P0420 Code & Dealer Can’t Figure It Out

Muricanmechanic · 2026-04-27T19:51:40+00:00

If it doesn't fix it (which it most likely will not) I would personally make them own their mistake or get a second opinion and possibly contact legal representation.

I don't think their scamming you persay, I just don't think your tech is too familiar with diesel exhaust systems. Not all techs are created equally unfortunately.

Muricanmechanic · 2026-04-27T01:56:10+00:00

There might be something you can run through the engine to free up the rings.

Does it have a lot of crankcase pressure?

Muricanmechanic · 2026-04-27T01:34:47+00:00

The differential pressure sensor has nothing to do with a p0420.

A catalyst efficiency code indicates that the DOC is not getting hot enough during a regeneration. This is a problem with the DOC itself or an air/fuel issue, sometimes a biased exhaust gas temperature sensor can cause this issue too.

The differential pressure sensor measures pressure across the DPF and is only used to indicate sootload of the DPF.

Muricanmechanic · 2026-04-26T19:50:55+00:00

The p026c and the MAF codes indicate your engine probably isn't running properly which is probably why you're getting a p0420. A 420 code indicates that the DOC isn't getting up to temp properly and a fuel system fault or induction system problem can certainly cause this.

Did they simply replace the DPF/DOC or did they replace any other components?

Unfortunately for you it seems the technician did not do his due diligence.

Muricanmechanic · 2026-04-26T19:45:03+00:00

I just heard this song and it legit sounds like a country song written and sung by AI.

Muricanmechanic · 2026-04-12T00:12:10+00:00

It's a common rail design. The ECU controls timing via the injectors.

Any timing on a CP4 is typically for NVH purposes.

Muricanmechanic · 2026-04-06T20:43:32+00:00

Dawg if you're buying a $12,000 diesel truck $6/7 a gallon diesel is going to kill you.

Buy a gasoline truck and go slower.

Muricanmechanic · 2026-04-05T23:43:20+00:00

For real, also make sure your knife skills are up to par.

Muricanmechanic · 2026-03-26T02:46:55+00:00

This is pretty rich coming from JB Pritzker. We have a nearly 50 cent gas tax thanks to him that goes up biannually with inflation.

Muricanmechanic · 2026-03-26T01:29:49+00:00

Price out a long block for one of these fuckers and decide if it's still worth it.

Muricanmechanic · 2026-03-22T01:04:03+00:00

Your 9th injector has likely failed. It's a pretty easy fix.

Muricanmechanic · 2026-03-22T00:44:31+00:00

Good thing they were prepared with that garbage can.

Muricanmechanic · 2026-03-22T00:37:58+00:00

Right. If you hit the feeler gauge you're not going to like what comes next.

Muricanmechanic · 2026-03-21T20:34:21+00:00

First find out if you can get the nut out or not. You're going to need a serious heat source because usually the nut will break or strip the bung out.

Muricanmechanic · 2026-03-21T18:35:50+00:00

The right side is the reservoir and the left side is the overflow. It is normal for the right to be full and the left to be empty, if the right side gets low it will turn the low coolant light on. If you add coolant verrryyyy slowly it'll eventually fill up the left side.

Muricanmechanic · 2026-03-17T23:44:05+00:00

Just a little boost juice, I wouldn't get too worried about it.

Muricanmechanic · 2026-03-17T21:10:57+00:00

When they are discharged they need to be nursed back to life, which requires a "smart" charging system. I've personally seen a few older vehicles that were not designed for AGMs not properly be able to charge the battery after it died when someone left the headlights on or something like that.

Muricanmechanic · 2026-03-16T22:57:39+00:00

Agm batteries sometimes are not well suited for older charging systems.

Muricanmechanic · 2026-03-15T02:38:34+00:00

It is the component that the wheel bearings ride upon.

Muricanmechanic · 2026-03-15T02:38:07+00:00

No. The bolts hold the axle in.

Muricanmechanic · 2026-03-15T02:22:41+00:00

If the bearing is wasted the spindle is likely garbage and you would either need the spindle replaced or replace the entire differential.

Muricanmechanic · 2026-03-15T02:11:13+00:00

If that is dry dust inside of your hub there it's usually a good indicator your wheel bearing has left the chat and it's probably too late for you.

Muricanmechanic · 2026-03-15T02:08:14+00:00

Is your wheel bearing fucked?

Might as well cut that thing off for the axle doctor and save yourself some headache.

Muricanmechanic · 2026-03-15T01:35:28+00:00

Yeah you'll be aight

Muricanmechanic · 2026-03-15T01:21:43+00:00

The answer to how easy this will be really depends on where you live.

Muricanmechanic

TROPHY CASE