Diagnosing faults¶

The dashboard reads; the diagnosis is yours. This page is the method, fault by fault - the exhaust gas temperature sensors, then the particulate filter: what obd-tui can show about one, how to read it, and what usually fixes it. Each section assumes the basics from Usage: an adapter that connects, the PID catalogue (p) to check what the vehicle answers, and --record to keep a drive for later.

Two habits pay off whatever the fault. Read the sensors with the engine completely cold, ideally left overnight: every temperature should then sit near ambient, and one that does not stands out. And record the first drive after a repair, so the next time the code comes back there is a healthy trace to compare against.

Exhaust gas temperature sensors¶

What the dashboard reads¶

The exhaust panel (6) reads mode 01 PIDs 0x78 and 0x79, one per bank, each answering the whole bank in one frame: a bitmap of the sensors fitted, then four temperatures. obd-tui shows the fitted ones, up to four per bank, upstream first: sensor 1 sits before the turbine on most diesels, the last one past the particulate filter. Rows are named the way a trouble code names a sensor - B1S2 is bank 1 sensor 2.

python-obd's own table stops at PID 0x5F, so these are declared by obd-tui itself, and so is the supported-PID bitmap that covers them, PID 0x60. On connect the vehicle is asked for that bitmap; only a bank it names there is ever asked for. The PID catalogue (p) lists EGT_BANK_1 and EGT_BANK_2 as [x] or [ ] accordingly. Most four-cylinder diesels have one bank; a V engine has two.

Both PIDs are optional in the standard. Diesels with a particulate filter usually have the first, since the filter's regeneration is controlled from those sensors; many petrol engines do not, and the panel then says No data reported by the vehicle. A manufacturer that exposes the temperatures some other way can fill in through its profile; the panel reads the same either way.

A recording writes the banks as egt_banks, one list of four sensor slots per bank number, null for a slot not fitted. The banks are read every fifth sweep, and every sweep while the panel is open.

The codes¶

The ECU raises one code per sensor and per failure, and the code names both:

Low and high describe the signal, not the exhaust: a sensor whose circuit is shorted reads one end of the scale, and one whose circuit is open reads the other. P2033 - sensor circuit high, bank 1 sensor 2 - is the ECU saying the second sensor reports a temperature it cannot believe.

Reading the panel¶

1. Connect the adapter and switch the ignition on, engine off and completely cold.
2. Open the PID catalogue (p) and check that the bank the code names, EGT_BANK_1 or EGT_BANK_2, is marked [x]. If it is not, the vehicle does not report that bank and there is nothing more to see here.

3. Open the exhaust panel (6). Cold, every sensor should read close to the ambient temperature shown on the engine panel:

   B1S1 °C           19.0
   B1S2 °C           18.0
   B1S3 °C           19.0
   

   A sensor whose circuit is open or shorted reads one end of the scale instead, and gets a note:

   B1S1 °C           19.0
   B1S2 °C         1000.0  ⚠ far from the other sensors
   B1S3 °C           18.0
   

4. Start the engine and watch the bank warm up. The sensors should climb together, the upstream one first and fastest; one that stays put, or stays pegged, is the one the code names.

The note is a hint for the eye, not a verdict. It marks a sensor sitting more than 300 °C from the median of all the others, which under load a healthy exhaust never does, and a failed circuit always does. With only two sensors fitted it cannot tell which of the two is wrong - both get the note, and the ambient temperature settles it.

Remediation¶

A sensor that reads one end of the scale on a cold engine is almost always the sensor or its wiring, not the exhaust:

• Connector first. Exhaust sensors live in heat and road spray; a corroded or loose connector is the cheapest find. Unplug, inspect, and reseat. Watch the panel while wiggling the harness - a reading that jumps is a wire.
• Then the sensor. Most EGT sensors are thermistors; the workshop manual gives a resistance at room temperature. One that reads open or shorted is done. They screw into the exhaust and seize; penetrating oil and patience beat force.
• Then the loom. A sensor that measures fine off the car but reads wrong on it points at the wiring to the ECU.

After the repair, clear the codes from the faults panel (x), let the engine cool, and read the bank cold again: every sensor near ambient, no note. Then drive it and watch the sensors climb together. A particulate filter whose regeneration was blocked by the bad reading may need a few drives, or a forced regeneration from a workshop tool, before its own codes clear.

Particulate filter¶

What the dashboard reads¶

The DPF panel (7) gathers everything the vehicle says about its particulate filter and, beside it, what the dashboard works out from that. Three standard mode 01 PIDs feed it, each optional and each asked for only when the vehicle names it in its supported-PID bitmap:

A temperature inside the filter, DPF_TEMP_INTERNAL, has no standard slot either, and comes only from a manufacturer.

The soot load has no standard PID. It comes from the manufacturer's own identifiers, which differ from one engine's ECU to the next, so it is read only when the engine is declared and a profile has a cited table for it. Compatibility says what is declared today.

Two rows are guesses when the vehicle gives nothing better, and say so. A filter temperature marked (exhaust sensor) is an exhaust gas sensor the manufacturer profile placed at the filter, not the filter's own. A regeneration reading probable with estimated on the source row was inferred from a hot filter at moderate load, not reported. Neither ever replaces what the ECU said when it said something.

Rows marked (derived) were worked out by the dashboard: the restriction per gram per second of air, the inlet-to-outlet temperature delta, and the time since a regeneration the dashboard saw end.

The codes¶

The pressure sensor codes are the ones to rule out first: a filter reads as blocked, or as empty, through a sensor that has failed.

Reading the panel¶

The restriction rises with the flow through the filter, so a number on its own means little; the panel puts the engine speed and air flow beside it, and works out the restriction per unit of flow so two moments compare.

1. At idle, warm, note the differential pressure and the ΔP / MAF row. A clean filter puts a few kPa in the way.
2. Hold a steady 2500 rpm and note both again. The differential rises with the flow; the per-flow figure should not rise much.
3. Compare the DPF ΔT row across the two: a filter doing nothing runs the outlet a little cooler than the inlet. During a regeneration the outlet runs hotter, and the exhaust sensors climb past 550 °C.

A differential marked (elevated) sits past 20 kPa, a wide bound that a healthy filter under load rarely reaches. It is a reason to look, not a diagnosis: the per-flow figure, the soot load when the vehicle gives one, and a recorded drive to compare against are what turn it into one. A differential marked (inconsistent) reads more pressure after the filter than before, which is a sensor or a hose, not a filter.

Watch a regeneration through: SINCE LAST restarts when it ends, and the differential should drop afterwards. One that does not drop after a completed regeneration is ash, not soot, and P242F will say so in time.

Remediation¶

• A pressure that does not move with the flow is a blocked hose or a failed sensor. The two hoses to the differential pressure sensor clog with soot; clearing them is a five-minute job, and the sensor's own codes point at it.
• A load that keeps climbing between regenerations on a car driven short distances is the driving, not the filter. A regeneration needs the exhaust hot for a quarter of an hour; a longer drive lets it finish. A workshop tool can force one.
• A restriction that survives a completed regeneration is ash. Ash is not burnt off; the filter is cleaned or replaced.
• An estimated regeneration that never reads probable on a vehicle that should regenerate every few hundred kilometres is worth a look at the exhaust temperatures: a sensor reading low keeps the ECU from ever seeing the filter hot enough.

After a repair, clear the codes from the faults panel (x), record the next drive with --record, and compare its dpf_pressure and diesel columns against the one before.