The question customers ask is simple and the answer depends entirely on which warning behavior they are seeing. Both of these codes describe conditions that get worse with continued operation, and both can turn a moderate repair into a major one over the course of a few hundred miles. Understanding what each code protects explains why the advice differs.
What Each Code Is Reporting
A p0300 code means the control module detected misfire events across more than one cylinder, or could not attribute them to a single cylinder. Because it is a random or multiple misfire, the cause is usually something shared: fuel delivery, ignition supply, a vacuum leak, low compression across cylinders, or a sensor feeding bad data to the fuel and spark calculations.
A p20ee code applies to diesel vehicles with selective catalytic reduction. It means the module compared upstream and downstream NOx sensor readings and concluded the SCR catalyst is not converting nitrogen oxides at the required rate. Diesel exhaust fluid quality, dosing hardware, NOx sensor accuracy and catalyst condition all feed into that calculation.
Driving With a Misfire: The Honest Answer
If the check engine light is flashing, the vehicle should not be driven. A flashing light indicates a misfire rate high enough to send unburned fuel into the exhaust, where it ignites in the catalytic converter and drives substrate temperature past its design limit. Damage happens in minutes, not months.
If the light is steady, short and gentle driving to a repair facility is generally acceptable. Keep engine load low, avoid highway speeds and heavy throttle, and keep the trip short. A misfire under load produces far more unburned fuel than the same misfire at idle.
Beyond the converter, a persistent misfire washes cylinder walls with fuel, dilutes engine oil, loads spark plugs and stresses catalytic and ignition components that were fine before the fault appeared. The repair bill grows with every mile.
Driving With an SCR Efficiency Fault
Diesel emissions faults follow a different logic. Regulations require the vehicle to warn, then restrict, when aftertreatment performance falls out of compliance. Many platforms display a countdown in miles or engine hours, followed by a speed limited derate that caps the vehicle at a low road speed until the fault is corrected.
That means the immediate mechanical risk is lower than with a misfire, but the operational risk is high. A work truck that derates on the interstate is a serious problem, and for fleet operators the downtime usually costs more than the repair. Treat the countdown as a hard deadline rather than a suggestion.
There is also a component risk. Contaminated or diluted diesel exhaust fluid, or a dosing injector that has stopped atomizing properly, can crystallize inside the decomposition tube and the catalyst face. Left alone, that turns a fluid or injector repair into a catalyst replacement.
Working Through the Misfire
Start with the shared systems, since a random misfire points to a common cause. Check fuel pressure and volume under load, inspect for vacuum leaks at the intake and the brake booster, review long term fuel trims for a lean or rich bias, and scope the crankshaft and camshaft position signals for noise or dropout.
Then move to the mechanical side. A relative compression test across all cylinders is faster than a wet and dry test on each one and catches worn timing components, a stretched chain or valvetrain wear. On direct injection engines, carbon buildup on the intake valves is a frequent cause of multiple cylinder misfire at low load.
Record misfire counters per cylinder while the engine runs through idle, cruise and load. Even a random misfire usually shows a pattern once you watch the counters long enough, and that pattern narrows the search considerably.
Working Through the SCR Fault
Test the diesel exhaust fluid first. A refractometer check for concentration takes a minute and rules out the most common cause, which is fluid that was diluted with water or contaminated in a dirty container. Replace suspect fluid and flush the tank rather than topping it off.
Next verify dosing. Command the injector with a scan tool and confirm both electrical function and actual spray, then check the delivery lines and the pump pressure. Inspect the decomposition tube and the catalyst inlet for white crystalline deposits, which indicate poor atomization.
Validate the NOx sensors before condemning the catalyst. These sensors drift with age, and a downstream sensor reading high produces this code on a healthy catalyst. Compare both sensor readings at a steady operating point and against expected values in the service data.
Practical Guidance for Customers
Give a clear instruction rather than a general caution. Flashing light means park it and arrange a tow. Steady light with a misfire means drive it directly to the shop, gently, and do not take it on the highway. A diesel with an SCR fault and an active countdown means book the appointment now, before the derate takes the vehicle out of service.
Frame the cost comparison plainly. Spark plugs, coils, an injector or a fluid flush are modest repairs. A catalytic converter or an SCR catalyst is not. Customers who understand that difference make the right decision about the next two hundred miles.
