Testing the EGR System
Command, feedback and flow — proving whether the valve actually moves and whether the passages are clear.
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What you will learn
- Command the EGR valve
- Interpret position/flow feedback
- Recognise coking faults
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EGR reduces nitrogen oxides by recirculating inert exhaust gas into the intake. The valve meters the flow and is usually monitored. Diagnosis means commanding the valve with a scan tool and watching whether the feedback and the engine behaviour change accordingly.
Carbon build-up is the classic failure: the valve sticks, flow is wrong, and the engine runs poorly or stores a P040x code. On many engines the passages and cooler also coke up, so cleaning the valve alone is not always enough.
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Key points
- Command and feedback must agree.
- Coking affects valve, passages and cooler.
- Excess EGR makes combustion unstable — rough idle, misfire.
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Key points
- Command and observe feedback together
- Carbon build-up is the common failure
- Too much EGR causes misfire
All values shown are training examples. Real thresholds depend on the manufacturer, engine, temperature and system — always confirm with the vehicle service documentation.
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