Ignition, Oxygen Sensor and Emissions Codes

What does P0430 catalyst efficiency below threshold bank 2 mean?

The engine computer compared the upstream and downstream oxygen sensors on bank 2 and decided the catalytic converter is not storing oxygen as well as it should; the converter may be worn, but sensors, leaks or engine problems can cause it too.

What does P0351 ignition coil A primary or secondary circuit mean?

Short answer: The engine computer detected a fault in the control circuit for ignition coil A, usually cylinder 1, such as an open or shorted driver circuit, a bad connector or a failed coil.

Coil circuit codes are electrical codes rather than misfire codes, though a misfire often comes with them. The computer monitors the coil driver circuit and sets the code when it sees a problem with current flow or feedback.

Check the coil connector for corrosion, a broken lock or oil contamination. Back-probe the power feed and check it with the engine cranking, then check the control circuit. A scope or current probe on the coil feed shows whether the coil charges normally and compares well with other cylinders.

Swapping the coil with another cylinder is a quick test: if the code moves, the coil is the likely cause. If the code stays, look at wiring and the computer driver. Do not disconnect coils with the engine running, and keep hands away from ignition components while cranking, since secondary voltage can cause a painful shock.

What does P0133 oxygen sensor slow response mean?

Short answer: The upstream oxygen sensor on bank 1 is switching between rich and lean more slowly than the engine computer expects, often due to an aging or contaminated sensor, exhaust leaks or fuel control problems.

Upstream oxygen sensors switch rapidly when fuel control is working. Over time, contamination from oil, coolant or silicone, plus normal aging, slows the sensor's response. The computer measures switching time and sets P0133 when it is too slow.

Exhaust leaks ahead of the sensor, misfires and incorrect fuel pressure can also distort the signal. Check for these before replacing the sensor, especially if fuel trims are far from zero.

Graph the sensor while varying engine speed between idle and about 2500 rpm. A healthy narrowband sensor swings well above and below the midpoint quickly. A snap test, adding a little propane or creating a brief lean condition, can confirm response, though modern tools make graphing easier. If replacing the sensor, use the correct part and anti-seize only if the sensor does not come pre-coated, and allow the exhaust to cool to avoid burns.

What does P0138 oxygen sensor high voltage bank 1 sensor 2 mean?

The downstream oxygen sensor on bank 1 is reading higher voltage than expected for too long, which can indicate a rich condition, a shorted sensor or a wiring fault.

What does P0410 secondary air injection system mean?

The engine computer did not see the expected oxygen sensor response when it commanded the secondary air pump during cold start, pointing to a failed pump, stuck valve, blocked passages or wiring problems.

What does P0446 EVAP vent control circuit mean?

Short answer: The engine computer detected a problem with the EVAP vent valve or its circuit, often a sticking vent solenoid, a blocked vent line or filter, or a wiring problem near the fuel tank.

The vent valve opens the EVAP system to the atmosphere through a filter and closes it during leak tests. When the computer commands the vent closed or open and does not see the expected pressure change or circuit response, it sets P0446.

Vent valves often live near the fuel tank or charcoal canister, where they are exposed to dirt and water. Mud wasp nests and debris in the vent line are common. A canister saturated with fuel from overfilling can also restrict venting.

Use the scan tool to command the vent valve and listen or feel for it clicking. Check power and ground, and inspect the connector for corrosion. Use a smoke machine or hand pump to check that the vent line flows when open and seals when closed. Work on EVAP components with the engine off in a ventilated area away from sparks, since fuel vapors are present.

What does P0457 EVAP leak detected fuel cap loose or off mean?

Short answer: The EVAP system found a large leak that the computer attributes to the fuel cap or capless filler, often after refueling; check and reseat the cap, inspect the filler neck seal, then smoke test if it returns.

P0457 typically sets when the EVAP monitor detects a large leak soon after a refueling event. The most common cause is a cap that was not tightened or a capless filler that did not seal. Many vehicles show a check fuel cap message along with the code.

Inspect the cap seal for cracks or debris and the filler neck for rust or damage, which is common on older vehicles. On capless systems, check that the flap closes fully and is not held open by dirt.

If the cap and filler are good and the code returns, smoke test the EVAP system to find the leak, and test the purge and vent valves. Clear the code after repair and let the EVAP monitor run. Perform EVAP work with the engine off and away from ignition sources, since fuel vapors are flammable.

What does P0496 EVAP high purge flow mean?

Short answer: The engine computer saw more vapor flow into the intake than expected when purge should be closed or limited, which usually means a purge valve stuck open or leaking.

The purge valve controls vapor flow from the charcoal canister into the intake. When it sticks open, engine vacuum pulls vapor and air into the intake at the wrong time, which can cause rough idle after refueling, hard starting and P0496.

Command the purge valve closed with a scan tool and check whether vacuum still passes through it. Many technicians remove the valve and test it with a hand vacuum pump. Inspect the canister for liquid fuel or charcoal debris, which can come from overfilling the tank.

Replace a leaking purge valve and check for charcoal contamination in the lines. Work with the engine off when disconnecting EVAP lines, keep ignition sources away, and clean up any fuel spills.

What does P2096 post catalyst fuel trim too lean bank 1 mean?

Short answer: The downstream oxygen sensor reported a lean condition that the post-catalyst trim could not correct, often caused by an exhaust leak near the sensor, a failing sensor or an overall lean condition.

Many engines use the downstream oxygen sensor to fine-tune fuel control. If the downstream sensor stays lean and the computer reaches its correction limit, P2096 sets.

Exhaust leaks between the engine and the downstream sensor are common causes, since outside air makes the sensor read lean. Check for leaks at flanges, gaskets and the converter. Check upstream fuel trims too; a lean engine can cause this code.

Compare upstream and downstream sensor data. A downstream sensor that never moves may be faulty. Repair leaks before replacing sensors, and let the exhaust cool before working on it to avoid burns.

What does P0403 EGR solenoid control circuit mean?

Short answer: P0403 means the engine module found a fault in the EGR valve or solenoid control circuit, such as an open, short or failed valve; test the circuit and valve before replacing parts.

Many engines control exhaust gas recirculation with an electric solenoid or an electronically controlled EGR valve. P0403 sets when the module sees an electrical problem in the control circuit, not necessarily a flow problem.

Common causes include a damaged connector, broken or shorted wiring, a failed solenoid or valve coil, and in some cases a module driver fault. Carbon buildup usually causes flow codes rather than circuit codes, but it can make a valve stick.

Command the valve with a scan tool and watch its position or response. Check power, ground and the control signal with a meter or scope, and compare coil resistance with service information. Let the exhaust cool before touching the EGR valve to avoid burns.