What Does P1166 Mean?
This code indicates that the long-term fuel trim (LTFT) for bank 1 at higher load/RPM conditions (range 2) has exceeded its maximum rich correction limit. Unlike P1165 which covers idle/low-load, this code triggers under part-throttle cruise and moderate load conditions. The PCM is subtracting fuel beyond its calibration limit, indicating a rich condition at higher airflow conditions on bank 1.
Common Causes
25%
Excessive fuel pressure under load (weak fuel pressure regulator vacuum reference)
25%
MAF sensor under-reporting airflow at higher flow rates (contaminated or failing)
20%
Leaking fuel injector(s) that become more apparent under higher pulse widths
15%
Restricted exhaust (partially clogged catalytic converter) creating high backpressure
15%
Bank 1 upstream O2 sensor degraded, causing incorrect rich/lean feedback
Diagnostic Steps
1
Monitor bank 1 LTFT at cruise speeds (2,000-3,000 RPM, partial throttle); values exceeding -15% to -20% confirm the rich limit is exceeded at higher loads.
2
Compare LTFT at idle (range 1) vs. cruise (range 2); if only range 2 is affected, focus on load-dependent causes like fuel pressure under vacuum and MAF accuracy.
3
Test fuel pressure under load conditions; verify the regulator responds properly to manifold vacuum changes.
4
Check MAF sensor readings at various RPM points against known-good airflow tables; clean the MAF sensor element with appropriate MAF cleaner.
5
Perform an exhaust backpressure test at the upstream O2 sensor bung; excessive backpressure (above 2 PSI at idle, 5 PSI at 2,500 RPM) indicates a restriction.
Estimated Repair Cost
$150 - $700
Parts + labor, varies by vehicle and location
This code indicates that the long-term fuel trim (LTFT) for bank 1 at higher load/RPM conditions (range 2) has exceeded its maximum rich correction limit. Unlike P1165 which covers idle/low-load, this code triggers under part-throttle cruise and moderate load conditions. The PCM is subtracting fuel ...
The most common cause of P1166 (Bank1, Long Term Fuel Trim, Range 2 Rich Limit Exceeded) is: Excessive fuel pressure under load (weak fuel pressure regulator vacuum reference)
Typical repair costs for P1166 range from $150 to $700, depending on the vehicle, location, and whether you do it yourself or go to a shop.
Rich running under load accelerates catalytic converter deterioration and increases the risk of converter overheating. Extended driving with this condition can lead to reduced power and potential converter failure.
Start by connecting an OBD2 scanner to read the code and any freeze frame data. Then follow the diagnostic steps specific to P1166 to identify the root cause.
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Quick Info
Category
Powertrain
System
Fuel / Air Metering
Difficulty
Type
Manufacturer
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