NOx Sensor Circuit Bank 1 High Input (Generic)
The P4027 code means the vehicle's onboard diagnostic system has detected: NOx Sensor Circuit Bank 1 High Input (Generic). This is a moderate severity code.
- Keep driving?
- Short distances only
- DIY difficulty
- hard
- Estimated cost
- $300 - $1500 (parts and labor)
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Common Symptoms
- Check Engine Light on
- High NOx reading despite SCR operation
- DEF warning active
- Potential speed derate countdown
- Failed emissions test
Probable Causes (Ranked by Likelihood)
- SCR catalyst efficiency loss Very High
- Failed NOx sensor reading high High
- DEF injector clogged or failed Moderate
- Contaminated DEF fluid Low
Step-by-Step Diagnostic Procedure
-
Check post-SCR NOx levels against pre-SCR levels
-
Test DEF injector operation
-
Verify DEF quality with refractometer
-
Inspect SCR catalyst for damage or contamination
Frequently Asked Questions
Does P4027 mean my SCR is bad?
Not necessarily. While a failing SCR catalyst is one cause, a faulty NOx sensor or clogged DEF injector are also common causes. Proper diagnosis is important.
Can I drive with P4027?
You can drive short distances but a derate timer may be active. On some vehicles, after a set number of miles or hours, speed will be limited to 5 mph.
How much does SCR catalyst replacement cost?
An SCR catalyst replacement is very expensive, typically $2000-5000. Make sure to rule out the NOx sensor and DEF injector first.
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