Diagnosing a Hot Start Problem

Print-ready PDF export — generated on 05/10/2026

911Tech
Model:Porsche 911
Type:964
Category:Electrical
Difficulty:Difficult

Difficult hot starts, abnormally high idle speeds, or engine stalling can have several causes. The original article suggests checking the cylinder head CTN sensor located near cylinder No. 6. Before drawing any conclusions, also check the DME relay, power supplies, ground connections, fuel pressure, and wiring.

Figure 12.1 — Locating Terminals 30 and 45 on the ECU's female connector.
Figure 1 — Locating terminals 30 and 45 on the ECU's female connector.

Overview

Difficult hot starts, abnormally high idle speeds, or engine stalling can have several causes. The original article suggests checking the cylinder head CTN sensor located near cylinder No. 6. Before drawing any conclusions, also check the DME relay, power supplies, ground connections, fuel pressure, and wiring.

Figure 12.2 — Complete pinout diagram for the ECU connector for the electrical control system.
Figure 2 — Complete pinout diagram for the ECU connector for the electrical control system.

Control Values

The measured values must fall within the following ranges:

• at 0 °C: from 4.4 to 6.8 kΩ;

• 15 to 30 °C: 1.4 to 3.6 kΩ;

• at 40 °C: 1 to 1.3 kΩ;

• at 80 °C: 250 to 390 Ω;

• at 100 °C: 160 to 210 Ω.

Note: These ranges are taken from the original article. They should be compared with the values in the manual for the specific ECU and sensor installed.

Figure 12.3 — Exploded view of the cylinder head for cylinder No. 6, with the CTN sensor labeled.
Figure 3 — Exploded view of the cylinder head for cylinder No. 6, with the CTN sensor labeled.

Explanation

Any value outside these ranges — whether higher or lower — indicates a component failure. The engine control unit then receives incorrect information and may enrich the air-fuel mixture when it should be leaning it out, or vice versa. The vehicle may come to a complete stop and be unable to restart until the temperature has dropped.