Common P-Codes Explained

What does P0300 mean and where do I start?

P0300 means the computer detected random or multiple-cylinder misfires; start by checking for cylinder-specific misfire codes and counts, then look for causes common to all cylinders.

How do I diagnose a single-cylinder misfire code like P0302?

Short answer: Swap the coil, then the plug, to the neighboring cylinder and see if the misfire follows; if it stays, test the injector, compression and wiring for that cylinder.

A cylinder-specific code narrows your search to ignition, fuel or mechanical problems on that cylinder. Start with the easy swap test. Move the coil from the misfiring cylinder to a neighbor, clear codes, and run the engine while watching misfire counters. If the misfire moves, the coil is the problem. Repeat with the spark plug, and inspect it for oil, carbon, cracks or a worn electrode.

If the misfire stays put, check the injector. A noid light confirms the injector is being pulsed, and a balance test or swap shows whether it is flowing correctly. On direct injection engines, follow the maker's procedure because injector work involves high fuel pressure.

Next, do a compression or leak-down test. A burned valve, broken ring or cam problem shows up here. Also check for an intake leak near that runner and for wiring damage at the coil and injector connectors.

What causes P0171 system too lean bank 1?

P0171 means the computer is adding more fuel than its limit to keep bank 1 at the right air-fuel ratio, usually from unmetered air, low fuel delivery or a skewed airflow reading.

What does P0420 mean and is it always a bad catalytic converter?

Short answer: P0420 means the downstream oxygen sensor shows the bank 1 catalyst is not storing oxygen as well as it should, but misfires, exhaust leaks, sensor problems or engine faults can cause it too.

The computer compares the upstream and downstream oxygen sensor signals. A working converter stores and releases oxygen, so the downstream signal stays fairly steady while the upstream one switches. When the downstream signal starts copying the upstream one, efficiency is judged low and P0420 sets.

Before condemning the converter, rule out other causes. Fix any misfire or fueling code first, because those can damage the converter and skew the test. Check for exhaust leaks ahead of or near the downstream sensor, since outside air can fool it. Confirm both oxygen sensors respond correctly, and check for software updates or service bulletins, since some manufacturers updated the monitor strategy.

If the converter is truly worn out, find out why. Oil consumption, coolant entering the combustion chamber, or long-term rich running can kill a new converter quickly. Replace with a converter that meets your state's emissions rules.

What causes P0128 coolant thermostat below regulating temperature?

Short answer: P0128 means the engine took too long to reach normal operating temperature, usually from a thermostat stuck open, but low coolant, a bad sensor or a cooling fan running constantly can also cause it.

The computer models how fast the engine should warm up based on ambient temperature, load and run time. If coolant temperature does not reach a set threshold within that window, it sets P0128.

The most common cause is a thermostat that is stuck partly open. Watch coolant temperature on a scan tool during a cold start. It should climb steadily and level off near the normal range. If it creeps up slowly, or drops at highway speed, suspect the thermostat. Also confirm the coolant level is full and the system is bled, because air pockets can give strange readings.

Rule out a cooling fan that runs all the time and a coolant temperature sensor that reads low. Compare the sensor's reading to an infrared or contact thermometer on the thermostat housing. Never open a hot cooling system; let it cool before removing the cap.

How do I find the leak behind a P0442 or P0455 EVAP code?

P0455 points to a large leak and P0442 to a small one in the fuel vapor system; check the gas cap and filler neck first, then use a smoke machine to find the leak.

What does P0401 insufficient EGR flow usually mean?

Short answer: P0401 means the computer did not see the expected change when it commanded EGR flow, most often from carbon-clogged passages, a failed EGR valve or a faulty position or pressure sensor.

The EGR system routes a measured amount of exhaust back into the intake to lower combustion temperatures and NOx. To test it, the computer opens the valve under certain conditions and watches for a change in manifold pressure, temperature, or a dedicated sensor. If the change is too small, P0401 sets.

Carbon buildup in the EGR passages or ports is very common, especially on engines with many miles of short trips. On some designs the passages clog while the valve itself is fine. Use the scan tool to command the valve open at idle; a working system usually makes the engine stumble or idle rough, because exhaust is diluting the intake charge.

Check vacuum supply on older vacuum-operated valves and the position sensor signal on electronic valves. On diesels, EGR coolers can also clog. Allow the exhaust to cool before you remove parts, since EGR components get very hot.

What does P0016 crankshaft and camshaft position correlation mean?

Short answer: P0016 means the computer sees the camshaft out of its expected relationship to the crankshaft on bank 1, which can come from a stretched timing chain, oil problems affecting variable valve timing, or sensor and wiring faults.

The engine computer compares cam and crank position signals constantly. When the relationship drifts outside the allowed window, it sets a correlation code. P0016 is bank 1 intake on many engines; related codes cover other banks and exhaust cams.

Start with the oil. Variable valve timing actuators run on engine oil pressure, so low level, the wrong viscosity, sludge or a clogged screen at the oil control valve can cause this code. Check service bulletins too, since some engines have known timing chain or actuator issues.

Next, use the scan tool to compare desired and actual cam position, and command the cam phaser if bi-directional controls support it. A scope with crank and cam signals on two channels shows the physical relationship directly and can reveal a stretched chain or jumped tooth without tearing the engine apart. Compare the pattern to a known-good reference for that engine.

What does P0101 mass airflow sensor range or performance mean?

P0101 means the airflow reading does not make sense compared with what the computer expects from engine speed, throttle and manifold pressure, often from a dirty sensor, an air leak or a restricted intake.

What causes P0335 crankshaft position sensor circuit?

Short answer: P0335 means the computer is not seeing a usable crank signal, from a failed sensor, damaged wiring, a bad connector, or a damaged reluctor ring.

The crankshaft position signal is the heartbeat of the engine management system. Without it, most engines will not start or will stall, and you may get no tachometer movement while cranking. The code means the signal is missing, erratic or out of range.

Check the obvious first: connector seated, pins clean and tight, harness not chafed against a pulley or exhaust. Many sensors sit low on the engine where oil, heat and road debris attack the wiring.

Then check the signal. On a two-wire magnetic sensor, the AC signal should grow with cranking speed. On a three-wire Hall-effect sensor, confirm reference voltage, ground and a clean square wave. A scope is the best tool here, because a meter cannot show missing teeth, dropouts or noise. If the signal is damaged but the sensor and wiring test good, inspect the reluctor ring and its air gap.

What does P0507 idle control system rpm higher than expected mean?

Short answer: P0507 means idle speed is higher than the computer is commanding, most often from a vacuum leak, a dirty or unlearned throttle body, or a sticking idle control valve.

The computer sets a target idle and adjusts airflow to reach it. If idle stays above target and the computer cannot pull it down, P0507 sets. Extra air getting into the engine is the usual reason.

Look for vacuum leaks first, since air that bypasses the throttle raises idle. A smoke test of the intake finds cracked hoses, leaking gaskets and a stuck-open PCV valve quickly. Then inspect the throttle body. Carbon buildup changes airflow at the throttle plate, and the computer learns around it. After cleaning, many electronic throttles need an idle or throttle relearn, which your scan tool can run.

On older engines with an idle air control valve, check that it moves freely and responds to commands. Also confirm no aftermarket intake or air filter changes are affecting the reading. Check coolant temperature data too: a sensor reading cold makes the computer raise idle on purpose.

What does P0700 transmission control system malfunction mean?

Short answer: P0700 is a flag in the engine computer saying the transmission computer has stored its own fault; read the transmission module directly to see the real code.

Generic code readers often show only P0700 because they read the engine computer. The code just tells you the transmission control module has requested the check engine light and holds one or more detailed codes of its own.

Use a scan tool that reads the transmission module and look at those codes, such as shift solenoid faults, pressure control problems, ratio errors or input and output speed sensor issues. Those point to the actual diagnosis.

Then check basics. Fluid level and condition matter a great deal; low or burned fluid causes many transmission codes. Follow the maker's procedure for checking fluid, since many transmissions need a specific temperature and have no dipstick. Check for software updates, since many transmissions receive updated shift logic. After repairs or fluid service, some units need a relearn or adaptation reset that a scan tool performs.

What does P0135 oxygen sensor heater circuit mean?

Short answer: P0135 means the heater inside the bank 1 upstream oxygen sensor is not drawing the expected current or is out of range, usually from a failed heater element, a blown fuse or a wiring fault.

Oxygen sensors only work once they are hot, so they have a built-in heater that brings them up to temperature quickly after a cold start. The computer monitors the heater circuit and sets a code when current or resistance is wrong.

Check the fuse for the heater circuit first, since one failed heater can blow a shared fuse and set codes on several sensors. Then unplug the sensor and measure heater resistance across the heater terminals; compare with the specification in service information. An open reading means the element has failed.

If resistance is good, check for battery voltage on the heater supply with the key on and verify the computer's control side. Look closely at the harness near the exhaust for melted or chafed wiring. Let the exhaust cool before working near it, and use an oxygen sensor socket to avoid crushing the sensor or its wiring.

What does P0299 turbocharger underboost mean?

Short answer: P0299 means the engine is not reaching the boost pressure the computer requested, most commonly from boost leaks, a sticking wastegate or variable vane mechanism, or a sensor problem.

The computer commands a boost target and watches the boost or manifold pressure sensor. If actual boost stays below target for a set time, P0299 sets and the engine may go into reduced power.

Boost leaks are the most common cause. Cracked intercooler hoses, loose clamps and leaking intercoolers let pressurized air escape. Pressure testing the intake tract with a turbo system tester or a smoke machine adapter finds them quickly. Listen for hissing under load and look for oily residue at joints.

Next, check the wastegate or variable geometry mechanism. Many scan tools can command the actuator so you can watch it move. Carbon can stick variable vanes on diesels. Also verify the boost sensor reads correctly by comparing key-on engine-off readings with barometric pressure. Turbochargers and exhaust parts get extremely hot, so let them cool before handling.

What does P0087 fuel rail pressure too low mean?

Short answer: P0087 means measured fuel rail pressure is below the target, from a weak supply pump, a restricted filter, a failing high-pressure pump, a leaking regulator or a bad pressure sensor.

On port injection systems the code points toward supply pressure, while on direct injection and diesel engines it usually refers to the high-pressure rail. In both cases, the computer compares desired rail pressure with the sensor reading.

Start with the scan tool. Compare desired and actual pressure at idle and during a snap throttle. Many direct injection systems also report low-side pressure, which helps you decide whether the problem is the in-tank pump or the high-pressure pump. Check for restricted filters and for service bulletins.

Confirm the supply side with a mechanical fuel pressure gauge if possible. High-pressure direct injection and diesel rails run at pressures that can cause serious injury, so never open those lines with the engine running or before following the maker's depressurizing procedure. Wear eye protection, keep ignition sources away, and use gauges and adapters rated for the system.