P0420: P0420 Diagnostic Guide

What this code means

P0420 may indicate the PCM is seeing catalyst efficiency lower than expected during aftertreatment operation.

What the vehicle may do

The vehicle may show a warning light.
The vehicle can appear to drive normally in some cases.
Aftertreatment operation may be affected depending on the underlying concern.

Possible fault areas

Possible catalyst system performance concern.
Possible downstream injector or fuel injection concern.
Possible fuel quality or reductant quality concern.
Possible excessive oil consumption concern.
Operating conditions may contribute on heavily loaded or high temperature use.

Diagnostic path

Set up the P0420 diagnosis

On this 6.7L Power Stroke, P0420 may mean the PCM is seeing catalyst efficiency lower than expected during aftertreatment operation. The truck may only show a warning light, and in some cases it can seem to drive normally, but aftertreatment operation may be affected depending on the underlying concern. Think broad first: this can involve a catalyst system concern, downstream injector or fuel injection issue, fuel quality, reductant quality, excessive oil consumption, or operating conditions that keep the system from seeing the temperature change it expects. For this fault, the PCM sets P0420 when it detects the temperature increase is less than 40% of the expected maximum increase. During regeneration, values can be biased, so if regeneration starts while you are testing, let it finish before continuing. You can monitor the EGT11 PID and EGT12 PID to watch the temperature change across the oxidation catalyst area.

Confirm the target fault is present

Start by confirming P0420 is actually present. If it is present, move on to the next check. If it is not present, treat it as an unable-to-duplicate concern for this path. Check for a loose connection, damaged terminals, or corroded terminals, and wiggle the harness while trying to recreate the concern. Repair anything you find as necessary, but do not guess at the catalyst if the target fault is not there.

Handle other codes first

Next, check for other self-test codes. If other codes are present, set P0420 aside for the moment and diagnose the next code first. If no other codes are present, continue with the P0420 catalyst path.

Inspect for exhaust and sensor connection concerns

Now inspect the exhaust side. Look at the oxidation catalytic converter, the downstream injector, and the exhaust gas temperature sensors. Make sure the exhaust tube fittings and sensor exhaust ports are connected correctly, and look for leaks, cracks, pinches, or punctures. If you find a concern, repair it as necessary, clear the PCM codes, and repeat the self-test. If that inspection is clean, keep going.

Check fuel and reductant quality

After the exhaust inspection, check the fuel and reductant. Verify the fuel level, grade, and quality are sufficient for normal engine operation, and carry out a sufficient clean fuel check. Confirm with the customer that ultra low sulfur diesel fuel is being used. Only use ultra low sulfur diesel fuel that is 20% or less biodiesel with this vehicle. Using a different fuel type may damage the vehicle or cause an emissions concern. Also check reductant level and quality. If there is a concern here, repair it as necessary, advise the customer on the correct fuel type, clear the PCM codes, and repeat the self-test. If fuel and reductant check out, move on.

Ask about operating conditions

With the ignition off, talk with the customer about how the truck is being used. Ask about excessive trailer weight, steep uphill operation, high engine load operation, and high ambient temperature operation. Some high temperature conditions before the oxidation catalyst can limit the temperature increase across it and may set P0420. If the truck is consistently operated under those high load or high temperature conditions, advise the customer that those conditions may be causing the fault, clear the PCM codes, and repeat the self-test. If that is not the pattern, continue to the oil consumption check.

Check oil consumption and finish the P0420 path

Next, carry out the oil consumption test. If excessive oil consumption is found, repair that concern as necessary, clear the PCM codes, and repeat the self-test. If no oil consumption concern is found for P0420, the path ends with installing a new oxidation catalytic converter. After that, run a diesel particulate filter manual regeneration with the scan tool, perform the required DPF parameter reset, reset and clear the oxidation catalyst function, and initialize SCR learning with the scan tool. Then carry out the drive cycle, clear the PCM codes, and repeat the self-test.

Verify the repair

Keep verification separate from the testing. Any time this path has you repair a found concern, clear the PCM DTCs and repeat the self-test. At the end, verify the repair and confirm P0420 stays gone. The takeaway is simple: do not condemn the catalyst until you have checked code priority, exhaust integrity, fuel and reductant quality, operating conditions, and oil consumption in order. For more diagnostic training, visit stepdiagnostics.com.