas is neither cooled nor flowing freely. The cooler is a small heat exchanger with narrow gas channels — exactly the geometry that clogs fastest — and it sits at the dirtiest point in the system.
It fails in two ways that present quite differently. Blocked, it restricts flow and delivers hot gas, which shows as low EGR flow, a temperature correlation fault between the sensors either side of it, or raised nitrogen oxide output.
Split internally, it does something far more serious. The gas channels run through engine coolant, so a crack lets coolant into the intake — which shows as white exhaust smoke, coolant disappearing with no visible leak, and eventually hydraulic damage if enough collects in a cylinder. Unexplained coolant loss on a diesel with EGR faults is worth taking seriously as this.
Cleaning is possible but less rewarding than on a valve, because the channels are narrow and the deposits are baked. Many are replaced.
As with everything in this family, short cold journeys are the underlying cause — a cooler that never gets hot accumulates what a hot one would burn off.
Typically a moderate job.
Codes this can cause (6)
Codes where this is a likely explanation first, rather than merely a possible one.
- P040B Exhaust Gas Recirculation Temperature Sensor A Circuit Range/Performance Likely for this code Moderate
- P040F Exhaust Gas Recirculation Temperature Sensor A/B Correlation Likely for this code Moderate
- P04D9 Closed Loop Exhaust Gas Recirculation Control At Limit — Flow Too Low Likely for this code Moderate
- P2457 Exhaust Gas Recirculation Cooler A Efficiency Below Threshold Likely for this code Moderate
- P2493 Exhaust Gas Recirculation Cooler Bypass Position Sensor Range/Performance — Bank 1 Moderate
- P2498 Exhaust Gas Recirculation Cooler Bypass Position Sensor Range/Performance — Bank 2 Moderate
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