Fuel and Air Metering Codes

What does P0172 system too rich bank 1 mean?

The engine computer has reduced fuel trim as far as it can on bank 1 and the mixture is still rich, which points to excess fuel or a sensor reporting less air than is actually entering the engine.

What causes P0174 system too lean bank 2?

Short answer: P0174 means bank 2 is running lean even with maximum positive fuel trim; common causes are vacuum leaks on that bank, intake gasket leaks, weak fuel delivery, or an airflow reading that is too low.

On V-type engines, bank 2 is the side opposite cylinder 1. If only bank 2 is lean, look for a problem that affects that side, such as an intake manifold gasket leak, a cracked vacuum hose or a leaking PCV connection near that bank, or restricted injectors on that side.

If P0171 and P0174 set together, the cause usually affects the whole engine, such as low fuel pressure, a mass airflow sensor reading low, or a large vacuum leak at the throttle body or brake booster.

Use a smoke machine to find intake leaks, and compare fuel trims at idle and at 2500 rpm. Trims that improve at higher engine speed suggest a vacuum leak, while trims that worsen under load suggest a fuel delivery problem. Check fuel pressure and volume, and wear eye protection when connecting a fuel pressure gauge.

What does P0106 MAP sensor range or performance mean?

Short answer: The engine computer saw a manifold absolute pressure reading that does not make sense compared with other inputs, such as throttle position, engine speed or barometric pressure.

The MAP sensor measures intake manifold pressure, which changes with engine load. With the key on and engine off, MAP should read close to barometric pressure. At idle, it should show vacuum, and it should rise quickly as the throttle opens.

P0106 is a rationality code, meaning the reading is within electrical limits but does not match expected values. Causes include a cracked or disconnected vacuum hose to the sensor, a plugged port, intake leaks, engine mechanical problems that reduce vacuum, or a sensor that responds slowly.

Compare key-on engine-off MAP to the baro reading, then check readings at idle and during a snap throttle. A vacuum gauge connected to the intake can confirm whether the engine really has normal vacuum. Check the 5 volt reference and ground at the sensor before replacing it, and inspect the connector for corrosion.

What does P0113 intake air temperature sensor circuit high mean?

Short answer: P0113 means the IAT signal voltage is high, which the engine computer reads as extremely cold air; it is usually an open circuit, a disconnected sensor or a failed sensor.

Most intake air temperature sensors are thermistors whose resistance drops as temperature rises. The engine computer supplies a reference voltage through a resistor and reads the voltage across the sensor. An open circuit makes the voltage rise to near reference, which looks like very cold air.

Check the scan tool reading first. A value at the bottom of the range, often a very low temperature, with the engine warm suggests an open. Unplug the sensor and check the connector, then measure resistance and compare it with the chart in service information at known temperatures.

On many vehicles the IAT sensor is built into the mass airflow sensor, so check the wiring and connector at that assembly. Jumper the signal to ground briefly with a fused jumper and see if the reading changes to a very high temperature, which confirms the wiring and computer input are working.

What does P0122 throttle position sensor circuit low mean?

Short answer: The throttle position sensor signal voltage dropped below its expected minimum, usually caused by an open reference circuit, a shorted signal wire, a bad connector or a failed sensor.

Throttle position sensors typically receive a 5 volt reference and ground and return a signal that changes with throttle angle. A signal near zero volts is outside the normal range and sets the low code.

On electronic throttle bodies, the code may affect throttle control, so the engine may go into a reduced power mode. Check for related codes, including pedal position and reference voltage codes, which can point to a shared reference circuit problem.

With the key on, back-probe the connector and check the reference, ground and signal. Wiggle the harness while watching the signal on a scope or graphing meter, since intermittent opens show up as dropouts. Inspect for damaged pins and water intrusion. Electronic throttle bodies may need a relearn after replacement. Keep fingers out of the throttle bore when the key is on, since the blade can move without warning.

What does P2135 throttle position sensor A/B correlation mean?

Short answer: The engine computer compared the two throttle position signals and found they disagree more than allowed, which can be caused by a failing throttle body sensor, connector or wiring problems, or a reference voltage issue.

Electronic throttle bodies use two position sensors that change in a known relationship. The engine computer compares them constantly. If they disagree, it cannot be sure where the throttle is, so it sets P2135 and usually limits power.

Common causes include worn sensor tracks inside the throttle body, corrosion or loose pins at the connector, chafed wiring and shared reference circuit faults. Contaminated throttle bodies can sometimes contribute.

Graph both sensor signals while slowly opening the throttle, with the engine off and key on if the procedure allows. Look for dropouts or a sudden step in one signal. Check the connector carefully and do a wiggle test on the harness. If the throttle body is replaced or cleaned, perform the throttle relearn in service information. Keep hands clear of the throttle plate when the system is powered.

What does P0234 turbocharger overboost mean?

Short answer: P0234 means boost pressure exceeded the commanded or maximum value, often due to a sticking wastegate or actuator, a faulty boost control solenoid, a blocked control hose, or a bad boost pressure sensor.

The engine computer controls boost by opening the wastegate or adjusting variable geometry vanes. When actual boost rises above the target for a set time, it sets an overboost code and may limit power to protect the engine.

Look at live data for commanded versus actual boost and the wastegate or vane position command. A wastegate that sticks closed, a seized actuator rod, a split vacuum or pressure hose or a failed control solenoid are common. On variable geometry turbos, soot buildup can cause the vanes to stick.

Verify the boost sensor reading at key-on engine-off against baro. Use a scan tool to command the wastegate actuator if supported and watch it move. Check the control hoses for cracks. Let the turbo and exhaust cool before working near them, since they stay hot enough to cause serious burns.

What does P0068 MAP/MAF to throttle position correlation mean?

Short answer: The engine computer saw airflow or manifold pressure readings that do not match what the throttle position says should be happening, often due to vacuum leaks, a dirty or faulty airflow sensor, or throttle body problems.

The computer predicts airflow from throttle angle and engine speed. If the measured airflow or manifold pressure is much higher or lower than expected, it sets P0068. On electronic throttle systems this can trigger reduced power mode.

A large intake leak after the throttle body lets in unmeasured air, which raises manifold pressure at a given throttle angle. A contaminated mass airflow sensor can under-report. A dirty throttle body can make the throttle position and actual airflow drift apart.

Check for intake leaks with a smoke machine, especially at the intake boot, PCV system and brake booster hose. Compare MAF readings at idle with specifications. Clean the throttle body if service information allows, and perform the throttle relearn afterward. Check related codes and bulletins, since some vehicles have known software fixes for this code.

What does P0191 fuel rail pressure sensor range or performance mean?

Short answer: The fuel rail pressure reading did not match what the engine computer expected for the conditions, which can be caused by a faulty sensor, wiring problems, or a real fuel pressure problem from a pump or regulator.

Many engines, especially direct injection and diesel engines, use a fuel rail pressure sensor so the computer can control fuel pressure precisely. P0191 means the reading is out of the expected range or changes in a way that does not make sense.

Start by checking live data. Compare desired and actual rail pressure at idle and during a snap throttle. Check the sensor's reference voltage, ground and signal. A reading that is fixed or jumps unrealistically suggests a sensor or circuit problem. If the reading tracks smoothly but is wrong, the fuel system may have a real pressure problem.

Direct injection systems operate at very high pressure. Do not open the high-pressure side or use a standard fuel gauge on it. Follow the service procedure to test the low-pressure side and to depressurize safely before replacing parts.

What does P0175 system too rich bank 2 mean?

Short answer: P0175 means bank 2 is running rich and fuel trim cannot pull out enough fuel, which points to leaking injectors, excess fuel pressure, purge flow or a sensor reading on that side.

When only one bank runs rich, the cause is often on that bank: a leaking injector, an upstream oxygen sensor that reads incorrectly, or an exhaust leak ahead of the sensor. An exhaust leak can pull in outside air, making the sensor report lean so the computer adds fuel and the bank actually runs rich.

If both banks show rich codes, look at causes shared by the engine, such as high fuel pressure, a failing mass airflow sensor, a purge valve stuck open or fuel diluted oil.

Compare fuel trims between banks, check the upstream oxygen sensor response, and inspect for exhaust leaks ahead of the sensor. Do an injector balance test if the scan tool supports it. Keep the area ventilated and free of ignition sources when testing fuel system components, and wear eye protection.

What does P0627 fuel pump control circuit open mean?

Short answer: The engine computer detected an open in the circuit that controls the fuel pump or fuel pump control module, often from a connector, wiring, relay or module problem.

Many vehicles control the fuel pump through a module or relay commanded by the engine computer. P0627 indicates the control circuit is open, so the pump may not run, causing a no-start or stalling.

Check fuses and relays first, then check for power and ground at the fuel pump control module and the command signal from the engine computer. Look for corrosion at connectors near the tank, which are exposed to water and road salt.

Use a scan tool to command the pump on if possible and listen for it. A current probe on the pump feed can show whether the pump runs and how it draws current. Relieve fuel pressure before disconnecting fuel lines, and never create sparks near the fuel tank.

What does P0505 idle air control system mean?

Short answer: P0505 means the idle speed is not matching the target or the idle control circuit has a problem; causes include a dirty throttle body, vacuum leaks or a faulty idle control valve.

P0505 sets when the engine module cannot control idle speed as expected. On older vehicles with an idle air control valve, the valve or its circuit may be faulty. On electronic throttle vehicles, a dirty throttle body or vacuum leak can cause similar problems.

Symptoms include high, low or surging idle and stalling.

Check for vacuum leaks, inspect and clean the throttle body if recommended, and test the idle control circuit with a scope or meter. Perform any idle or throttle relearn required after cleaning. Keep hands clear of moving parts with the engine running, and follow service information.

What does P0102 mass airflow sensor circuit low mean?

Short answer: P0102 means the MAF sensor signal is lower than expected, often from a wiring problem, a faulty sensor or an air leak after the sensor; check power, ground and signal first.

The mass airflow sensor measures incoming air. P0102 sets when the signal is below the expected range.

Common causes include an open or shorted signal wire, a missing power or ground, a damaged connector, or a failed sensor. A large air leak between the sensor and throttle can also lower the reading.

Check live data, then test power, ground and signal at the connector with a meter or scope. Inspect intake ducts for cracks or loose clamps. Clean or replace the sensor only after confirming the circuit is good. Keep hands clear of moving parts with the engine running.

What does P2187 system too lean at idle bank 1 mean?

Short answer: P2187 means fuel trims show a lean condition at idle on bank 1, often caused by vacuum leaks, PCV problems or a dirty MAF sensor; the condition may improve at higher speeds.

At idle, small vacuum leaks have a larger effect on air-fuel ratio because airflow is low. P2187 sets when fuel trims show the engine is adding fuel at idle to correct a lean condition.

Common causes include vacuum leaks, PCV valve or hose leaks, intake gasket leaks and MAF sensor problems. Fuel delivery problems can also contribute.

Check fuel trims at idle and at higher rpm. If trims improve with rpm, look for vacuum leaks with a smoke machine. Inspect PCV components and intake gaskets. Keep hands clear of moving parts and fuel components with the engine running.