P0174: P0174 Diagnostic Guide

What this code means

P0174 means the PCM may see Bank 2 running too lean and may add fuel correction to compensate.

What the vehicle may do

The vehicle may have a check engine light.
It may idle rough, hesitate, or feel weak under some conditions.
It can also run normally while the code is stored.

Possible fault areas

Possible unmetered air, intake, vacuum, PCV, EVAP purge, or exhaust leak concerns.
Possible fuel delivery, injector, fuel pressure, or fuel quality concerns.
Possible oxygen sensor circuit, MAF sensor, wiring, connector, or PCM-related concerns.

Diagnostic path

Open and set up the diagnostic path

On a 2015 to 2025 F-150 with the 2.7L EcoBoost (238kW/324PS), P0174 means the PCM may see Bank 2 running too lean, so fuel correction may be driven rich trying to compensate. The truck may have a check engine light, rough idle, hesitation, weak feel, or it may run fairly normal with the code stored. Broadly, this can come from unmetered air, vacuum or intake leaks, PCV or EVAP purge issues, fuel delivery problems, injector concerns, fuel quality, oxygen sensor circuit issues, MAF or connector problems, and in some cases PCM-related concerns. Start with the basic system checks, then check for stored codes. If other codes are present, check what they mean first. Do not clear the codes or reset KAM yet. Record the ECT or ECT1 value from the captured conditions, because the goal is to reproduce the operating conditions that set the code.

Do the visual inspection before chasing data

With the ignition off, make the first real pass a careful visual inspection. Check the air filter and housing for restriction. Inspect the intake tract from the MAF sensor to the intake manifold for leaks, damaged tubes, loose connections, hose problems, restrictions, or other damage. Look at the throttle plate for sludge or obstruction. Check vacuum hoses and tees for correct routing, cracks, and damage. Inspect EVAP lines, the intake manifold and gasket area, the EGR system for leaks, and the PCV system integrity and correct PCV valve part number if equipped. Also look for exhaust leaks at flanges and gaskets, inspect the oxygen sensor wiring and pins for damage or contamination, and make sure the dipstick, dipstick tube, and oil fill cap are seated correctly. If the concern is clearly found here, handle that concern and then verify it. If it is specifically a vacuum leak concern, move to the vacuum leak repair verification path.

Check injector balance and purge behavior

If the visual inspection does not show the cause, run the relative injector flow test using the scan tool process. If that test is not available, continue down the passing path. A failed result points to the injector in question; after installing the new fuel injector, reset KAM, clear PCM codes, and repeat the self-test. Next run the power balance test the same way. If it fails, install the applicable fuel injector for that fuel injection type, reset KAM, clear PCM codes, and repeat the self-test. If power balance passes, check the EVAP system for a stuck open purge valve. Use the scan tool purge check if available. If it is not available, record the FTP value, start the engine, command EVAPCP to 0%, command EVAPCV ON at 100% duty cycle, and watch whether the FTP value stays within 0.5V of the recorded value within 30 seconds after closing the purge valve. If it does not, install a new EVAP canister purge valve, clear PCM codes, and repeat the self-test.

Use fuel trims to separate vacuum leaks from fuel delivery

For P0174, if the purge check passes, use fuel trims to decide whether this is acting like a vacuum leak. Use normal hot-engine safety while working around the engine. Start the engine and monitor CHT, ECT, ECT1, IAT, LONGFT1, SHRTFT1, LONGFT2, and SHRTFT2. Stabilize the engine at the captured temperature if you have it. If you do not, keep coolant temperature between 82°C - 101°C (180°F - 215°F) and intake air temperature less than 46°C (115°F). Add long term and short term fuel trim together for each bank at idle. Then raise engine speed to 3,500 RPM for 10 seconds and record the same trims again. If the difference in total fuel correction between idle and 3,500 RPM is less than 15 percent of the idle total, continue toward fuel pressure testing. If it is not, go after a vacuum leak. That makes sense because trims at idle are more sensitive to unmetered air; the same leak is a bigger share of total airflow at idle than it is off idle. The next check shown in the path is MAP voltage behavior: with the ignition on, monitor MAP_V and record it, then start the engine, record MAP_V again, and make sure the PID value changes. If it does not change, shift into the MAP diagnostic path before continuing.

Locate and verify a vacuum leak

When the trim comparison points toward a vacuum leak, locate the intake air and PCV vacuum tees and monitor SHRTFT1 and SHRTFT2 with the engine running. Restrict vacuum lines one at a time for 30 seconds. Do not clamp or pinch a hard plastic hose; use a vacuum cap or equivalent. If a leaking circuit is being restricted, short term fuel trim should drop. A decrease greater than 15 percent identifies the leak path. If none of the hoses produce that drop, inspect the intake air system, intake manifold, and intake gaskets for a vacuum leak and repair as necessary. After the vacuum leak repair, keep verification separate: run the engine at the temperature needed to recreate the concern, record SHRTFT1 and SHRTFT2, shut the engine off, make the repair, then restart and stabilize it again. Compare before and after. A decrease greater than 15 percent confirms the vacuum leak repair; then reset KAM and repeat the self-test. If the decrease is not there, a vacuum leak is still present, so go back through the leak search instead of guessing.

Check oxygen sensor circuit integrity, fuel pressure, and FRP bias

Some branches in this fuel-control path check the universal oxygen sensor pump-current circuit before moving into fuel pressure. Only diagnose the suspect universal HO2S. With the ignition off, disconnect the Universal HO2S connector and the PCM connector. On the 2.7L EcoBoost (238kW/324PS), measure resistance between C172 Pin 3 and C1232T Pin 57, and between C171 Pin 3 and C1232T Pin 59. Resistance should be less than 5 Ω. If it is not, repair the open circuit, reset KAM, and repeat the self-test. For the P0174 path that moves into fuel pressure testing, use fuel system safety precautions because the system may still be pressurized after the engine is switched off. Relieve fuel pressure, connect a mechanical fuel pressure gauge, pressurize the system, start the engine, and let pressure stabilize. Then run the fuel pump with the FP command to obtain maximum fuel pressure. If fuel pressure is not within range for the vehicle, stop and diagnose the fuel pressure system before continuing. If it is within range on this 2.7L EcoBoost, check for a biased FRP sensor. With the ignition off, disconnect the Fuel Injection Pump connector, start the engine, monitor FRP in kPa, and command FUEL_MASS_DI to the maximum value. The FRP PID should be between 345-827 kPa (50-120 PSI ). If it is not, install a new Fuel Rail Pressure sensor, clear PCM codes, and repeat the self-test. Where the path uses the FRP-to-FLP comparison, monitor FRP and FLP together and make sure the FRP PID is within 138 kPa (20 PSI ) of the FLP value. If it is not, install a new Fuel Rail Pressure sensor, clear PCM codes, and repeat the self-test.

Check pressure stability, external leakage, and electrical noise when that branch applies

Next, reconnect the Fuel Injection Pump connector and check fuel pressure stability. Turn the ignition on, run the fuel pump to maximum pressure, command the pump off, let the pressure stabilize, and record that stabilized reading. For the fast leakdown check, pressure should stay within 34 kPa (5 psi) of the recorded reading after 10 seconds. If it does not, inspect the fuel tank, lines, and filler pipe for an external fuel leak. If a leak is found, repair as necessary, reset KAM, and repeat the self-test. If no external leak is found, the diagnostic path leaves this fuel-control routine and moves into deeper fuel system testing. If the fast leakdown check passes, continue watching pressure for the slow leakdown check. After 1 minute, pressure should remain within 34 kPa (5 psi) of the recorded reading for MRFS, or greater than 275 kPa (40 psi) for ERFS. If it does not, continue with deeper fuel system testing before replacing unrelated parts. On the branch that checks electrical noise, turn the ignition off and inspect the CKP sensor harness routing, alterations, shielding, and possible interference from other systems. Also consider radio frequency interference and excessive audible generator noise. If a concern is found, repair as necessary, clear PCM codes, and repeat the self-test. If no concern is found, pause and avoid forcing a repair that is not proven.

Evaluate fuel quality and ethanol learning

If fuel pressure stability passes, check the fuel for ethanol, water, and gasoline separation. Use a 200 ml beaker with 5 ml of clean water. Drain 22 ml of fuel into an approved clean container, pour 20 ml of that fuel into a 25 ml graduated cylinder, then add water to bring the total volume to 24 ml. Stopper the cylinder, shake it, and let it separate for approximately 3 minutes. Record the separation level where the ethanol and water mixture meets the gasoline. If the fuel does not separate, it is either 100% ethanol or an ethanol and water mixture; replace the contaminated fuel, clear PCM codes, and repeat the self-test. If it does separate, subtract the amount of water added from the recorded separation level, then multiply the result by 5 to calculate ethanol percentage. On a flex fuel vehicle, compare the FF_INF PID to that calculated ethanol percentage. It should be within 20% of the calculated value. If it is not, reset KAM, start the engine, monitor FF_LRND, and drive approximately 11.3 km (7 miles) or until FF_LRND indicates yes. Then cycle the ignition and compare FF_INF again. If it now matches within 20%, return the vehicle and advise correct fueling practices. If it does not, continue with fuel system testing. On a non-flex fuel vehicle, if calculated ethanol is not less than 25%, repair as necessary, advise the customer on the correct fuel type, reset KAM, and repeat the self-test. Certain fueling habits, like only adding small amounts or repeatedly switching between gasoline and an ethanol blend greater than E15, can keep the PCM from learning ethanol content correctly.

Check oxygen sensor condition, MAF response, and sensor activity

If the oxygen sensor side of the path is reached, turn the ignition off, disconnect the Universal HO2S connector, and inspect for pinched, shorted, or corroded wiring and pins, oil or water contamination, crossed sensor wires, and a contaminated or damaged sensor. If a concern is present, repair as necessary, clear PCM codes, and repeat the self-test. If no concern is found and the next step is not clear, pause, recheck the earlier diagnostic path, and avoid guessing. Next, check MAF functionality. Start the engine and monitor RPM, MAF_V, MAF_HZ, and MAF PIDs. Run the engine up to 1,500 RPM for 5 seconds, return to idle, and repeat. At idle, the PID value should be within 30% of the normal PID value. If it is not, move into MAF and IAT diagnosis before continuing. Then connect the Universal HO2S connector and check sensor activity in closed loop. Monitor FUELSYS_CL, RPM, EQ_RAT11, and EQ_RAT21. Increase engine speed to 2.000 RPM for 1 second, then return to idle. The equivalence ratio should switch from greater than 1 to less than 1, or from less than 1 to greater than 1. If it switches, the concern may not be duplicating at that time. If it does not, continue on the branch for the code group being tested. On the rich-condition branch, check for fuel leakage into the crankcase by recording fuel trims at normal operating temperature, temporarily plugging the intake manifold port after disconnecting the PCV line, then comparing the current trims to the recorded trims. If the trims shift and the rich condition is no longer present, install a new Fuel Injection Pump, clear PCM codes, and repeat the self-test. If not, continue to the universal HO2S operation check.

Use the HO2S electrical checks when sensor-response codes are present

If the path is dealing with oxygen sensor response codes along with the fuel-control concern, run the engine at approximately 2,000 RPM and hold it there for 3 minutes, then see whether the specific KOER oxygen sensor response codes are present. Do not read a long code list into the job; use the code information on the scan tool to identify the suspect sensor. If that branch is active, check the HO2S harness for shorts between circuits. With the ignition off, disconnect the PCM connector, disconnect the suspect HO2S12 or HO2S22 connector, and measure the specified sensor connector circuits for the 2.7L EcoBoost. The resistances should be greater than 10K ohms. If not, repair the short circuit, clear PCM codes, and repeat the self-test. If the short check passes, check the HO2S circuit for an open using the specified HO2S12 or HO2S22 connector-to-PCM and power-feed circuits. Those resistances should be less than 5 ohms. If not, repair the open circuit, clear PCM codes, and repeat the self-test. Then check for a short to voltage. With the ignition on, measure the specified HO2S12 or HO2S22 signal circuits and determine whether any voltage is present. If voltage is present, repair the short circuit, clear PCM codes, and repeat the self-test. If no voltage is present, reconnect the PCM and HO2S connectors, start the engine, let it idle for 2 minutes, monitor the O2S12 or O2S22 PID, and record the value. The HO2S PID voltage should be less than 1.5 V. If it is, the concern may not be duplicating at that time. If it is not, continue to the HO2S operation check.

Final connector checks and repair verification

For the final module checks, do not jump straight to a controller or sensor. First disconnect the PCM connectors, inspect for pushed out pins and corrosion, then reconnect the connectors and make sure they seat correctly. If the PCM concern is still present after that check, the path supports installing a new Powertrain Control Module and programming it correctly. If the concern is gone, the system is operating correctly at that time and the issue may have been a loose or corroded connector. For the universal HO2S check, disconnect the PCM connectors and the universal HO2S11 and HO2S21 connectors, inspect for pushed out pins and corrosion, reconnect everything securely, reset KAM, and verify whether the concern is still present. If it is still present, install a new Universal HO2S, reset KAM, and repeat the self-test. For the HO2S12 and HO2S22 check, do the same connector inspection and reseating process, reset KAM, and verify whether the concern is still present. If it is still present, install a new HO2S, reset KAM, and repeat the self-test. The main takeaway with P0174 is to keep the order clean: confirm the code context, inspect the air path, prove injector and purge behavior, use trims to separate vacuum leakage from fuel delivery, then verify pressure stability, fuel quality, and sensor circuits only where the path takes you. For more diagnostic training, visit stepdiagnostics.com.