Check engine light

Solid or flashing, and why it matters

A solid check engine light means the engine computer has stored a fault, usually something affecting emissions or how the engine runs. It is not an emergency on its own. If the car drives normally, keep driving gently and book a scan in the next few days rather than the next few months.

A flashing light is different. It means an active misfire, bad enough that unburned fuel is being pushed into the exhaust, where it burns inside the catalytic converter and overheats it. If the light is flashing, ease off the gas and get off the road as soon as you safely can. Driving across town on the 417 with it flashing can turn a coil or spark plug repair into a converter replacement.

We do not tow, so if the car is shaking badly or the light will not stop flashing, arrange a tow to 1541 Star Top Road and call (613) 744-1521 first. And if a red oil pressure or engine temperature light comes on with it, stop and shut the engine off. Those are not check engine light problems.

Check the gas cap first

The cheapest cause is worth ruling out yourself. A loose, cross threaded or cracked gas cap lets fuel vapour out, and the evaporative emissions system, called EVAP, sets a leak code. Codes like P0455 and P0457 often come down to the cap. In an Ottawa winter the rubber seal on an older cap goes hard and stops sealing, and filling up at minus 20 with gloves on is when caps get left loose.

Tighten it until it clicks. The light will not go off right away, because the EVAP system only tests itself under certain conditions, often after the car has sat overnight. Give it a few days of normal driving. If the light is still on after a week, book a scan.

Misfires

A misfire means a cylinder is not burning its fuel properly. P0300 is a random misfire, and P0301 to P0308 point to a specific cylinder. The car may shake at idle, hesitate when you accelerate or feel down on power.

The common causes are a worn spark plug, a failing ignition coil, or a cracked coil boot that lets moisture reach the plug. That last one is very Ottawa. Slush spray on the 174, or a car wash in January, gets water into the plug wells and the misfire shows up the next cold morning. Fuel injectors and, on some direct injection engines, carbon build up on the intake valves are next on the list. Low compression from a mechanical fault is less common, and if the testing points there, it moves over to engine repair.

Catalytic converter codes

P0420 and P0430 mean the converter’s efficiency has dropped below what the computer expects. The converter itself is often not the root cause. It fails because something upstream damaged it, like a misfire, an engine burning oil, or coolant getting into the cylinders. Replace the converter without fixing that and the new one goes the same way.

The code can also be a false alarm. The computer judges the converter by comparing the oxygen sensors before and after it, so a lazy rear sensor or an exhaust leak ahead of the sensor can set the same code. Salt rotted flex pipes and cracked manifolds are common on Ottawa cars, and those are worth checking before anyone prices a converter. Exhaust leaks and rusted pipes are covered on the muffler and exhaust page.

Other common causes here

A few more codes come up often enough in this climate to mention.

• P0128, coolant below thermostat temperature. A thermostat stuck open sets this in winter, usually along with weak heat. The no heat page covers that side of it.
• Oxygen sensor heater circuit faults, which tend to show up after a cold snap.
• A dirty or failing mass airflow sensor, which can set lean codes and make the car stumble.
• A stuck EVAP purge valve, which can make the car hard to start right after filling up.
• Low system voltage from a weak battery, which can set strange codes after a hard cold start.

What happens at the scan

The scan is the same $50, before tax, as any computer diagnostic here. We read the codes and the freeze frame data, which shows what the engine was doing when the light came on, look at the parts the code points to, and tell you what it means and what the repair would involve. The computer diagnostics page explains the process in more detail.

One thing not to do is clear the code and hope. A fault that is still there sets the code again, and clearing it resets the readiness monitors, which shows up on the next scan anyway.

Bringing it in

Book the scan online any hour and pick computer scanning or diagnostics. In the description, say whether the light is solid or flashing, when it came on, and anything the car did at the same time, like a shake, a smell or a fill up. A photo of the dash helps. You can drop the car off until 2pm.

SA Auto Repair is at Unit 1, 1541 Star Top Road, Ottawa, ON, just off the 417 at Exit 112 (Innes Road). We are open weekdays 8:30am to 6pm and Saturday 9:30am to 2pm, and we answer in English or French.

Questions people ask