Cause
Failed PCV valve or hose
The crankcase ventilation valve and the hoses around it, which draw blow-by gases out of the crankcase and feed them back into the intake to be burnt. It appears on lean codes and on idle control codes for the same underlying reason: every part of this circuit sits downstream of the airflow sensor, so anything that opens it admits air the engine never measured.
A split hose is the commonest version and the easiest to miss. The rubber hardens with heat and age and cracks on the underside where nobody looks, or splits only when it flexes at certain engine loads — which produces a lean condition that comes and goes and reads like a failing sensor. A valve stuck open does the same thing continuously; stuck closed it does the opposite, pressurising the crankcase until oil is forced past seals that were perfectly good.
The distinction worth holding is between the valve and the plumbing. The valve is a cheap part and gets replaced first, but a new valve into a perished hose changes nothing. Smoke-test the whole circuit rather than replacing the valve on suspicion — it finds the split in minutes and it also finds the intake leaks elsewhere that present identically.
Typically an easy fix. Often worth checking before paying anyone to look.
Some of these codes mean you should stop driving
1 of the 5 codes this can cause indicate a fault where continuing to drive risks damage or is unsafe. Which one you have matters more than the cause does — read the code's own page before deciding.
Codes this can cause (5)
Codes where this is a likely explanation first, rather than merely a possible one.
- P0171 System Too Lean — Bank 1 Likely for this code High
- P0174 System Too Lean — Bank 2 Likely for this code High
- P0507 Idle Air Control System RPM Higher Than Expected Likely for this code Moderate
- P0505 Idle Air Control System Moderate
- P2101 Throttle Actuator A Control Motor Circuit Range/Performance Stop driving High
Information, not instruction
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Have anything you intend to act on checked by a trained person against the manufacturer's own procedure for your vehicle. Nothing here replaces that procedure.
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