What this code means
P0171 may indicate the PCM is seeing a lean fuel correction trend on Bank 1.
What the vehicle may do
- The vehicle may run rough, hesitate, or lack power.
- The MIL may be illuminated.
- The vehicle can sometimes appear to drive normally.
Possible fault areas
- Possible air or vacuum leaks
- Possible fuel delivery concern
- Possible exhaust leak affecting oxygen sensor feedback
- Possible oxygen sensor or oxygen sensor circuit concern
- Possible MAP sensor input concern
- Possible engine mechanical concern
Diagnostic path
Opening context
On this 2019 to 2024 Ram 1500 Classic with the 5.7 HEMI gas engine, P0171 means the PCM may be seeing the Bank 1 fuel system trend lean. In plain terms, the computer may be adding fuel because the exhaust feedback suggests there is too much air, not enough fuel, or the feedback signal is leading it that way. The truck may run rough, hesitate, or turn the MIL on, but it can also drive fairly normally. Broadly, this can involve air leaks, fuel delivery, exhaust leaks, oxygen sensor feedback, MAP sensor input, wiring, or an engine mechanical issue. The diagnostic is built to separate those areas instead of jumping straight to an oxygen sensor.
How the monitor sets the code
The fuel system monitor uses upstream oxygen sensor feedback for short term and long term fuel correction, and it also watches downstream oxygen sensor feedback for catalyst efficiency information. This diagnostic is looking for rich or lean fuel system abnormalities that could cause emissions to exceed 1.5 times any of the standards. For this code, the PCM multiplies short term compensation by long term adaptive, and when the calibrated percentage is exceeded, the fault can set. The monitor runs continuously with the engine running in closed loop, ambient or battery temperature above -6.7°C (20°F), and altitude below 2590.8 m (8500 ft). The MIL comes on after the second consecutive failed trip, and turns off after the third consecutive passing trip.
Start with the basic system checks and confirm the fault
Start with the basic system checks before going into the code path. Then connect the scan tool, read and record the codes, and save the captured operating data. Start the engine and verify the fuel system is in closed loop fuel control. Use the captured data and the monitor gates to operate the vehicle under the same kind of conditions that set the code, then read codes again. If P0171 is active or pending, continue the diagnostic. If it is not active or pending, treat it as an intermittent condition and do not force a repair based on code history alone.
Check for related fuel or air handling faults first
Next, look at the recorded codes. If other active or pending faults are present in the fuel or air handling systems, handle those first because they can drive the lean calculation. If there are no related active or pending fuel or air handling faults, move on to the sensor and system checks in order.
Check MAP sensor operation
Check MAP sensor operation using the MAP sensor operation test for this diagnostic path. If that test finds a MAP-related problem, repair the MAP sensor related issue and then verify the repair. If the MAP sensor operation checks out, keep moving down the lean diagnostic path.
Check engine mechanical and air path causes
With the ignition off, inspect the engine mechanical and air handling side. The air induction system needs to be free of leaks. Check the engine and vacuum hoses for vacuum leaks. Check the power brake booster for an internal vacuum leak by watching engine speed and vacuum while pinching the booster vacuum hose. If a restricted injector is suspected, remove the fuel rail with the injectors intact, energize the fuel pump to maintain rail pressure, actuate the suspect injector with the scan tool, and compare its spray pattern with the others. Also check that the purge flow system flows freely with no leaks or restrictions, including the hose and connections between the purge solenoid and intake manifold. Engine compression must be within specifications. If you find an engine mechanical issue, repair that issue and verify the repair. If not, continue to fuel delivery.
Check fuel delivery and fuel content
Now check the fuel delivery system using the fuel delivery test for this path. If a fuel delivery problem is found, repair the fuel delivery related issue and verify the repair. If fuel delivery passes, check the ethanol content in the gasoline using the gasoline ethanol level test. If the gasoline test finds an issue, repair the ethanol level related issue and verify the repair. If the gasoline checks out, continue to the exhaust system.
Check for exhaust leaks
Check the exhaust system for leaks using the exhaust leak test for this diagnostic path. A leak can affect oxygen sensor feedback and push the fuel trim calculation lean. If an exhaust system problem is found, repair the exhaust related issue and verify the repair. If no exhaust leak problem is found, move to the upstream oxygen sensor circuits.
Check upstream oxygen sensor wiring
For the upstream oxygen sensor wiring check, disconnect the upstream oxygen sensor harness connector and disconnect the PCM C2 harness connector. Connect the GPEC Diagnostic 10436 adapter to the engine harness side of the PCM C2 connector, but do not connect the adapter to the PCM. Do not probe the PCM harness connectors; probing them can damage the PCM terminals and create a poor terminal-to-pin connection. With the adapter in place, check the upstream oxygen sensor circuits between the oxygen sensor connector and the adapter for a short to ground, and for an open or high resistance. If a circuit problem is found, repair the open, high resistance, or short to ground, then verify the repair. If the circuits check good, continue to oxygen sensor operation.
Check oxygen sensor operation
Check oxygen sensor operation using the oxygen sensor operation test for this path. If that test finds a problem, repair the oxygen sensor related issue and verify the repair. If oxygen sensor operation checks out, the next step is a connector and terminal inspection through the related harness connections.
Inspect related harness connections and retest
Disconnect all PCM harness connectors, any related in-line harness connectors if equipped, and the related component connectors. Inspect the harness connectors, component connectors, and male and female terminals for correct connector installation, damaged locks, corrosion, signs of water intrusion, weather seal damage if equipped, bent terminals, overheated or discolored terminals from poor connection, and terminals pushed back in the connector cavity. Check for spread terminals and make sure terminal tension is correct. Repair any conditions found. Then reconnect the PCM connectors, in-line connectors if equipped, and related component connectors, making sure every connector is fully seated and every lock is fully engaged. Clear the codes with the scan tool, operate or test drive the vehicle under the monitor conditions, and read codes again.
Final decision and repair verification
If P0171 returns after the connector and terminal inspection or repair, the next action in this path is to replace and program the PCM, then verify the repair. If the code does not return, the wiring or poor connection problem has been corrected, and the final step is to verify the repair and confirm the code stays gone. The takeaway is simple: on this lean code, prove closed loop operation, rule out related fuel and air handling faults, then work through MAP input, mechanical air and fuel issues, exhaust leakage, oxygen sensor wiring, oxygen sensor operation, and connector integrity before condemning the PCM. For more diagnostic training, visit stepdiagnostics.com.
Final check
P0171 often requires checking whether the lean calculation may be related to air, fuel, exhaust feedback, sensor input, wiring, or mechanical conditions before making a final repair decision.
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