What this code means
P0138 may indicate that an oxygen sensor signal is staying higher than expected for the circuit being monitored.
What the vehicle may do
- The MIL may be illuminated.
- The vehicle may seem to run normally.
- The vehicle can have emissions-related or fuel-control symptoms depending on the underlying issue.
Possible fault areas
- Possible oxygen sensor signal circuit concern.
- Possible wiring or connector issue.
- Possible ground or heater-related concern.
- Possible exhaust or fuel-control condition affecting sensor feedback.
- Possible PCM interpretation or monitoring concern.
Diagnostic path
Opening and code context
For this Ram, the target code is P0138. In general diagnostic terms, P0138 may point toward a high voltage signal concern in an oxygen sensor circuit. The truck may only have the MIL on, or it may show driveability or emissions-related symptoms depending on what is actually happening. Possible fault areas can include the sensor signal circuit, wiring or connector problems, ground or heater-related issues, exhaust or fuel-control conditions, or the PCM’s interpretation of the signal. One important pause before testing: the code label and the test path here do not line up cleanly. The step-by-step path is for an EVAP purge performance problem using the fuel tank pressure sensor and purge flow monitor. Confirm the exact DTC path before applying these EVAP tests to a P0138 complaint.
What this EVAP path is trying to prove
If the correct lookup does point you into this purge-performance path, understand the logic before chasing parts. The purge flow monitor watches the Fuel Tank Pressure Sensor and looks for tank vacuum to increase as purge flow increases, then decrease as purge flow is reduced. For purge performance diagnostics, the Fuel Tank Pressure Sensor is the key feedback input; the ESIM switch is not used to diagnose purge performance itself. Broadly, this path is separating a 5-Volt supply problem, a vacuum leak or restriction, a restricted purge or canister line, a purge solenoid problem, or a fuel tank pressure sensor problem. Where the purge solenoid hand-vacuum check is used, the amount called out is to apply approximately 10 - 15 in. Hg.
Monitor gates before running the test
Before relying on the purge-flow result, make sure the operating window makes sense. This monitor is gated by Battery voltage above 11.0 volts, Fuel level is between 12% and 88%, Ambient temperature is between 4°C (39.2°F) and 35°C (95°F), and Engine run time since start is less than 30 minutes. The fault logic is based on the PCM seeing too little change in fuel tank pressure during the purge ramp-up or ramp-down portions of the test.
Start with other codes
First, turn the ignition on, read the codes with the scan tool, and record them. If there are active or pending purge solenoid control circuit codes, or small or large EVAP leak codes, handle the applicable diagnostic path first. If none of those are active or pending, continue with the purge-flow routine.
Run the forced monitor and choose the branch
Next, check for an active purge-performance condition. With the ignition on, vent any pressure or vacuum in the fuel tank before starting the ESIM/Purge Flow Forced Monitor. Open the capless fuel filler with the vent, or remove and reinstall the fuel cap, so the tank pressure is closer to atmosphere and the routine gives a cleaner result. Then run the forced monitor with the scan tool and match the result to the condition you see. If all the monitors pass and the pressure changes are well beyond the pass thresholds, the purge system is passing at that time. If the passes are close to threshold, think about a possible partial restriction in the intake manifold vacuum port or manifold hose. The example shown uses a -50 Pa Pass Threshold: a pressure decrease of -20 Pa fails, and a pressure decrease of -60 Pa passes. If side 1 passes but side 2 fails with very high vacuum, well below approximately -500 Pa, and the vacuum holds or bleeds off very slowly after purge is turned off, look at the fresh-air inlet filter or hose restriction; if removing that restriction path does not bring vacuum back toward approximately -500 Pa, the ESIM internal vacuum vent valve may be stuck closed. If side 1 or side 2 fails and the fuel tank pressure reading changes little or not at all, use the large-leak result to decide whether to inspect the canister and tube side first, or continue to the fuel tank pressure sensor check. If the ESIM switch does not close, or the purge solenoid is suspect, skip ahead to the intake manifold vacuum-port check.
Check the canister-to-FTP sensor path
If the forced monitor points you toward the canister-to-fuel-tank-pressure side, turn the ignition off, remove the recirculation tube from the Fuel Tank Pressure Sensor, and connect a hand vacuum pump to the sensor. Watch the FTP Sensor reading on the scan tool while you apply vacuum. If the FTP value changes, the sensor is responding and the restriction is in the canister tube, recirculation tube, or multi-function control valve area. Repair that restriction, then run the ESIM/Purge Flow Forced Monitor again to confirm the system is repaired. If the FTP value does not change, continue to the 5-Volt supply test.
Check the FTP Sensor 5-Volt supply
Now check the Fuel Tank Pressure Sensor supply circuit. With the ignition off, disconnect the FTP Sensor harness connector. Turn the ignition on and measure the 5-Volt Supply circuit at the FTP Sensor connector. The voltage should be between 4.9 to 5.1 volts. If it is in that range, check for good pin-to-terminal contact at the sensor connector. If the connector checks out and no terminal issue is found, replacement of the Fuel Tank Pressure Sensor is the supported next move in this path, followed by another ESIM/Purge Flow Forced Monitor to confirm the repair. If the voltage is not between 4.9 to 5.1 volts, repair the 5-Volt Supply circuit for an open or high resistance, then rerun the forced monitor. Keep in mind, if that 5-Volt supply is lost, the sensor signal can stick mid-range, and the PCM may not see the FTP signal change it expects during purge operation.
Check the vacuum supply side
If the path sends you to the vacuum side, start at the intake manifold port. Remove the manifold hose vacuum line at the intake manifold vacuum port and inspect inside the port for restriction or blockage from flashing or carbon buildup. If the port is restricted, repair the restriction and rerun the forced monitor. If the port is clear, reinstall the manifold hose at the intake manifold, remove the manifold hose at the purge solenoid inlet, start the engine, let it idle, and check whether sufficient engine vacuum is reaching the purge solenoid. If vacuum is not sufficient, repair the broken or plugged manifold hose, then verify with the forced monitor. If vacuum is sufficient at the inlet, turn the ignition off, reconnect the inlet hose, remove the outlet line that leads toward the evaporative canister, connect a vacuum gauge to the purge solenoid outlet, and start the engine. Let it idle long enough for the purge solenoid to cycle on, then check for sufficient vacuum at the outlet. If outlet vacuum is present, inspect the purge tube for damage, including broken, disconnected, or plugged sections, then rerun the monitor. If outlet vacuum is not present and the connectors check out, replacement of the purge solenoid is supported by this path, followed by the forced monitor to confirm the repair. A restriction can still reduce purge flow even when a vacuum reading is present, so inspect the manifold port and manifold hose closely instead of judging by vacuum alone.
Takeaway and CTA
Takeaway: do not force an EVAP purge performance routine onto P0138 unless the code lookup confirms that this is the correct path for the vehicle. If it does, follow the purge-flow result, verify the FTP sensor response and 5-Volt supply, then work the vacuum path in order and confirm the repair with the forced monitor. For more diagnostic training, visit stepdiagnostics.com.
Final check
The supplied diagnostic path does not cleanly support a P0138 oxygen sensor diagnosis, so the correct action is to confirm the proper code-specific diagnostic path before testing.
For more guided automotive diagnostics, visit STEP Diagnostics.
